Keithley 181 User Manual

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Operator’s Manual

Model 181

Digital Nanovoltmeter

01982. Keithley Instruments, Inc Cleveland, Ohio, U.S.A.

Document Number 32421

181

NANOVOLTMETER

IEEE-488 BUS IMPLEMENTATION MULTlLlNE COMMANDS: DCL, LLCJ, xc, GET. “NlLINE COMMANDS: IFC, REN, EOI, SRQ, ATN. PROGRAMMABLE PARAMETERr3

Front Panel Controls: Range, Filter, Zero, Damping, Hi

Resoluli”“.

Internal Pnrametea: SRQ Rcsponsc, Trigger Modes, Data Ter-

minators.

GENERAL

DISPLAY: Seven 0.5 in. LED digits with appropriate decimal point

and polarity.

NOISE: <30”” p-p on lowest range wit,, Filter on.

lNP”T CAPACITANCE: 5OOOpF on mV ranges.

SETTLING TIME: 0.5 sec. to within 25 COLMS of final reading with

Fikr on, Damping off. FILTER: 3-polcdigital; RC = 0.5.l.or2 wands dcpcndingonrangc. CON”CRSlON WEBI): 4 readings/second. OVERLOAD INDICATION: Display indicates polarity and OFI.0. ANALOG OUTPUT: Accuracy: +(0.15% of displayed reading +

ImW Time Constant: 400mscc. Level: f2V full scale on all rongcs;

xl or xl000 gain.

ISOLATION: Input LO to Output LO or power line ground: ,400”

peak, 5 x IOW*Hz, >I09 paralleled by 150OpF. WARM-UP: 1 hour to r&cd accuracy. ENVIRONMENTAL LIMITS: Operating: 0”35”C, O%-80% *da-

tive humidity. Storage: -25” to +WC. POWER: 105-123’ or 21~25OV (internal switch selected), 50-6OHz,

30Vh maximum.

ADDRESS MODES: TALK ONLY and ADDRESSABLE. TRIGGlX MODES: One Shot: Updatcs output buffer once at first

valid conversion after triggeronTALKand/arG~T. Continuous: Updates output buffer at al, valid ~onwr~ion~ after trigger.

INFUT CONNECTOR% Special low thermal for 2”OmV and lower

ranges. Binding posts for 2V to lOO”V ranges.

DIMENSIONS, WEIGHT: 127mm high x 21hmm wide x 359mm

deep (5 in. x 8.5 in. x 14.125 in.). Net weight 3.85kg (8.5 Ibs.). ACCESSORY SUPPLIED: Model ,506 Low ‘Thermal fnput Cable. ACCESSORfES AVAILABLE:

Model 262: Low Thermal Voltage Divider

Model 1019A-1: 5%-i,,. Single Fixed Rack Mounting Kit

Model 1019A-2: 5’/rin. Dual Fixed Rack Mounting Kit

Model 1019SI: 5’,~-in. Single Slide Rack Mounting Kit

Model 1019S.2: Y/1-in. Dual Slide Rack Mounting Kit

Model 1483: Low Thermal Connechan Kit

Model 1484:

Madcl1485: Fcmalc Low Thermal Input Conneck!r

Model 1486: Male Low Thermal Input Connector

Model 1488: Low Thermal Shorting I’lug

Made, 1506: Low Thermal Input Cable (4 ft., clips)

Model 1507:

Model ,815: Maintcnancc Kit

Model 8003: Low Resistance Test Fixture

&fill Kit for 1483 Kit

Low Thermal Input Cable (4 ft., plugs)

TABLE OF CONTENTS

Paragraph

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

2.10

2.11

3.1

3.2

3.3

3.4

3.5

3.6

3.7

Title

SECTION l-GENERAL INFORMATION

Introduction.. ...............................................................................

Model181 Features.. .........................................................................

Optional Accessories

Warranty Information .........................................................................

ManualAddenda..

SafetySymbolsandTerms .....................................................................

ScopeofOperator’sManual...........................................................~.~~ .....

Specifications ................................................................................

SECTION 2-OPERATION

Introduction.. ................................................................................

Unpackingandlnspection

PreparingforOperation.................................................................~...~

Operating Conrtrolsand Connections

BasicVoltageMeasurement..........................................................~.~

Nanovolt and Microvolt Measurements

Special Measuring Situations

Additional Front Panel Controls. .................................................................

UsingtheAnalogOutput .......................................................................

Source Resistance Considerations ..........................................................

Microvolt and Nanovolt Measurement Consideration SECTION 3-APPLICATIONS

Introduction.. ................................................................................

StandardCellComparisons .....................................................................

Low Resistance “Lindeck” Measurements

TemperatureMeasurements .........................................................

ResistanceThermometry .......................................................................

SemiconductorTesting .........................................................................

JosephsonJunctionStudies ....................................................................

..........................................................................

...........................................................................

............................................................

.............................................................

....................................................... ...

....................................................................

...............................................

..................................................... ..

........

..........

....

...

Page

l-l 1~1 1~1 l-l 1~2 1~2 1~2 1-2

2~1 2~1 2~1 2-2 2-2 2~4 2-4 2-5 2~6

2-J

2-8

3-1

3~1 3-2 3-3 3~3 3-3 3-3

4.1

4.2

4.3

4.4

4.6

4.7

4.8

4.9

SECTION 4-IEEE OPERATION

Introduction to the IEEE-488 Bus .................................................................

Descriptionof BusLines ........................................................................

IEEE-488Set-UpProcedure.. ...................................................................

BusCommands.. .............................................................................

Device-Dependentcommands ..................................................................

Data Format.. ................................................................................

StatusByte Format.. ..........................................................................

StatusWordFormat ...........................................................................

ProgrammingExample .........................................................................

4-1 4-l 4-2 4-3 4~5 4-7 4-8 4-9 4-9

LIST OF ILLUSTRATIONS

Figure

l-l 2-1 2-2 2-3 2-4 2-5 2-6 2-7 2-8 2-9 2-10 2-11 2-12 2-13 2-14 3-l 3-2 3-3 3-4 4-1 4-2 4-3

4-4

4-5

4-6

4-7

4-8

Model 181 Front Panel View. .............

Line Voltage Switch Location ............

Front Panel Controls and Connections Rear Panel Controls and Connections. Basic Voltage Measurements.

mV and nV Measurements ...............

Common Ground Connections for V and mV

Filter Response Graph. ..................

Analog Output Connections ..............

Xl000 Analog Output ...................

Source Resistance Consideration .........

Thermal emf Generation .................

Power Line Ground Loops ...............

Ground Loop Voltage Generation .........

Eliminating Ground Loops ...............

Standard Cell Comparison ...............

Absolute Cell Measurement Connections. Low Resistance Measurement Connections. Minimizing Josephson Junction RFI Effects

IEEE Bus Configuration ..................

IEEEHandshakeSequence ...............

Primary Address and IEEE Mode Switches.

IEEE Contact Configuration ..............

IEEE Bus Data Format ...................

Status Byte Format .....................

Programming Example ..................

Timing Diagram .......................

............

.....

Title Page

......

.....

......

.....

......

.....

......

.....

..........

.......... 2-l

.......... 2-3

.......... 2-5

.......... 2-6

.......... 2-7

.......... 2-8

.......... 2-9

......... 2-10

..........

.......... 3-2

..........

.......... 4-l

.......... 4-2

..........

.......... 4-8

.......... 4-9

......... 4-11

l-2

2-4 2-4

2-9

3-l 3-2

3-3

4-3 4-3

4-13

Table

2-l

2-2

2-3

4-l

4-2

4-3

4-4

4-5

4-6

4-7

4-8

4-S

Fuse Selection .........................

Settling Times .........................

Analog Output Parameters. ..............

IEEE Contact Designations ...............

Bus Command Summary ................

Device-Dependent Command Summary

Range Commands ......................

Default Conditions. .....................

Data String Exponent Values .............

Error and Data Code Summary ...........

Status Word Example ...................

HP-85 BASIC IEEE-488 Statements .......

LIST OF TABLES

...

......

., ....

Page

2-l 2-6 2-7 4-4 4-4 4-6 4-6 4-7 4-8 4-9 4-9

4-10

SECTION 1

GENERAL INFORMATION

1.1 INTRODUCTION The Keithley Model 181 is a highly sensitive nanovoltmeter

with a large, easy to read 5 K or 6 ‘/ digit display. The Model 181 is unique in that it combines microprocessor technology with a new concept in low-noise, high-gain front ends, resulting in a programmable instrument with sensitivity down to 10nV. The Model 181 provides highly accurate, stable, low-noise readings on seven ranges for DC voltage measurements between 1OnV and 1OOOV. The mV ranges use a special low-thermal input connector, while connections for the higher voltage ranges are made through two S-way binding posts. Additional versatility is afforded by the inclusion of an IEEE-488 interface which allows the unit to communicate with other instrumentation.

1.2 MODEL 181 FEATURES The Model 181 includes the following features:

High Sensitivity. The resolution of the Model 181 on the

2mV range is IO ~* volts (10nV).

5% or 6% Digit Resolution. Normal 5% digit display

resolution may be increased to 6 % digits at the touch of a button.

IEEE-488 Interface. A built in IEEE-488 interface allows the instrument to communicate with other devices such as a central controller or printer.

Analog Output. An analog output, which accurately

reflects the displayed readings, is available from the rear

pallel.

3-p& Digital Filter. The internal 3-p& filter minimizes the

effects of noise in voltage readings and may be controlled from the front panel or IEEE bus.

Separate Inputs. A special input connector is used for the

mV ranges to minimize thermal emf generation.

Isolated Low Terminals. The low signal connections for

both inputs are isolated from power line ground and

from IEEE low to minimize ground loop problems.

Color Coded Front Panel. Inputs, range switches, and

other front panel controls are marked to form color-coded

groups for easier operation.

1.3 OPTIONAL ACCESSORIES

Model 1483 Low-Thermal Connection Kit. The Model

1483 kit contains a crimp tool, pure copper lugs, Lowe thermal cadmium solder, copper alligator clips, and assorted hardware. It may be used for constructing experimental circuits with low-thermal connections to minimize thermal emf effects.

Model 1484 Refill Kit. The Model 1484 kit contains replacement parts for the Model 1483.

Model 1485 Low-Thermal Female Connector. The Model 1485 connector is used for the mV INPUT on the front panel of the Model 181.

Model 1486 Low-Thermal Male Connector. The Model 1486 connector mates with the Model 1485 female connector. It can be used to construct custom cables of various lengths. This connector is used with the Model 1506 and 1507.

Model 1488 Low-Thermal Shorting Plug. The Model 1488 provides a means of shorting the mV INPUT to check instrument offset and drift.

Model 1503 LawThermal Solder. The Model 1503 kit

contains low-thermal cadmium solder to make solder connections for low voltage measurements.

Model 1506 Low-Thermal Input Cable. The Model 1506 cable is supplied with the unit It is a specially designed, four foot triaxial cable that provides excellent shielding for sensitive measurements. The Model 1506 has two color coded alligator clips on one end, and a Model 1486 low-thermal male connector at the other end.

Model 1507 Low-Thermal Cable. The Model 1507 cable is similar to the Model 1506. except that the alligator clips are replaced by spade lugs. The Model 1507 is cons strutted of a four foot triaxial cable and has a Model 1486 low-thermal male connector on one end.

Model 1815 Maintenance Kit. The Model 1815 kit cons tains a calibration cover and extender cables that are helpful when making service adjustments to the Model

181. The calibration cover replaces the top cover while making these adjustments. The extender cables allow individual PC cards to be partially removed from the unit during maintenance.

Model 1019 Rack Mounting Kit. The Model 1019 kit allows the Model 181 to be conveniently mounted in a standard 19 inch rack.

A summary of the many optional Model 181 accessories is listed in the following paragraphs. These accessories are designed to enhance the capabilities of the instrument and are described in more detail in the Model 181 Service Manual, Document Number 30816. Contact the nearest Keithley representative or the factory to obtain accessories.

1.4 WARRANTY INFORMATION

Warranty information may be found inside the front cover of this manual. If warranty service is required, contact the

Keithley representative in your area or the factory to detw

mine the correct course of action. Keithlev maintains service

l-l

facilities in the United States, West Germany, Great Britain,

France, the Netherlands, Switzerland and Austria. lnforma-

tion concerning the application, operation or service of your

instrument may be directed to the applications engineer at any of the previously mentioned locations. Check inside front cover of this manual for addresses.

1.5 MANUAL ADDENDA

Because of a policy of constant improvement, it may

become necessary to make changes to the unit. Any modifi-

cations will be listed in an addendum attached to the inside

back cover of this manual. Be sure to note these changes

before attempting to operate the instrument.

1.6 SAFETY SYMBOLS AND TERMS

Safety symbols used in this manual are as follows:

The WARNING used in this manual explains dangers that could result in personal injury or death.

The CAUTION used in this manual explains hazards that could damage the instrument.

1.7 SCOPE OF OPERATOR’S MANUAL

This manual is intended to familiarize the operator with the operating controls and features of the Model 181 nanovolto-

meter. Some of the items covered in this manual include: basic and nanovolt measurement techniques, possible problems that could result when making measurements, addi-

tional Model 181 uses, operation of the Model 181 on the

IEEE-488 bus, and programming examples. For technical information including performance verification, theory of operation, and maintenance procedures, refer to the Model

181 Service Manual.

The symbol

A on the instrument denotes that the

user should refer to the operating instructions.

The symbol 1/2)

on the ,nstrument denotes that 1OOOV

or more may be present on the terminal(s).

1.8 SPECIFICATIONS

For Model 181 detailed specifications, refer to the specifications that precede this section.

1-2

Figure l-l. Model 181 Front Panel View

SECTION 2

OPERATION

2.1 INTRODUCTION This section contains information needed for basic Model

181 operation. Be sure to read this entire section before

attempting to operate the unit.

2.2 UNPACKING AND INSPECTION The Model 181 was carefully inspected before shipment.

Upon receiving the unit, unpack all the items from the shipping carton and check for any damage that might have occurred during shipment. Report any damage to the shipping agent at once. Save the original packing material for

possible future reshipment. Contact your nearest Keithley

representative or the factory if the unit fails to function

properly. The following items are included with every Model 181

shipment:

1. Model 181 Nanovoltmeter

2. Model 181 Operator’s Manual

3. Model 181 Service Manual

4. Model 1506 Low-Thermal Input Cable

5. List of computer programs.

6. Additional accessories as ordered.

2.3 PREPARING FOR OPERATION

transformer must be installed. Contact your Keithley representative or the factory for information.

To remove the top cover. remove the two screws securing the cover to the rear panel. Then lift off the cover from the back until the tabs at the front of the cover clear the front panel. Then remove the cover entirely.

Refer to Figure 2-1 for the location of the voltage switch. Set the switch to the appropriate voltage. Also make sure

the proper fuse is installed; refer to Table 2-l for the proper

type. Replace the top cover in the reverse order. Make sure the

tabs at the front of the cover mate with the slots in the front panel. Finally, install the two screws that secure the top cover to the rear panel.

Table 2-1. Fuse Selection

3AG. SLO BLO

Before operating the Model 181, the appropriate line voltage must be selected and the unit must be plugged into a proper power source. This section covers each of these steps; be sure to observe any precautions that are given.

1. Line Voltage Selection. The operating voltage of the Model 181 was set at the factory as indicated on the rear panel. Do not attempt to operate the unit with power line voltages outside the indicated range. If it is necessary to change the operating voltage, the top cover of the instrument must be removed to allow access to the line voltage selection switch.

WARNING

These instructions are intended for use only by qualified service personnel. Do not

remme the top cover unless qualified to do so because of the possibility of electric shock.

NOTE

The Model 181 is designed to operate with

105.125V or 210.250V as selected by the inter-

nal switch. For operation on 90.IlOV and

180.22OV power sources, a special power

POWER

TRANSFORMER

REAR PANEL

FRONT

WV=

VOLTAGE VOLTAGE

SWITCH SWITCH

Figure 2-l. Line Voltage Switch Location

2-l

Power Line Connection. The Model 181 power cord is supplied with a 3.prong plug that is designed to be used with grounded outlets. Connect the female end of this cord to the power receptacle on the rear panel of the unit. Connect the other end to an appropriate power SO”,lX.

CAUTION

Make sure the proper line voltage is selected as described in the last section.

Failure to do so may result in damage to the instrument,

possibly voiding the

warranty.

Power-up Procedure. Once the power connections have been made, the unit may be turned on by depressing the front panel power switch. The Model 181 display should show the line frequency and software revision level (e.g.

F60 b7) for approximately one second. After that, the

unit will revert to the normal display mode. In addition, the 1OOOV range indicator light should be on. This is one of the power-on default conditions that are explained

more fully in paragraph 4.5.

2.4 OPERATING CONTROLS AND CONNECTIONS Front Panel Controls. The front panel controls are shown

in Figure 2-2. In addition to the power switch previously described, the Model 181 has a number of other front panel switches. The 2mV. 20mV. and 200mV switches are used to select one of the mV measurement ranges. The 2V. 2OV. 2OOV. and 1OOOV switches are used to select one of the normal voltage ranges. The light above the selected range will turn on when the appropriate switch is depressed. Note that these switches may be superseded by IEEE commands as outlined in Section 4.

In addition to the range switches, the Model 181 has several other front panel controls. These include: the HI RES switch to select 5% or 6% resolution, the ZERO

switch to enable baseline suppression, and the FILTER and DAMPING switches, which alter the response of the internal 3-pole filter. These features will be described in more detail in later sections.

Front Panel Connections. The front panel has two input connectors. The two 5.way binding posts are used for measurements on the 2V through 1OOOV ranges, while the low-thermal mV INPUT connector is used for measurements on the 2mV through 200mV ranges.

When using the mV INPUT, be sure to use the supplied

low-thermal cable to minimize errors caused by thermal emfs.

Display. The 6% digit display is used to make Model 181 voltage readings. The display may be switched to either

5% or 6% digits at the touch of a button. A leading

minus sign appears when negative voltages are measured, and the decimal point is automatically placed. Overrange is indicated by an “OFLO” message.

4. IEEE Status Lights. The TALK, LISTEN, and REMOTE indicator lights show the present IEEE status of the Model

181. For complete IEEE information, refer to Section 4.

5. Rear Panel Controls and Connections. The rear panel Controls and connections are shown in Figure 2-3. An analog output is available through the two 5-way binding posts. The switches and connector shown in the lower left corner are for use with the IEEE-488 bus. The functions and operation of these connectors and switches will be covered in more detail in later paragraphs.

6. Tilt Sail. The tilt bail is useful for elevating the front panel of the instrument to a convenient height. To extend the

tilt bail, rotate it 90’ away from the bottom cover; then

push the bail upward until it locks into place. To retract the bail, first pull the bail down away from the front cover

to release the locking mechanism; then rotate the bail until it is flush with the bottom cover.

2.5 BASIC VOLTAGE MEASUREMENT Normal voltage measurements are made on the 2V through

IOOOV ranges. To use one of these ranges, the source to be

measured must be connected to the V INPUT. The follow-

ing paragraphs describe the basic procedure for making

these voltage measurements.

Turn on the Model 181 by depressing the front panel power switch. As previously described, the unit should momentarily display the line frequency and software revision level. Allow a one hour warm-up period to obtain rated accuracy. Four hours are required for minimum drift.

Select the desired voltage range by depressing the appropriate range button. Select a range that can easily handle the maximum voltage to be measured.

Select other front panel operating modes, such as HI RES, ZERO, DAMPING,and FILTER, as required. Refer to paragraph 2.8 for further information on these controls.

Connect the source to be measured to the V INPUT terminals es shown in Figure 2-4. Note that circuit ground is normally connected to the LO terminal, while the HI terminal should be connected to the point to be measured.

CAUTION

Do not exceed IOOOV between the HI and LO V INPUT terminals or the instrument might be damaged. Note that the LO INPUT terminal floats and is not connected to power line ground. Therefore, it is important that the potential between the LO input terminal and power line ground not exceed 14OOV. or the instrument might be damaged.

WARNING

Observe normal safety precautions when connecting the Model 181 to potentially

lethal voltage sources. Failure to observe these precautions may result in serious personal injury because of electric shock.

2-2

mV RANGES

DISPLAY

IEEE STATUS LIGHTS

- mV INPUT

ANALOG

OUTPUT

POWER

ON/OFF

II I

RESOLUTION

Figure 2-2. Front Panel Controls and Connections

^ ^ ^ .

U,SLUNNC

DISPLAY

* ,\

ZERO

FILTER CONTROLS

V HANGES

ENABLE

AC RECEPTACLE

I ,Nt ClArlNG

.,,,,,. ,,, :r,. ,...

,. t.;,

,.,. ,,I_, . “,.L

ANALOG OUTPUT

RANGE SWITCH

IEEE CONNECTOR

Figure 2.3. Rear Panel Controls and Connections

2-3

5. Observe the display; if an “OFLO” is shown, switch to the next higher range. Use the lowest range possible to make the measurement. This procedure will achieve the best resolution.

6. Make the voltage reading. The display shows the reading directly in DC volts with a leading minus sign for negative voltages. No conversion is necessary as the decimal point is automatically placed on all ranges.

7. The Model 181 input impedance is greater than 10% on

the 2V range and equal to lOMl7 on the 20V through

IOOOV ranges. Thus, loading should not be a problem except with very high source resistance values. Refer to paragraph 2.10 for precautions to be taken under those conditions.

VOLTAGE SOURCE

,100” MAX,

CIRCUIT GROUND

,WHERE APPLICABLE1

-L

-I-

4. Connect the low-thermal cable to the mV input. Connect the alligator clips of the cable to the voltage source to be measured as shown in Figure 2-5.

CAUTION

Do not exceed 120V momentary. 35V con-

tinous, between the mV INPUT terminals.

or 1400V between the mV low terminal and ground. Failure to observe these precau-

tions may result in damage to the unit.

5. Observe the display reading; if the unit is in overflow, select the next higher range. If an overflow condition exists on the 200mV range, use the V INPUT and appropriate range as outlined in the preceding paragraph.

6. Take the voltage reading. The reading may be made directly, in millivolts,

automatically placed. A leading minus sign will be displayed for negative voltages.

7. Because of the very low signal levels involved, unwanted no,se, as CleSCrlDea I” paragrapn z.11, may upset the ac-

curacy of the measurement.

since the decimal point is

Figure 2-4. Basic Voltage Measurements

NANOVOLT AND MICROVOLT MEASUREMENTS

2.6

The Model 161 may be used to make very low voltage

readings down to a resolution of 10nV. These readings are made on one of the mV ranges by using the mV INPUT on

the front panel.

The following paragraphs describe the basic procedure for making these measurements.

1. Turn on the Model 181 with the front panel POWER switch. Allow the unit to warm-up for at least an hour for rated accuracy. To guarantee low drift, allow at least four hours.

2. Select the desired mV range with the appropriate front panel switch. Use a range appropriate for the voltage to be measured.

3. Select other parameters such as HI RES, DAMPING, FILTER, and ZERO as needed. Refer to paragraph 2.8 for more details on these controls.

2-4

Figure 2-5. mV and nV Measurements

2.7 SPECIAL MEASURING SITUATIONS

Some situations may call for a wide range of voltage measurements that neither the V input nor mV input can handle alone. In those cases, it may be convenient to use a

common ground for both the V and mV inputs. Since the

LO terminals of the mV and V inputs are internally connected together, it is only necessary to connect the mV Lo

terminal (black lead of the Model 1506 low-thermal cable) to

common of the circuit under test, as shown in Figure 2-6. Using this method, either the V HI or mV HI terminal can be used as the test probe, depending on the voltage to be measured.

CAUTION

Do not exceed the maximum input limit for the Model 181, especially when the mV HI terminal is connected. or damage to the in-

strument may occur. Never parallel the mV

and V leads to prevent accidental overload

to the mV input or inadvertent loading of

the circuit under test.

Figure 2-6. Common Ground Connection for V and mV

The zero function is especially useful for nulling out offset voltages, including internal offsets of the Model 181. To use the zero in this manner, short the test leads together with the instrument on the desired range and depress the ZERO switch; the ZERO indicator light should turn on. This stores the residual voltage level as the baseline. All voltage reading taken with zero enabled will then be the actual voltage level since the unwanted voltage will be subtracted from the reading.

Note that baseline suppression for the V and mV ranges operates separately. Switching the unit between a mV and V range, for example, will cancel the ZERO, also causing the front panel ZERO indicator light to turn off.

Controlling the Filter. The Model 181 has an internal 3-p& digital filter that can be controlled by the front panel FILTER and DAMPING controls. Normally, the filter is switched on and off as a function of the rate of change in input signal. Depressing the FILTER button increases the RC time conk stant of the filter. At the same time, the front panel FILTER light will turn on. The digital filter cannot be totally disabled by the front panel controls. However, it may be disabled by commands given over the IEEE bus. Operating with the filter disabled allows the user to customize Model 161 response by using external filtering. For further information on IEEE commands that control the filter, consult Section 4 of this manual.

2.8 ADDITIONAL FRONT PANEL CONTROLS The Model 181 has additional front panel controls that can

be used to enhance the capabilities of the unit. These

switches which include HI RES, ZERO, FILTER, and DAMP-

ING, are shown in Figure 2-2. The following paragraphs will

describe the operation of these controls in more detail.

HI RES. The display resolution of the Model 181 upon power-up is 5% digits. The display resolution may be increased to 6 % digits by depressing the HI RES switch. Once the unit is in the 6% digit mode, the display may be returned to the 5% digit mode by depressing the HI RES switch a second time. Readings made in the 5% digit mode have the

least significant digit rounded off. HI RES switch affects

only the data on the display; data transmitted over the IEEE

bus always contains 6% digit information. For further infor-

mation on IEEE operation, refer to Section 4.

Zero. The Zero mode serves as a means for baseline sup-

pression. The front panel ZERO indicator light will turn on

when the zero mode is enabled. All readings taken with the

zero enabled will be the difference between the stored

baseline and the actual voltage level.

The baseline is obtained by connecting the instrument to the voltage to be zeroed. For example, if the baseline

voltage is IOmV, all subsequent readings will have 1OmV

subtracted from the actual voltage level.

The DAMPING button controls whether or not the filter is continuously enabled. When the DAMPING is off, the microprocessor automatically disables the filter when the input voltage changes to permit rapid display update. Once the reading is within 25 digits of the final value on the 2mV range, and within 6 digits on the remaining ranges. the microprocessor then enables the filter to minimize noise in the final reading. When the DAMPING is on. the digital filter is permanently enabled. The unit would normally be operated in this mode only for signals that vary slowlv, or with extremely noisy ambient signals.

Through careful use of the FILTER and DAMPING controls, the user can optimize the Model 161 to the required perfw mance, keeping in mind the resulting speed/ noise compromises. Figure 2-7 shows four curves resulting from operating the unit with various combinations of the DAMPS

ING and FILTER controls. Curve A shows the fastest response time because the filter RC time constant is at a minimum. Also, with DAMPING off, the microprocessor initially disables the filter as previously described.

Depressing the FILTER switch as with curve 6, has little effect on the response time since the filter is initially off.

Curves C and D, on the other hand, show that enabling the

DAMPING slows the response down considerably. This can be seen in more detail in Table 2-2, which lists the settling times of the various control combinations.

2-5

Table 2-2 Settling Times

~~~~~-~~~~“~~~~~

(The readings all settle to within 0.002% of the Full Range in the specified time.)

2.9 USING THE ANALOG OUTPUT

The analog output of the Model 181 is useful for monitoring

the input signal with an external device such as a chart recorder. The analog signal is reconstructed from digital data (supplied by the internal microprocessor) by a 12 bit

D/A converter. Because of this configuration, the analog output will accurately reflect the display until an overflow condition is reached. The analog output is optically isolated from the front panel LO terminal to avoid potential ground loop problems. The following paragraphs describe the basic procedure for using the analog output.

1. Connect the measuring device to the two analog output terminals on the rear panel as shown in Figure 2-8.

CAUTION

The potential between the analog output

LO terminal and ground must not exceed

30V. Make sure the external device does

not exceed this voltage on its common or ground connections. Failure to observe this precaution may damage the Model

181. possibly voiding the warranty. IEEE

common is connected to analog output

IOW.

Select the Xl or Xl000 range by using the analog output gain switch on the rear panel. This switch is combined

with those used to set the IEEE mode in the lower left cor-

ner of the rear panel and is clearly marked. (See Figure

2.3.) In the Xl position, the most significant +2000 counts of the display reading can be covered, while the Xl000 position will change the range to cover the least significant f2000 counts. In this manner, the entire 6% digits of the display may be represented.

If necessary, the analog output may be zeroed with the front panel ZERO control. To do so, depress the ZERO button.

The Model 181 will display an “OFLO” message when the

capability of a specific range is exceeded. When this message is displayed. the analog output value will be + 2V if the polarity of the input voltage is positive, and -2V if the input voltage polarity is negative.

An analog output range overflow can occur when the Model

181 analog range switch is in the Xl000 position. An example of the analog ouput voltage under these conditions is

shown in Figure 2-9.” The conditions shown are for the 2mV

range. but the output will react similarly on the other voltage ranges if the proper scaling factor is applied. For each ten-

fold increase in voltage range, the scale of the horizontal

axis must also be multiplied by a factor of ten.

2-6

Figure 2-8. Analog Output Connections

The horizontal axis of Figure 2-9 has an input voltage range between -10&V and +lOpV. The vertical axis shows an analog output voltage between -2V and +ZV. which is the maximum range of the analog output. Beginning at the OV point on the graph, the analog output follows the input voltage linearly until the input voltage reaches +2pV. The analog output will then suddenly switch to the maximum negative output value of -2V. Thus, for each 4uV increment

* Units with B-7 software.

in input voltage, the output pattern repeats. ihe action of the analog output for negative input voltages is the same, except that the slope of the graph is negative for these negative-going input voltages.

Figure 2-9. Xl000 Analog Output

By counting the number of repeating waveforms on a chart

recorder, the user can easily determine the actual voltage at the input, even though the range of the analog output was exceeded. If, for example, the +lV point on the second

peak with a positive-going slope is noted, it can be clearly determined that the input voltage was +5@V at that particular time.

A summary of analog output information is shown in Table

2.3. Each range of input values corresponds to the incre-

ment necessary to cause the output to go through its entire 0 to 2V range. Note that the sensitivity is increased by a fac-

tor of a thousand on the Xl000 range. For example, when

the Model 181 is in the 200mV range, and the analog switch

is in the Xl position, the output voltage will swing from 0 to 2V in a smooth manner as the input voltage increases

gradually from 0 to 200mV. When the analog output switch

is changed to the Xl000 position, the input need only swing

between 0 and 2OOpV to obtain the same voltage swing at

the analog output. Beyond those input limitations, the out-

put voltage will repeat as shown in Figure 2-9.

The output resistance of the analog ouput is Ikll for all

voltage ranges regardless of the position of the analog range

switch. Thus, loading problems caused by external devices

are minimized. To keep loading errors below I%, the input

resistance of any device connected to the analog output

should be greater than lOOk0.

Table 2-3. Analog Output Parameters

2.10 SOURCE RESISTANCE CONSIDERATION The Model 181 has an input resistance greater that IGIl

flO% on the 2mV. 20mV. 200mV. and 2V ranges. The in-

strument will meet this input resistance specification on the mV ranges even when in overflow with voltages up to 1V. The input resistance on the remaining voltages ranges is

lOML2. Thus, the Model 181 input resistance is sufficiently

high to minimize loading errors in most measuring situations. For voltage sources with very high source resistance, two precautions should be observed when using the Model

181.

Shielding becomes more critical when the source resistance is very high. Otherwise, interference signals may be picked up by the test leads. Noise picked up in this manner can affect the mV ranges more severely. but shielding might be necessary for connections to the V INPUT in extreme situations.

Loading of the voltage source by the Model 181 can become

important with high source resistance values. As the source resistance increases, the error due to loading increases.

Figure Z-10 shows the method used to determine the Peru

cent error due to loading. The voltage source has an internal resistance R,, while the internal resistance of the Model 181 is represented by R,. The source voltage is E, while the voltage actually measured by the meter is E,.

The voltage actually seen by the meter is attenuated by the voltage-divider action of R and R, and can be found by using the relationship: E, = &R,IIRL + I?,).

We can modify this relationship to obtain a formula for per-

cent errors as follows: Percent Error = lOOR,/(R, * R,i~

From the above, it is obvious that the input resistance of the Model 181 must be at least 99 times greater that the source resistance if the loading error is to be kept to 1%. This maximum 1% error limitation will be achieved on the 2mV through 2V ranges with sources resistances up to lO.lMI1, while the source resistance should be no greater than IOlklI if the same 1% error limitation is to be maintained on the 2OV through IOOOV ranges. If lower errors are required, the source resistance must be correspondingly less.

20mV

200mV

20 v

200 v

1 kV

*IV Full Range Maximum

INPUT FOR

IG OUTPUT

Figure 2-10. Source Resistance Considerations

2-7

2.11 MICROVOLT AND NANOVOLT MEASUREMENT CONSIDERATIONS

Low level voltage measurements are subject to various types of noise that can make it difficult to obtain accurate voltage readings. Since the measuring instrument cannot distinguish between signal and noise voltages, the presence of unwanted low level signals can seriously affect a measurement. Some of the phenomena that can cause unwanted noise include: thermocouples (thermoelectric effects), flexing of coaxial cables (triboelectric effects), and the battery action of two terminals (galvanic action). The following paragraphs will discuss potential noise sources in more detail.

Source Resistance Noise, Noise that is present in the source resistance itself is frequently the determining factor in the ultimate resolution of a measurement system. The amount

of noise in a given resistance is given by the Johnson Noise

‘wE”= ~+:;;Js:

Noise Bandwidth in Hertz

. L’

At a room temperature of 293’=K (20°C). the above can be simplified to read: E.,,= 1.27~ IO- lm

It has been statistically shown, that p-p n&e is aPProximately five times the rms noise 99% of the time. From this relationship, we E

=6.35~1O-‘~F

0~”

From the preceding equations it is immediately obvious

that the noise voltage can be reduced by lowering the

temperature, reducing the resistance, or narrowing the

bandwidth. Reducing the resistance is not very useful

because the signal voltage will be reduced more than the

noise. For example, decreasing the resistance of a current

shunt by a factor of 100 will reduce the signal voltage by a factor of 100 as well; the noise, however, will be reduced

only by a factor of 10. Very often, cooling is the only practical method available to

reduce the noise. Here again, the reduction available is not as large as it seems because the noise reduction is related to the square root of the change in temperature. For example,

to cut the noise voltage in half, the temperature must be

decreased from ‘293°K to 73.25OK. a fourfold decrease.

Source Resistance in Ohms

Temperature OK

Boltrman’s Constant Il.38 x 10-23)

can

equate

the following:

Thermoelectric potentials. Thermal emf’s are small electric potentials generated by differences in the temperature at the junction of two dissimilar metals.

Thermal emf’s are particularly troublesome at the low signal levels measured by the mV ranges. To minimize thermal emf drift, use copper leads to connect the circuit to the instrument. The Model 1506 low-thermal cable supplied with the

Model 181 is ideal for this purpose. Other suitable lowthermal items are listed in paragraph 1.3 Optional Accessories.

Even with low-thermal cables and connectors, thermal emf’s can still be a problem in some cases. It is especially

important to keep the two materials forming the junctions at the same temperature. Keeping the two junctions close together is one way to minimize such thermal problems. In some cases, connecting the two junctions together with good thermal contact to a common heat sink may be required.

Most good electrical insulators have good thermal insulation characteristics as well. In cases where such low-thermal conductivity may be a problem, special insulators that combine high electrical insulating properties with high thermal conductivity may be used. Some examples of materials with low electrical conductivity and high thermal conductivity are: hard anodized aluminum, beryllium oxide, specially filled epoxy resin, sapphire, and diamond.

Oxidation of leads and connectors can also lead to thermal

emf problems. When copper oxidizes, for example, the

resulting copper to copper oxide junction can cause thermal

emf’s as high as lOOOpV/OC. Thus, it is imperative that all connections be kept as clean as possible.

Figure 2-l 1 shows a representation of how thermal emf’s

between two dissimilar metals are generated. The test leads

are made of the A material, while the B material is ~the source under test. The temperatures between the junctions are represented by T, and T,. To find the thermal emf for the circuit, the following relationship pay be used:

E, = Q,,(T, T>,

-Temperature of the A to B junction in OC or OK.

Temperature of the B to A

junction in OC or OK.

Thermoelectric coefficient of material A

with respect to B, given in aVI°C.

As an example of determining noise voltage generation, assume that the Model 181 is connected to a voltage source with an internal resistance of lOk!l. At a room temperature

of 20°C (293’K). the p-p noise voltage generated cwer a bandwidth of 0.5Hz will be:

=6.35x 10~~‘“jUOx 103)(0.5)

P’P

E,~,=4.5x lo-*V=45nV

2-6

Figure Z-11. Thermal EMF Generation

In the unlikely avant the two junction temperatures are identical, the thermal emf’s will exactly cancel, since the generated potentials oppose one another. More often, the two junctions temperatures will differ, and considerable thermal emf’s will be generated.

A typical test set up might have one or more copper to cadmium-tin junctions. Typically. such a junction has a thermoelectric coefficient of O.~FV/~C. Since the two materials will frequently have a several-degree temperature differential, it is easy to see how thermal potentials of several microvolts can be generated, even if reasonable precautions are taken.

Magnetic fields. When a conductor cuts through magnetic

lines of force, a very small currant is generated. This

phenomenon will frequently cause unwanted signals to occur in the test leads of a measuring instrument. If the conductor has sufficient length, even weak magnetic fields such as the earth’s can create sufficient signals to upset voltage measurements in the nanovolt or millivolt ranges. Thus, several precautions may be taken if magnetic-field in-

duced signals become a problem.

Reducing the length of the leads or minimizing the exposed circuit area are two ways these effects can be minimized. In extreme cases, magnetic shielding may be required. Special metals with high permeability at low flux densities (such as mu metal) are effective in this application.

Figure Z-12. Power Line Ground Loops

To see how a ground loop can affect the voltage readings,

refer to Figure Z-13. The source to be measured is connected to the nanovoltmeter through the customary HI and LO leads. The resistance of the LO terminal connection is

represented by R,. This resistance is usually very low about 0.111, but even this low value can be significant if the circulating current is high enough.

Even in cases where the conductor is stationary, magnetically induced signals may be a problem. Fields may be produced by various signals such as the AC power line voltage. Large inductors such as power transformers are very good magnetic field generators, so care must be taken to keep the measuring circuit a good distance away from these potential noise sources.

At high current levels, even a single conductor can generate significant fields. These effects can be minimized by using twisted pairs; using this method, the resulting fields will be largely cancelled out.

Ground Loops. When two or more devices are connected together, care must be taken to avoid unwanted signals caused by ground loops. Ground loops usually occur when

sensitive instrumentation, such as the Modal 181. is con-

nected to other instrumentation with more than ona Signal return path. One of these return paths may be power lina ground. The resulting ground loop causes CUrrant t0 flow

through the instrument LO signal leads and than back through the power line ground (See Figure Z-12). Bacausa of this circulating currant, a small but undesireable voltage is developed between the LO terminals of the two instruments. This voltage will be added to the swrca voltage.

upsetting the measurement.

GROUND

Figure Z-13. Ground Loop Voltage Generation

The source voltage is Es, while the ground loop currant is I,. The actual voltage seen by the nanovoltmeter is the sum of the source voltage and the IR drop across the LO lead connections, and can be found by using the relationship:

E,,=Es+I,R,

Thus, for a IOOnV source voltage, an R, value of 0.111, and a

IOOnA ground loop currant, the total voltage actually seen

by the instrument will be 1lOnV. creating an error of 10%.

Figure Z-14 shows a configuration that will eliminate this type of ground loop problem. Here, only the center instrument is connected to ground. Ground loops are not normally a problem with the Model 181 because the LO input terminals are isolated from power line ground. However, the mV INPUT and V INPUT LO terminals should not be externally connected together as this will create a ground loop. Also, since other instruments may not be designed in the same way, they may cause ground loop problems even though the Model 181 is isolated. When in doubt, consult the manual for other instrumentation in the test setup.

2-9

ments. With either type of RFI, the affect on instrument performance can be considerable if enough of the unwanted signal is present.

P”WGR LINC mO”No

Figure 2-14. Eliminating Ground Loops

~RFI. Radio Frequency Interference (RF11 is a general term to describe electromagnetic interference over a wide range of frequencies across the spectrum. RFI can be especially troublesome at the low signal levels measured on the mV

ranges, but it may also affect readings on the higher voltage ranges in extreme cases.

RFI can be caused by steady-state sources such as TV or radio broadcast signals, or it can result from impulse sources, as in the case of arcing in high voltage environ-

RFI can be minimized by taking one or more of several

precautions when operating the Model 181 in such environ-

ments. The most obvious method for minimizing these effects is to keep the instrument as far as possible away from the source. Shielding the instrument, voltage source, and test leads will often reduce RFI to an acceptable level. In

extreme cases, a specially constructed screen room may be

required to sufficiently attenuate the troublesome signal. The internal 3-p& filter within the Model 181 may help

reduce RFI in home situations. For more serious RFI pro-

blems, the user is encouraged to try more effective external

shielding.

2-10

SECTION 3

APPLICATIONS

3.1 INTRODUCTION The high sensitivity and very high input impedance of the

Model 181 makes it ideal for critical measurements ordinary

DVM’s are unable to handle. Some of these applications, including standard cell comparison and low resistance measurements, are covered in the following paragraphs.

3.2 STANDARD CELL COMPARISONS The Model 181 may be used for making standard cell com-

parisons without the usual problems caused by different ground connections. The input low terminals on the instru-

ment are isolated from ground, eliminating the possibility of

shorting out one of the cells when the input connections are

reversed.

The greatest concern when making such comparisons is the

effect of the measuring instrument on the standard cell

voltage. The Model 181 input characteristics minimize these

effects because of high input impedance and very low offset

current.

The equivalent circuit for making standard cell comparisons

is shown in Figure 3-l. V, and V, represent the two standard

cells being compared, and are connected to the HI and LO

mV INPUT terminals as shown. The common terminals of

the two cells are normally connected to earth ground to pre-

vent electrostatic pickup. Because of the high impedance

nature of the measurement, it may be necessary to shield

the two standard ceils. This shield should also be connected

to earth ground as shown in Figure 3-l.

The amount of current and charge drawn from the cells in

Figure 3-1 is determined by the common-mode and normalmode impedances. The common-mode impedance appears only across V, and is represented by the parallel comblna-

tion of R,, (> 109% and Cc, (O.O015,,F.) Under typical

laboratory conditions lless than 60% relative humidity). the value of R,, ally be assumed to be infinite. The capacitance of the Model

1506 low-thermal cable makes up the larger portion of C,,. If the input capacitance should have a detrimental effect on standard cell performance, its value may be essentially balanced out by connecting a 15OOpF capacitor between the input HI terminal and earth ground. Since C,, will rarely affect cell performance, this balancing technique is not usually necessary.

The normal-mode impedance is represented by the parallel combination of R,, and C,,. This normal-mode impedance appears between the LO and HI input terminals. The actual amount of charge and current drawn from the two cells depends on the voltage difference between them (V,~V,I. Typically, the potential difference between V, and V, is 1mV or less, resulting in a charge of approximately 10 l2 coulombs that must be supplied by the two cells. In addi~ tion. an input offset current of less than 50pA is drawn from the cells under test. Any effects from internal spikes pm duced by the input multiplexing FET’s will be at a minimum, as the spikes will occur at the 4Hr multiplexing rate.

When on the mV ranges. the Model 181 will maintain its high input resistance characteristics ( ) 10%) for all voltages

up to lV, even during range changes or when in overload.

However, the input impedance drops substantially if the 1V

limitation is exceeded. To avoid possible cell degredation

under such conditions, it is recommended that a series

resistance 1 MR or greater be connected in series with one of

the cells, as shown in Figure 3-l. where R, is connected be-

tween V, and the input HI terminal. This safeguard will also

protect the cells from degradation in case of improper con-

nections. This resistance. Rs. can be shorted Out after an on scale indication is observed, and the reading can then be made in the normal manner.

is in the lo”12 to 1O’21l range. and may genw

Figure 3-I. Standard Cell Comparison

With this connection, it is assumed that the Model 181 is

connected to the power line with the standard 3.prong plug. Since the power line ground is also connected to earth ground, the complete ground connections will be made

through the ground wire in the line cord of the instrument.

The basic procedure for making standard cell measurements

involves connecting the two cells for comparison to the mV INPUT and sening up the Model 181 as follows:

1. Turn on the power to the Model 181 and allow a one hour warm-up period for the best accuracy. A four hour period is required to minimize drift.

2. Connect the cells to the Model 181 as shown in Figure 3-1, Use the low-thermal cable and mV INPUT on the ins strument. Include a lMI2 resistor for R, if the cells ate to be protected from accidental loading during set-up.

3-l

3. Select the appropriate mV range and observe the reading. If the display reading jumps around excessively, enable

the damping with the front panel DAMPING switch.

4. Observe the readings; once the display has stabilized,

short R, (if connected), and take the voltage readings.

5. Remove the short from R, and reverse the lead connec-

tions to V, and V,. The LO terminal should now be con-

nected to V, and the HI terminal should be connected to

V,. Disable the damping to reduce settling time. Once the

reading has stabilized, enable the damping again, if necessary. Once again, short R, and record the display

reading.

6. The standard cell difference may be found by averaging

the absolute values of the two readings as follows:

Cell Difference = (Reading In Step 41 + 1 Reading In Step 51

This averaging method is necessary to cancel thermal

effects, but either reading alone may be sufficiently accurate if precautions to minimize thermal emf generation are observed. The ZERO button may be used to null any

residual offset. Also, only copper to copper connections should be used to minimize such thermal effects. As always, the connections should be kept as clean as possible, or the

resulting copper to copper oxide junctions may create substantial thermal emf voltage, upsetting the measurement.

range for input voltages above 200mV; the input impedance on the mV ranges drops to IkR for inputs greater than IV.

Figure 3-Z. Absolute Cell Measurement Connections

3.3 LOW RESISTANCE “LINDECK” MEASUREMENTS

As with the standard cell comparisons, the high resolution of the Model 181 gives the instrument a definite advantage in speed and convenience over potentiometer systems traditionally used to make low resistance measurements.

The method just described is useful for comparing two standard cell voltages. However, some situations may call for the measurement of a single cell alone. For measuring absolute standard cell voltages, it is best to use the2V range

because the input impedance is greater than IOQ.

NOTE

Since the input impedance on the ZOV-1OOOV ranges drops to lOM12, those voltage ranges are unsuitable for making absolute standard cell voltage measurements.

Figure 3-2 shows the connections for making absolute standard cell voltage measurements. Once again, R, is to be left

in the circuit until the reading is actually taken. The basic

procedure for making the absolute voltage measurement is as follows:

1. Turn on the instrument and allow a one hour warm-up period. Four hours are required to minimize drift,

2. Connect the standard cell to the instrument as shown in Figure 3-2. Be sure to include R, to protect the cell,

3. Switch the instrument to the 2V range and observe the

reading. Be sure the reading is on range.

4. Short out R, and take voltage reading. Once the reading

is complete, remove the short from R,.

The method used with the Model 181 places a current source in parallel with the low resistance to be measured, as shown in Figure 3-3. The problem with such measurements

has generally been the trade-off between the power level required and the sensitivity of the instrument. In a circuit with a .Olfl resistance, a current source delivering only IOmA will provide a resolution of 0.01% with the Model 181 on the 2mV range. In contrast, a 1pV resolution DVM would require a current of IA. With the Model 181 measurement, the power level in the resistor will be only 1pW. whereas the measurement made with the DVM would result in a power level of IOmW in a .OlR resistance.

With this method, the resistance may be found simply by

dividing the voltage reading by the current source value.

With a 1OmA current source and ,010 resistance, for exam-

ple, a voltage of IOOpV will be developed across the

measured resistance.

MODEL 181

Care should be taken when making absolute readings in this

way. If higher voltage ranges are used, the lower input im-

pedance will degrade cell performance. Do not use the mV

3-2

Figure 3-3. Low Resistance Measurement Connections

3.4 TEMPERATURE MEASUREMENTS The Model 181 may be used with thermocouples or ther-

mopiles to monitor small temperature changes down to a

resolution of ~O.OOl°C. Connections should be made using appropriate low-thermal materials such as those supplied with the Model 1483 Low-Thermal Connection Kit. Although some shielding is recommended, the high AC re-

jection of the instrument eliminates most of the problems

normally caused nearby AC operated equipment such as heaters, fans, pumps, etc.

The rear panel analog output may be used along with a chart recorder to provide a continuous, permanent record of

temperature changes. Alternately, the Model 181 may be

connected to the IEEE-488 bus to allow easy transfer of

temperature data to a printer or computer. With a suitable

controller, the user may take full advantage of Model 181 capabilities, providing full automation of such temperature

tests.

appropriate controller and other instrumentation, virtually complete automation of the necessary instrumentation curve measurements may be obtained.

Josephson Junction measurements are especially vulnw able to the effects of high frequency EMI (Electromagnetic Interference) and RFI (Radio Frequency InterferenceI. Much of this interference may be generated by the microprocessor based instrumentation itself. Still other forms of interference may come from the outside environment. In either case, care must be taken to minimize the coupling of these unwanted signals to the device under test. The almost 1”~ evitable high-frequency coupling of unwanted RFI to the Josephson Junction itself will significantly affect the I-V characteristics of the device, rendering the data useless in many cases. The Model 181 has been carefully designed to minimize common-mode RFI that may be coupled through the input cable to the device under test. However. depend ding on the measuring environment, additional precautions may be required.

3.5 RESISTANCE THERMOMETRY The Model 181 may be used for resistance thermometry

where small deviations are measured with nW power dissipation levels. A stable current source may be used to provide the necessary constant low-level current. A typical resolution, using a IOOOfl germanium thermometer (at

4.2”K) is O.OOOZ°C, with a power dissipation of only 1nW. (lo-SW).

The floating input of the Model 181 eliminates the problems usually encountered when floating four-terminal measurements are made. Note that the Model 181 may be used with a wide range of source resistances, because of the high

input impedance of the unit. As with other Model 181 applications, thermometry measurements may be controlled over the IEEE-488 bus.

3.6 SEMICONDUCTOR TESTING The Model 181 may be used for semiconductor testing on

an automated production-line basis. Sensitive measure-

ments can be made on semiconductor devices to determine

gain stability, temperature coefficient, etc., without the

loading errors associated with many types of equipment. By applying proper programming techniques, a high level of automation through use of the IEEE-488 bus can be achieved.

Two methods of minimizing RFI effects are shown in Figure 3-4. The device under test is placed within a shielded booth, which is connected to earth ground. Also, a ferrite inductor is placed in series with the HI input lead of the lowthermal cable. This inductor is made up of 4 turns of #28 wire wound around a Keithley CT-7 ferrite core (Fai-rite #2643000701 I. Note that the inductor is placed just outside the shield, right at the point where the input cable enters the shielded area.

These precautions should eliminate all but the most stubborn RFI problems. In more extreme situations. a second inductor, identical to the first, may be connected in series with the LO input in a similar manner.

3.7 JOSEPHSON JUNCTION STUDIES The Model 181 can be used for Josephson Junction studies,

where speed and high sensitivity are a primary requirement. Josephson Junction I-V characteristics can be easily plotted

using the Model 181 in conjunction with other instrumenta-

tion connected to the IEEE-488 bus. Through the use of an

Figure 3-4. Minimizing Josephson Junction RFI Effects

3-313-4

SECTION 4

IEEE OPERATION

4.1 INTRODUCTION TO THE IEEE-488 BUS

The Model 181 has a built in IEEE-488 bus that allows the

user to give commands and read data via an external device.

All the front panel operating modes except power on-off

may be controlled by commands given over the bus.

The Model 181 may be commanded over the bus when the

rear panel TO/ADDRESSABLE switch is in the

ADDRESSABLE position. When in the TO (talk only) mode,

the Model 181 merely outputs data on the bus: no com-

mands may be given when the unit is in this mode. For fur-

ther information on changing the mode of operation, see

paragraph 4.3

A typical bus set-up for controlled operation is shown in

Figure 4-l. A typical system will have one controller and

several other instruments to which commands are given.

Generally, there are three catagories that describe device operation. These catagories are: controller: talker; listener. The controller does what its name implies: it controls the other instruments on the bus. A talker sends data, while a

listener receives data. Depending on the type of instrument, any particular device may be a talker only, a listener only. or

both a talker and a listener. The Model 181 is capable of

being both a talker and a listener, but does not have the

capability of being a controller.

Any given system can have only one controller, but any

number of talkers or listeners may be present up to the hardware constraints of the bus (see paragraph 4.2). Several

devices may be commanded to listen at once, but only one

talker can be active at any given time.

Before a device can talk or listen, it must be appropriately addressed. Devices are selected on the basis of their primary address. To avoid confusion, the addressed device is sent a talk or listen command derived from its primary address. The primary address of the Model 181 is set to 5 at the factory, but may be changed at the user’s discretion as outlined

in paragraph 4.3.

NOTE

Each device on the bus must have a unique primary address. Failure to observe this condition may result in erratic operation.

The IEEE-488 bus is made up of 16 signal lines and 8 ground lines. Eight of these signal lines are used for data, three of the lines control the handshake, and the remaining five lines manage the operation of the bus. The data lines are used for both data and commands. The three handshake lines ensure that all devices properly receive data, while the management lines control the remaining bus functions.

4.2 DESCRIPTION OF BUS LINES The IEEE-488 bus may have up to 15 devices connected at

the same time. Each signal line is inverted so that low is true. The following paragraphs briefly describe the purpose of these lines, which are shown in Figure 4-1.

1. Bus Management Lines. These 5 lines are used to control the bus and send certain single line commands. The single-line commands that affect Model 181 operation are explained in mcxe detail in paragraph 4.4

4-l

A. IFC, Interface Clear: Used to send the IFC command

to set the bus to a known state.

B. REN, Remote Enable: Used to send the REN com-

mand to set up instruments on the bus for remote, operation.

C. EOI, End or Identify: Used to send the END command

that usually terminates a multi-byte transfer sequence.

D. SRQ, Service Request: Used by an external device to

request service from the controller.

E. ATN, Attention: Used by the controller to indicate

whether the data bus contains data or commands.

2. Handshake Lines. The IEEE-488 bus uses three handshake lines that operate in an interlocked sequence. This method ensures reliable data transfer regardless of the

transfer rate. Generally, data transfer will occur at a rate

determined by the slowest active device on the bus.

The three handshake lines are: DAV (data validl, NRFD

(not ready for data), and NDAC (not data accepted). The device that is the source of the data controls the state of the DAV line, while the NRFD and NDAC lines are controlled by the device accepting data.

The complete handshake sequence for one byte of data is shown in Figure 4-2. Once the date byte is placed on the date bus, the source checks to see that NRFD is high, and NDAC is low, indicating that all devices on the bus are ready for data. Once this condition is met, the source sets the DAV line low, indicating the data is valid. The

NRFD line then goes low; the NDAC line will go high once all the devices have accepted the data. Each device will release the NDAC line at its own rate, but the NDAC

line will not go high until the slowest device has accepted the data.

DAV

SOURCE

NRFD

NDAC

I I

DATA

TRANSFER

BEGINS

DATA

TRANSFER

ENDS

ACCEPTOR

Figure 4-2. IEEE Handshake Sequence

After the NDAC line goes high, the source then sets the DAV line high, indicating that date is no longer valid. The NDAC line then goes low. Finally, the NRFD line is gradually released by each of the devices at their own rates, until the NRFD line finally goes high when the slowest device is ready, and the bus is set to repeat the sequence with the next data byte.

The sequence just described is used for both data transfer and the multiline commands. For further information on these commends, refer to paragraph 4.4.

The IEEE-488.1978 standard uses the terminology just described for the three handshake lines. In some cases,

DAC is substituted for NDAC, and RFD is used in place of NFRD when referring to those two bus lines. Except for that terminology, the operation of these lines is identical to the sequence just described.

Data Lines. The IEEE-488 bus uses B data lines that allow data to be transmitted or received one byte at a time. These lines, which use the convention DlOl through

D108 rather than the usual D0 through D7 terminology,

are used to transmit both data and the multiline com-

mands, and are bi-directional. Like the remaining bus lines, the date lines are inverted so that low is true.

4.3 IEEE-488 SET-UP PROCEDURE Before the Model 181 can be used with the IEEE-488 bus,

the IEEE mode and primary address selector switches must be set to the appropriate positions. Also, the instrument must be connected to the bus with a suitable IEEE-488 con-

nector as described in this section. The IEEE-488 connector

and associated switches may be found in the lower left cor-

ner of the rear panel.

1. IEEE Mode Selection. The Model 181 may be set for

either talk only (TO) or addressable operation by setting the TO/ADDRESSABLE switch on the rear panel to the desired position. In the addressable mode, the unit may be controlled by commands given over the bus. For a description of these commends, refer to paragraphs 4.4 end 4.5. When in the talk only mode, the Model 181 will ignore any commends given over the bus, but will transmit its normal data string to an external device one byte at a time, as requested. For formatting of the date string, see paragraph 4.6.

2. Primary Address Selection. If the Model 181 is to be used

in the addressable mode, the primary address switches must be set to the correct value. The method used to

determine the primary address depends on the controller

used, but, generally, the numeric value specified in the controller’s programming language must be the same as the numeric value set with the Model 181’s primary ad-

dress switches. As shown in Figure 4-3, the Model 181

primary address in set to 5 at the factory; however, any

value between O-30 may be used as long es the value

used in the controller program agrees with the selected

value on the instrument.

4-2

NOTE

Both the primary address switches and the TO/ADDRESSABLE switch are read only upon power-up. If the switch positions are changed, the instrument must be turned off and then powered-up again before it will recognize the new switch conditions

Contact designations for the rear panel IEEE connector (J1006 on schematic number 30583) are listed in Table 4-1.

Both the IEEE-488.1978 conventions and Keithley designa~ tions are shown in the table. For contact identification, refer to Figure 4-4. Contact 1 through 12 are along the top of the connector in sequence. while contacts 13 through 24 appear along the bottom edge.

cl 0

q 0

XIK

NOTE

The switching positions in the figure arc such that the Primary Address-5~001011 and the unit is in the Addressable Mode The Analog 0~1~~1 is in the Xl configuration

Figure 4-3. Primary Address and IEEE Mode Switches

3. Bus Connections. The Model 181 should be connected to the bus with a suitable IEEE cable and connector. The

IEEE connector on the unit is on the rear panel next to the primary address switches as shown in Figure 4-4. Note that the maximum cable length for any IEEE device mally 20 meters. If many devices are connected to the bus, shorter cable lengths may be required. The Keithley Model 7007 IEEE cable is ideal for connecting the instrument to the bus.

is nor-

Figure 4-4. IEEE Contact Configuration

4.4 BUS COMMANDS The Model 181 may be given a number of special bus corn

mands through the IEEE~488 interface. These commands are grouped into the following types: single line commands. universal commands, and addressed commands. These commands are summarized in Table 4~2 and are discussec in the following paragraphs.

1. Single Line Commands. Each of the single line con mands is sent by setting the appropriate bus line true (low) as follows:

A. ATN, Attention. The ATN command is sent when the

information on the data bus is a universal or addressed command. These commands will be described in more

detail in the following paragraph. When the ATN line

is false, the byte on the data bus is considered to be

data. The Model 181 will respond to the appropriate

universal and addressed commands when ATN is true and the device-dependent commands when ATN is false. assuming it is properly addressed.

B. END. The END command is sent by setting the

EOI line true during the last byte of data transfer. This command will be sent by the Model 181 during the last byte of its data string if prOQrammed to do so as outlined in paragraph 4.5.

C. REN, Remote Enable. The controller sends this come

mand to all devices on the bus when remote operas tion is desired. The Model 181 will respond by setting itself up for remote operation as indicated by the front panel REMOTE annunciator light.

D. IFC, Interface Clear. The IFC command is sent by set-

ting the IFC line true. It sets the bus to a known state. The Model 181 will respond by cancelling the TALK and LISTEN front panel indicator lights if the unit was previously in those modes

4-3

Table 4-l. IEEE Contact Designations

contact Number

1 2 3 4 5 6 7 8

9 10 11 12 13 14 15 16 17 18 19

20 21 22 23 24

*Numbers in parentheses refer to th

and REN signal lines return on contact 24

IEEE-488 Convention

DIOI

Dl02 cl103 D104 EOI (24)* DAV NRFD NDAC IFC SRQ ATN SHIELD Dl05 D106 D107 Dl08 REN 1241” Gnd, (6)” Gnd, (7)* Gnd, (8)” Gnd, (9)” Gnd. (10)” Gnd, (11)”

Gnd, LOGIC

-~ ’

Keithley Designation (J1006)

160 IBl 182 IB3 EOI DAV RFD DAC

IFC SRQ ATN BUS COMMON IB4 185 IB6 IB7 REN BUS COMMON BUS COMMON BUS COMMON BUS COMMON BUS COMMON BUS COMMON BUS COMMON

e signal ground return of the referenced contact number. EOI

Type

Data Data Data Data

Management

Handshake

Handshake Handshake Management Management Management Ground Data Data Data Data Management Ground Ground Ground Ground Ground Ground Ground

4-4

Table 4-2. Bus Command Summary

Comman

d i Affect On Model 181

ATN

END REN

IFC

SRQ

DCL

LLO

SPE SPD UNT UNL

SDC

GET GTL

Classifies Data Bus as Data or Commands. Set EOI low during last data byte. Set unit for remote operation. Cancel Talk and Listen. Sent by 181 to request service.

Return to default conditions. Lock out front panel controls. Send status byte. Disable serial poll sequence. Remove from talk mode. Remove from listen mode.

Return to default conditions. Trigger conversion in T2 and T3 modes. Return to local operation.

E. SRQ. Service Request. The SRQ command is sent to

the controller by external devices when service is required. The Model 181 will implement this command when in the appropriate bus response mode as described in paragraph 4.5.

2. Universal Commands. The IEEE universal commands are sent over the data bus when ATN is true. The following paragraphs describe the effect of these commands on Model 181 operation.

DCL, Device Clear. The Model 181 will return to the power-up default conditions as outlined in paragraph

4.5.

LLO, Local Lockout. When the controller sends a

LLO over the bus, all devices equipped to implement this command will respond by locking out their controls. After a LLO command has been sent, the Model

181 front panel controls are no longer operative.

Local control of the instrument may be restored by sending a GTL command as described in the next paragraph.

SPE, Serial Poll Enable. The SPE command is Norm mally sent by the controller after receiving a SRQ command to determine which device initiated the service request. The Model 181 will respond by sending its status byte when addressed to talk after the SPE command is sent.

SPD, Serial Poll Disable. This command disables the

serial polling sequence.

UNT, Untalk. The controller sends this command to remove any talkers from the bus. The Model 181 will return to its idle state if it was previously in the talk mode, and the front panel TALK light will go out.

UNL, Unlisten. The controller sends this command to remove all listeners from the bus. The Model 181 will

go to the idle state if previously set to listen, and the front panel LISTEN light will go out.

3. Addressed Commands. Each of the addressed commands is sent to a specific device on the bus. The device is selected on the basis of its primary address. The Model

181 will respond to these commands only if the primary address sent over the bus preceding these commands is the same as selected by the rear panel address switches. All these commands are implemented by addressing the

Model 181 to listen. A. SDC, Selective Device Clear. This command per-

forms the same function as DCL except that only the

addressed device will respond. The Model 181 will return to the default conditions outlined in paragraph

4.5.

B. GET, Group Execute Trigger. This command will trig-

ger a conversion within the Model 181. The GET command must be used before requesting data or the status word when in the T2 or T3 bus response mode.

C. GTL, Go To Local. The Model 181 will return to the

local mode if the LLO command was previously given. The front panel controls will once again function after this command is sent.

4.5 DEVICE-DEPENDENT COMMANDS

The device-dependent commands allow the user to send the

Model 181 commands that perform the same operations as all the front panel control switches except power on-off. ln addition, there are a number of commands that control parameters which are not available from the front panel,

Each command is entered as an ASCII character followed by a specific parameter that is then sent over the bus by the controller. The IEEE bus treats these commands as data in that the ATN line is false when the commands are transmitted.

A number of commands may be grouped together as long

as the total number of characters sent at one time is no greater than 18. Before a command or command string is executed, the ASCII character X must be sent. Commands sent without an X (execute) will be retained within the

Model 181 command buffer until the execute character is

received.

The device-dependent commands affect the condition of the status word within the Model 181. The status word may be obtained from the unit by using the commands covered in this section. For formatting of the status word. see paragraph 4.8. Illegal commands will cause no mode changes in the Model 181. However, the status byte condition will change as described in paragraph 4.7. Legal Model

181 commands are listed in Table 4-3, and are covered in the

following paragraphs.

Range. The voltage range of the Model 181 may be set to the desired value by sending the ASCII character R followed by a number from 1 through 7. Each number

represents one of the voltage ranges as described in Table 4-4. which also lists the readings rates for the various ranaes.

Bus Response Mode. The bus response mode detw

mines whether or not the Model 181 will send the SRQ

command when data is ready to be sent or an error cons

dition exists. The bus response mode may be program-

med with the following commands:

MO, Standard. Send no SRQ. The status byte will still

be updated and may be read as described in paragraph

4.7.

Ml, Interrupt. The Model 181 will send the SRQ cow

mand to the controller when a reading is triggered or if

an error condition exists. If more than one device is on

the bus, the user must do a serial poll sequence to deter-

mine which device has requested service.

3. Trigger Mode. The trigger mode affects the way the Model 181 updates its data output buffer. The instrument will update the buffer on a continuous or one-shot basis each time a talk or GET command is received, depending on the mode of operation. Once the data reading is in the buffer, the Model 181 will transmit the data string the next time a data request is made by another bus device. For formatting of the data string, refer to paragraph 4.6.

4-5

Table 4-3. Device-Dependent Command Summary

:omman

:haracte

U X

Any AS

Ispace)

**No parameter specified for this command

TO, Continuous On Talk. The data buffer will be continuously updated at a rate listed in Table 4-4. As indicated, the rate will depend on the voltage range used. When the unit is

addressed to talk, it will respond by sending its data string.

Table 4-4. Range Commands

Voltage Range Bus Response Mode

Trigger Mode

Terminator Filter

Damping

Display Resolution

Status Word Execute Other Command!

character except other (

Parameter Description

See Table 4-4 0, Non SRQ

1, Send SRQ

0, Continuous On Talk

1. One-Shot On Talk 2, Continuous On GET 3, One-Shot On GET

0, Disable Filter Entirely

1, FILTER Off

2. FILTER On 0, DAMPING

1, DAMPING

0, ZERO Out

1, ZERO In

0, 5% Digits

1. 6% Digits

0. Send EOI

1. Send No EOI

xx **

mmand characters and E, ., +, _,

Care should be taken when using the T2 or T3 modes. The Model 181 will not respond to a data request unless a GET command is received first, even if addressed to talk in the normal manner. Depending on the controller used, failure to observe this precaution may result in a bus “hang up”. For further details, consult the controller’s manual as to potential problems when using the IEEE-488 bus.

Off On

NOTE

Tl, One-Shot On Talk. The data output buffer is updated

only once each time the instrument is addressed to talk. The data string will then be sent when the unit is addressed to talk.

T2, Continuous On GET. The data buffer will be continuously updated after a GET command is received. When in this mode, the unit must receive a GET command before attempting to read data. T3, One-Shot On GET. The instrument will update its data output buffer once each time a GET command is received. As with the T2 mode, the GET command must be received before attempting to read data.

4-6

4. Programmable Terminator. The data string of the Model 181 is normally terminated with one or two ASCII

characters. The default terminator sequence is (CR LFI,

but this may be changed by sending the ASCII character

Y followed by the desired terminator. For example, if the desired terminator is the letter H, the sequence YHX

must be sent over the bus.

Other Model 181 command letters may not be used as terminator characters. These include: B,D,M,P,R,T.Y,X,K,U.

Special terminator characters must be sent for some sequences. For example, the ASCII (DEL) character will suppress the terminator entirely. An ASCII (CR) will change the terminator sequence to (LF CR), while as ASCII (LF) will

restore the terminator to the (CR LF) default sequence. For example, sending the command Y(LF)X to the Model 181 will restore the terminator to its default (CR LF) value.

5. Filter Commands. The operating mode of the internal

3.pole digital filter may be altered by commands given over the bus. Through the use of these commands, the user may change the RC time constant of the filter as described in paragraph 2.8. The filter commands are described in the following paragraphs.

PO, Filter Disabled. This command entirely disables the internal filter, and is not available from the front panel. The

Model 181 may be operated in this mode to obtain raw, unfiltered data readings, or if custom external filter designs are used. Care should be taken when operating the instrument in this mode as internally generated noise spikes may occur in the readings.

PI, Filter I Enabled. This Command will perform the same function as disabling the filter from the front panel. The

Model 181 FILTER indicator light will go out if Filter 2, as described below, was previously enabled.

P2, Filter 2 Enabled. This command performs the same operation as enabling the filter from the front panel. After the P2 command is given, the front panel FILTER indicator will turn on.

6. Damping Commands. The damping commands further control the operation of the internal filter. When the damping is off, the microprocessor within the Model 181 determines whether or not the filter is enabled. The operation of tne damping commands is described in the following paragraphs.

DO, No Damping. With the damping off, the internal microprocessor determines when the internal filter is enabled. For steady-state inputs, the filter will be continuously enabled. When the input voltage level changes, the microprocessor disables the filter to permit rapid display update. Once the reading is within 25 digits of the final value on the 2mV

range, and within 6 counts on the remaining ranges, the

MPU then enables the filter once again.

DI, Damping Enabled. In this mode, the filter is perma-

nently enabled. This mode of operation is normally used only for signals whose levels change relatively slowly.

The filter and damping commands may be used in various

combinations to achieve the desired instrument reponse.

For a more complete discussion of the interaction between these two commands, refer to paragraph 2.8.

7. Zero. The zero serves as a baseline suppression. Once the baseline is stored, all readings taken with the zero enabled will be the difference between the actual

reading and the stored baseline. This command is

especially useful for nulling out stray voltages picked up

by connections to the test set up. The zero is controlled by sending one of the following commands over the bus.

ZO, Zero Out. The zero will be disabled, as indicated by the off state of the front panel ZERO light.

Zl, Zero In. The zero will be enabled: the front panel zero

indicator light will turn on.

8. Display Resolution. The display may be set to 5% or 6% digit resolution by sending the appropriate command. These commands affect only the display; the bus always contains 6% digit information. In the 5K digit mode, the least significant digit will be rounded off.

60, 5% Digits.

61. 6% Digits.

9. EOI. The EOI line is usually set true by a device during the last bYte of data transmission. The Model 181 EOI operation may be programmed with one of the following commands.

K0. Send EOI during last byte of data. Kl. Send no EOI.

10. Status Word Command.The status word may be accessed with the following command sequence: UX. After this command is transmitted, the status word will be sent instead of the data string the next time data is rep

quested from the unit. Note that a GET command must be transmitted first if the unit is operating in the T2 or T3 trigger modes. For formatting of the status word, refer to paragraph 4.8.

II. Default Conditions. Upon power~up, the Model 181 will

assume the default conditions listed in Table 4-5. The unit will also revert to these conditions after receiving a DCL or SDC command over the bus. For a further des

cription of these commands, refer to paragraph 4.4. These conditions may be checked by accessing the status word with the UX command sequence. For for-

matting of the status word, refer to paragraph 4.8

Table 4-5. Default Conditions

YlLFI

Pl DQ zO 60 K0

1ooov range Non SRQ Continuous On Talk

Terminator is lCRllLFI

Filter out on front panel No Damping Contents of Zero Buffer equal zero 5% Digit Resolution Send EOI

4.6 DATA FORMAT The Model 181 has two modes of operation over the IEEE

bus. When in the TO (talk only) mode, the instrument will transmit its data string as requested by the external device. When the Model I81 is in the addressable mode, it must first

receive a talk command from the controller. That talk corn

mand is derived from the primary address set by the address

switches on the rear panel. Once the correct talk command

is received, the Model It31 will send its data in bit-parallel,

byte-serial fashion over the bus, as requested by the accep-

tor. The Model 181 data string contains between 16-16

ASCII characters as shown in Figure 4-5. The actual number

of characters will depend on the number of programmed

terminator characters (0-2).

4-J

The first string character will show the type of readings: an N will be transmitted if the reading is normal, while the 0 and Z characters indicate overflow and zeroed readings respectively. The next three characters indicate the func-

tion. Since the Model 181 reads only DC voltages, these

characters will always read DCV. The fifth character is the sign of the reading, while the next

seven characters form voltage reading itself. The data is

normalized so that only one digit appears to the left of the decimal point. For a normal reading, this digit can have only

the values 0 or 1. When the instrument is in overflow,

however, the most significant digit will be a 4; in addition, the remaining digits will show all zeroes while the overflow condition exists.

The next three characters show the exponent value. Since the reading is normalized, the voltage range of the Model

181 can be derived from the exponent value as shown in Table 4-6. On the 2mV range, the exponent will be -3. Switching to the 20mV range changes the exponent to -2. With each upwards range change, the exponent changes accordingly, until it reaches its maximum value of +3 on the

1ooov range.

Table 4-6. Data String Exponent Values

As an example of the data format, assume that the following data string is sent by the Model 181: NDCV-0.194557E.1 CR LF. A quick inspection reveals that a negative DC voltage is being measured. The data reading is 0.194557, but the exponent shows that the decimal point must be moved one place to the left, resulting in a final interpretation of -0.0194557 VDC. Finally, since the exponent has a value of -1, the instrument was on the 200mV range when the reading was taken.

4.7 STATUS BYTE FORMAT The Model 181 has a available status byte that will allow the

user to check certain error conditions, as well as SRQ and overflow status. The general format of the status byte is shown in Figure 4-6. Note that the IEEE-488 bits are desig-

nated DIOI through DIOE; these bits correspond to bit 0

through bit 7 in the usual binary convention. The status byte may be obtained by first sending the SPE

(Serial Poll Enable) command and then addressing the instrument to talk. The unit will then place the status byte on the the bus. After the status byte is read, the serial poll sequence should be disabled with the SPD (Serial Poll

Disable) command.

Range

-3

-2

-1 +o +1 i- 1

-I- 3

2mV

20mV

200mV

2 v

20 v

200 v

1000 v

The last two characters in the data string form the ter-

minator sequence. Figure 4-5 shows the default value of

(CR LF), but other programmed terminators will, of course, change the data string. No terminator will be sent if the terminator sequence was previously suppressed with the appropriate terminator command.

*[DCVj+jl 2 3 4 5 6 71E I 9lCR LF

N = NORMAL READING

0 = OVERFLOW z = ZEROED READlNG

READING

SliN+k&d LAS!

The status byte may be accessed whether or not an SRQ was generated by the Model 181. Care must be taken doing the serial polling sequence; if the command sequence is too slow, the instrument may send the wrong status byte. Also,

the byte must be read before the next data string is re-

quested or an incorrect value may be returned. If the bus response mode was previously set to Ml, the

Model 181 will send an SRQ command over the bus when

an error condition exists or when data is requested from the

unit. If more than one device is on the bus, the user must then use the SPE command to determine which device is requesting service. If the service request was initiated by the

Model 181, bit 6 (Dl07) of its status byte will be set. If this bit is cleared, the service request was not made by the

Model 181.

STRING

CHARACTER

4-8

Figure 4-5. IEEE-Bus Data Format

The status byte may be further checked to determine other operating parameters. If bit 5 of the status byte is set, an

error condition exists. Table 4-7 lists the conditions of the important bite along

with the resulting messages. Note that even if 5 bit is

cleared, bit 0 may be set if the Model 181 is in overflow,

The returned terminator status character has a slightly different format. Its value is derived by first masking off the four highest-ordered bits of the last terminator character by ANDing the byte with 00001111. The result is then ORed with 00110000. For example, the last byte in the default terminator is a LF, or ASCII 10. Masking with 00001111 yields

00001010. ORing this value with 00110000 gives the final result of 00111010, which is returned es an ASCII colon I:).

An example of returned values for the status word 5 0 1 0 0 0

1 1 : is shown in Table 4-8.

Table 4-8. Status Word Example

Returned

Function Range

Resolution

Zero

Filtering Damping SRQ

Trigger

EOI

Terminator

Two precautions must be taken when accessing the status

word. It must be immediately read by the controller, or the

present word will be lost. Also, since the terminator charac ters may not be printed out by some controllers, care should be taken when interpreting the terminator status character. Consult the controllers manual for further information.

Value

181 status

20 VDC 5 % digit Zero in

No filtering No damping No SRQ

One shot on talk

No EOI CR LF

4.8 STATUS WORD FORMAT 4.9 PROGRAMMING EXAMPLE The various modes of the Model 181 are controlled by the

conditions of the various bytes in the status word. Each mode such es range, resolution, etc. is assigned a number equal to its programmed value. The status word may be checked to determine the various operating modes of the unit.

When the UX command sequence is sent over the bus, the

Model 181 will transmit the status word the next time data is requested from the unit. This status word is sent es ASCII characters forming a string up to 24 bytes in length. The formet of the status word is es follows: R B Z P D M T K Y.

where R is the range, B is the resolution, etc. The EOI and

terminator modes during status word transmission remain as programmed by their separate commands.

NOTE

A GET command must be sent first in the T2 or T3 trigger modes.

The returned value for each mode except the Y (terminator)

is equal to the previously programmed number. For example, if the range was previously set to R5 (20VDC). the first character in the status word will be the ASCII character 5.

The programming example given in this subsection uses Hewlett Packard Model 85 BASIC computer language and is intended only to be an example of possible programming configurations. The HP-85 was chosen for these examples because it has a large number of BASIC commands that control IEEE-488 operation. Other controllers may be equals ly suitable for use with the Model 181; consult the controller’s operating manual for more information.

A partial list of important HP-85 BASIC statements that control bus operation is shown in Table 4-9. Many of these commands have a 3.digit argument that is essential to bus operation. The first digit in this 3.digit number is the HP-85 interface select code, and is set to 7 at the factory; the last two digits are the primary address of the external device. Each example in Table 4-9 assumes that the Model 181 primary address is et its factory set value of 5.

Many of the BASIC statements use only the interface select code. Other statements may be used with or without the primary address, depending on the desired command. CLEAR 7 for example, sends a DCL over the bus, while CLEAR 705 sends an SDC to device number 5 (in this case, the Model 1811. Note that the first statement will affect all

4-9

devices on the bus equipped to implement the DCL command. while the second statement will affect only the

addressed device.

Two of the more important statements in Table 4-9 are the ENTER and OUTPUT statements. The ENTER statement addresses device 5 to talk and then reads the entire data string into the computer, where it is stored as A$. The OUTPUT statement addresses device 5 to listen and then sends the device-dependent command string to the Model 181.

Table 4-9 lists only the statements most important to Model

181 operation. For a more complete list of HP-85stetements that affect the Model 181, consult the HP-85 operator’s manual.

A simple program to control the Model 181 with an HP-85 is shown in Figure 4-7. The program shown is by no means complete, but is merely intended to serve as a starting point for more complex programs. This program will allow the user to control the following aspects of Model 181 operation over the bus:

1. Send device-dependent commands.

2. Input the data string or status word into the computer

and display it on the CRT.

3. Send the following additional commands: REN. DCL,

SDC, LLO, GTL. G-ET.

4. Check for error conditions by accessing the Model 181

status byte.

Once the program is run, the user is prompted as to the desired course of action. Depressing the HP-85 kl key et this point will give access to the device dependent com-

mands. Once the appropriate command is sent, the data

string is read and then displayed on the CRT. At the same time, the status byte is checked, and any error messages are displayed on the screen. Note that the status word may be checked at this point by sending the UX command

sequence; the operation is very similar except that the returned string is the status word instead of normal data. Remember that a GET command must be sent first in the T2 or T3 bus response modes.

NOTE

The instrument should be placed in the remote mode with the REN command first.

The remainder of the commands may be asserted by depressing the appropriate k function key. For example, to lock out the front panel controls, the LLO command is

transmitted by using the k6 function key on the HP-85. The program in Figure 4-7 makes use of several HP-85

statements not listed in Table 4-9. The RESUME statement, for example, returns the ATN line to its false state after some commands that leave ATN true. Also, the SEND statement is used to send Untalk and Unlisten commands to the Model 181 after data or commands are transmitted. These commands are not really necessary unless other devices are used on the bus at the same time. For further details on the operation of these and other BASIC statements, refer to the HP-85 manual.

Some precautions are in order when using the program in

Figure 4.7. First of all, the ENTER statement used in lines 150 and 330 of the program assumes that the normal (CR

LF) terminator sequence will be sent by the Model 181 at the end of its data string. Other terminator sequences may require the use of the ENTER USING statement described in the HP-85 manual. Secondly, the HP-85 has no provision

for handling situations where an IEEE-488 device does not

respond to a bus command. Thus, the HP-85 computer may hang up if the program is used with a Model 181 set to a dif-

ferent primary address, or if the Model 181 is disconnected from the bus or turned off.

4.10

ENTER 705; A$

LOCAL LOCKOUT

OUTPUT 705; A$

REMOTE 705

Table 4-9. HP-85 BASIC IEEE-488 Statements

Action

Send IFC Send DCL.

Send SDC.

Device 5 addressed to talk.

Data placed in A$. Send GTL. Send LLO. Device 5 addressed to listen.

Transmit A$. Send REN. Send IFC, Cancel REN. Send SPE, obtain status byte,

send SPD. Send GET to all devices. Send GET to device 5.

~____

Affect on Model 181 Cancel Talk, Listen. Remote not affected.

Return to default conditions. Return to default conditions. Transmit data string or status word.

Return to local control. Front panel controls disabled. Receive device-dependent command string.

Set for remote operation. Cancel Talk, Listen, Remote. Send status byte.

Trigger conversion in T2 and T3 modes. Trigger conversion in T2 and T3 modes.

-..

DIM AS1251

ON KEY# l.“DEVICE” GOSUB 120

ON KEY# 2,“REN” GOSUB 540

ON KEY# 3, “GET” GOSUB 320

ON KEY# 4,“DCL” GOSUE! 380

ON KEY# 5,“SDC” GOSUB 420

ON KEY# 6,“LLO” GOSUB 460

ON KEY// 7,“GTL” GOSUE 500

CLEAR @ KEY LABEL DISP “SELECT OPTION”

100 110

GOT0 110

120

DISP “DEVICE COMMAND”

130

INPUT A$

140

OUTPUT 705 ;A$

150

ENTER 705 : ES

160

SEND 7 : UNT @ RESUME 7

170

s = SPOLL(705l

DISP @ DISP ES

180

IF S < > 0 THEN 230

190

DISP @I DISP “PRESS ‘CONT”

200 210

PAUSE

220

GOT0 20 IF BITIS.6) = 1 THEN DISP “SER

230

VICE REQUEST RECEIVED”

IF BIT(S,O)=l AND BIT(S,S)=II

240

THEN DISP “OVERFLOW” @ GOT0 290

250

IF BIT(S.51=0 THEN 290

IF BIT(S.ll=O AND BIT(S.0k0

260

THEN Dlii “ILLEGAL COMMAND” @ GOT0 290

270

IF BIT(S,1)=0 AND BITlS,Ol=l THEN DISP “ILLEGAL COMMAND OPTION” @ GOT0 290

280

IF BIT(S.ll=l AND BIT(S,0)=0

THEN DISP “ILLEGAL STRING L

ENGTH”

290

DISP “PRESS ‘CONT’ ”

300

PAUSE

310

GOT0 20

Figure 4-7. Programming Example

COMMENTS

SET AS FOR 25 CHARACTERS DEFINE KEY LABELS

WAIT FOR OPTION SELECTION TYPE IN DEVICE-- DEPENDENT COMMAND

TRANSMIT COMMAND TO 181 OBTAIN DATA STRING OR STATUS WORD

OBTAIN STATUS BYTE FROM 181

DISPLAY DATA ON CRT

IF STATUS BYTE < > 0 GOT0 LINE 230

CHECK FOR SERVICE REOUESl IS UNIT IN OVERFLOW?

ILLEGAL COMMAND?

ILLEGAL COMMAND OPTION?

ILLEGAL COMMAND STRING LENGTH,

4.11

PROGRAM 320 TRIGGER 705

330 ENTER 705 ; BS 340 DISP BS

350 SEND 7 ; UNL UNT 360 RESUME 7 370 RETURN 380 CLEAR 7 390 SEND 7 ; UNL 400

RESUME 7 410 RETURN 420 CLEAR 705 430 SEND 7 ; UNL 440

RESUME 7 450

RETURN 460 LOCAL LOCKOUT 7 470 SEND 7 ; UNL 480 RESUME 7 490 RETURN 500 LOCAL 705 510 SEND 7 ; UNL 520 RESUME 7 530 RETURN 540 REMOTE 705 550 RETURN 560 END

-~~

COMMENTS SEND GET

OBTAIN DATA STRING DISPLAY DATA ON CRT

SEND DCL

SEND SDC

SEND LLO

SEND GTL

SEND REN

Figure 4-7. Programming Example Cont.

4.12

Figure 4-8. Timing Diagram

4-1314-14

Figure 4-9. Nanovolt Preamp PC-526, Schematic

Diagram. Dwg. No. 30586D

4-151416

I N I m I .+ I I

-I

SERVICE FORM

Model No.

Serial No. Date

Name and Telephone No.

Company

List all control settings, describe problem and check boxes that apply to problem.

Intermittent

IEEE failure

OFrant panel operational DA11 ranges or functions are bad

Display or output (circle one)

aDrifts [7Unstable

Overload

Calibration only

Data required

(attach any additional sheets as necessary.) Show a block diagram of your measurement system including all instruments connected (whether power is turned on or not)

Also, describe signal source.

Analog output follows display

Obvious problem on power-up

Unable to zero

Will not read applied input

DC of C required

Particular range or function bad; specify

Batteries and fuses are OK

Checked all cables

Where is the measurement being performed? (factory, controlled laboratory. out-of-doors, etc.)

What power line voltage is used?

Relative humidity?

Any additional information. (If special modifications have been made by the user. please describe.)

Other?

Ambient Temperature?

“F

Model 1.81 Instruction Manual Addendum

Introduction

This addendum to the Model 181 Instruction Manual is being included in order to provide you with the latest in-

formation in the least possible time. Please read this information before using the Model 181.

Possible improper bus operation

Symptom: The Model 181 will not accept IEEE-488 commands.

Problem: Taking the Model 181 out of remote mode using the IEFIE48.1 Local command or issuing a Device Clear may cause problems. This problem appears to occur only with the National InAnments PCII and PCIIA IEEE-488 interface card software drivers. It has not been observed when using Rev. 2.11 of the CEC IEEE-driver or with Rev.

2.6 of the IOTech IEEE-488 bus driver, but the possibility of the problems occurring ruled out.

Solution: The Model 181 must be put back in the remote mode after sending Device Clear or Go to Local before any other IEEE commands will be accepted.

with these interfaces cannot be

Programming examples

National Instruments PCII and PCIIA Example

’ UNCLUDE: ‘qbdecl.bas’

CALL IBPINlJ (“GPIBO”, IBO%) CALL IBPIND (“KIlSl”, KIlSl%) CALL IBSIC (IBO%) CALL IBSRE uBo%, 1) CALL IBCLR KIlSl%) CALLIBLOC (KI181%)

’ MUST ISIJE THESE COMMANDS ’ OR ELSE YOU WILL HAVE TO TOGGLE THE POWER ON THE 181 TO RECOVER.

CALL IBSRE (lBo%, 1) CALL IBCMD (IBO%, “?%?“) ’ send UNL LISTEN 5 UNL to 181

BEFORE SENDING ANY COMMANDS

To THE l&31!

32421-B-1 / 12-91

(see Table 1 for LISTEN commands at other IEEE Addresses)

Table 1. Listen command characters

ASCII Equivalent listen commands

‘

SPACE

LISTEN 0

0 LISTEN 16

1 LISTEN1

I, LISTF.N 2 # LISTEN 3 $ LISTEN 4

% &

LISTEN 5 LISTEN 6 LISTImI 7

( LISTEN 8

LISTFN9

) I LISTFN10

+ LISTEN11 ;

LISTFN12 < LISTFN13 = LISTEN14 >

LISTBN15 ? IJNL

1 L.IsrEN 17 2 LISTEN 18 3 LISI-EN 19 4 LLSTEN 20 5 LISTEN 21 6 LIsrEN22 7 LISTEN23 8 LISTEN 24 9 LISTEN25

: LISTEN 26

LISTEN 27 Llsl-FN 28 LISTEN 29 LISTEN 30

I-o send a LISTEN 2 issue ‘?“+CHR$ (34) + “?”

CEC PC-488 card example

’ $INCLUDE: ‘IEEEQB.BI ’ CALL INITIALIzE01,0)

CALL TRANSMIT (“UNT UNL LISTEN 5 SIX UNL”, STATUS%) ’ clear 181 CALL TRANSMIT VJNL LISTEN 5 GI’L IJNL”, STATUS%) ’ Put 181 into Local

’ MAY HAVE TO ISSUE THIS COMMAND ’ OR ELSE YOU WILL HAVE TO TOGGLE THB POWER ON THE 181 TO RECOVER.

CALL TRANSMIT (“REN UNL LISTEN 5 UNL”, STATUS%)

BEFORE SENDING ANY CO-

TO THE 181!

lOTech~Personal488 card example

OPEN “\DEV\IEEEOUT” FOR OUTPUT AS #I OPEN “\DEV\IEEElW FOR INPUT AS #2 IOCTL #l, “BREAK” IWNT #I, “RESFY PRINT #1, “CLEAR 5” PRINT #l, ‘uxAL 5”

MAY HAVE TO ISSUE THIS COMMAND

OR ELSE YOU WILL HAVE TO TOGGLE THE POWER ON THE 181 TO RECOVER.

PRINT #1, “REMOTE 5”

BEFORE SENDING ANY COMMANDS TO THE 181!

Keithley 181 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual