B&B Electronics 485T User Manual

Size:
127.3 Kb
Download

Not Recommended for New Installations.

Please contact Technical Support for more information.

RS-422/RS-485Line Tester

Model 485T

Documentation Number 485T1995

This product

Designed and Manufactured

In Ottawa, Illinois

USA

of domestic and imported parts by

B&B Electronics Mfg. Co. Inc.

707 Dayton Road --P.O. Box 1040--Ottawa, IL 61350 PH (815)433-5100--FAX (815)433-5105

Internet: http://www.bb-elec.com orders@bb-elec.com support@bb.elec.com

1992 B&B Electronics --Revised August 1992

Documentation Number 485T1995 Manual

Cover Page

B&B Electronics --PO Box 1040--Ottawa, IL 61350

PH (815) 433-5100--FAX

(815) 433-5105

TABLE OF CONTENTS

INTRODUCTION ...............................................................................

1

RS-422ANDRS-485STANDARDS .................................................

1

CONTROLS AND CONNECTIONS ON THE 485T ..........................

6

A AND B TERMINALS ........................................................................

6

GROUND TERMINAL..........................................................................

6

THRESHOLD POT..............................................................................

7

POSITIVE THRESHOLD EXCEEDED LED .............................................

7

NEGATIVE THRESHOLD EXCEEDED LED............................................

7

RS-422/RS-485SWITCH.................................................................

8

TERMINATION SWITCH ......................................................................

8

POWER............................................................................................

8

RS-232DATA OUT CONNECTOR ......................................................

8

RS-422/485TEST .............................................................................

9

Documentation Number 485T1995 Manual Table of Contents i

B&B Electronics --PO Box 1040--Ottawa, IL 61350

PH (815) 433-5100--FAX (815)433-5105

INTRODUCTION

The Model 485T Tester can be used to check the condition of signals at any node on an RS-422orRS-485network. The tester determines if the maximum permissible negative or positive voltages are being exceeded. If these voltage levels are exceeded it is possible to damage theRS-422/485receivers and drivers. The tester also checks to see if minimum required differential voltages exist on the lines. The tester uses the normal data that is transmitted on the line to check the value of the minimum differential voltage. There is anRS-232output that can be used to view the network data.

RS-422ANDRS-485STANDARDS

The RS-232Standard (Interface Between Data Terminal Equipment and DataCircuit-TerminatingEquipment Employing Serial Binary Data Interchange) was developed many years ago by the Electronic Industries Association (EIA). It was used to connect modems to terminals fordial-upcomputer systems. Since the modem and terminal were usually located close to each other, wire length was not much of a factor in the standard.RS-232maximum wire lengths were set at 50 feet.RS-232lines can run farther with special (low capacitance) wire but there is a practical limit of 300 or so feet.

The EIA then developed two newer standards to allow serial data communications to go past the 50 foot barrier. The first is RS422, Electrical Characteristics of Balanced Voltage Digital Interface Circuits. The second is RS-485,Standard for Electrical Characteristics of Generators and Receivers for use in Balanced Digital Multipoint Systems. Using these standards, wire lengths of about 4000 feet are attainable. This is possible by using two wires for each signal instead of the one wire used inRS-232.

A typical RS-232system can have eight wires, seven signals and one ground, not all of which are used in every system. As each signal goes high or low (typically +12 and-12volts) it uses the one ground wire as a reference. Ground currents can cause confusion. Also, theRS-232drivers are not capable of driving much more than 2500 picofarads before the signal on the line is badly distorted.

Both RS-422andRS-485use two wires for each signal. These two wires are driven differentially. When one wire is low the other is

Documentation Number 485T1995 Manual

1

B&B Electronics --PO Box 1040--Ottawa, IL 61350

PH (815) 433-5100--

FAX (815) 433-5105

high and vice-versa.Also, the drivers are capable of driving a wire with as much as 25 times more capacitance before the signal is badly distorted. Most system designers try to minimize the number of signals used because these standards take two wires per signal. This is one reason mostRS-422andRS-485systems are eithertwo-wirewith ground orfour-wirewith ground.

There are two major differences between RS-422andRS-485.The first is that withRS-422,one driver can only drive 10 receivers. WithRS-485,one driver can drive 32 receivers. Secondly,RS-485drivers can be“tri-stated”or turned off. This allows you to put all 32 drivers and receivers on onetwo-wireline. When a device needs to output data, it first turns on its driver to seize the line and then sends its data. The other 31 devices will be listening.RS-422requires at least two pairs of wires, one pair to send data in one direction and the other for return data.

Typically, the RS-422orRS-485driver will switch between +5 volts and ground. Since devices can be long distances apart, they can be powered on different power lines or transformers that can force their “reference grounds” to be at different voltages. The RS422 Standard allows the signal lines to go as high as +7 volts and as low as-7volts. TheRS-485Standard allows +12 volts and-7volts.

Another requirement is that the differential input voltage at the receiver must be more than 0.2 volts. The differential voltage starts out from the driver at about 2 volts but the resistance, inductance, and capacitance of the wire combine to attenuate this signal. As the wire gets longer, this voltage will get smaller until it falls below the 0.2 volt level and then communications can no longer be guaranteed.

Both specifications discuss termination of the two-wireline. A typicaltwo-wiretwisted-pairline looks like a 100 ohm transmission line. In general, the line does not need to be terminated for slow data rates or forshort-wirelengths.

More information and the RS-232,RS-422,andRS-485Standards are available from:

ELECTRONICS INDUSTRIES ASSOCIATION

Engineering Department

2001 Eye Street N. W.

Washington, DC 20006

Phone: (202) 457-4900---FAX: (202)457-4985

2 Documentation Number 485T1995 Manual

B&B Electronics --PO Box 1040--Ottawa, IL 61350

PH (815) 433-5100--FAX (815)433-5105

Documentation Number 485T1995 Manual

3

B&B Electronics --PO Box 1040--Ottawa, IL 61350

PH (815) 433-5100--

FAX (815) 433-5105

4 Documentation Number 485T1995 Manual

B&B Electronics --PO Box 1040--Ottawa, IL 61350

PH (815) 433-5100--FAX (815)433-5105

Documentation Number 485T1995 Manual

5

B&B Electronics --PO Box 1040--Ottawa, IL 61350

PH (815) 433-5100--

FAX (815) 433-5105

CONTROLS AND CONNECTIONS ON THE 485T

A and B Terminals

The two signal lines used in an RS-422orRS-485network should be labeled A and B. These lines should be connected to the A and B terminals of the 485T Tester. If your lines are not marked (or are marked using another method) you can use a DC voltmeter to determine which is which. With no data being sent, the A line will be negative with respect to the B line. On anRS-485network, if all of the drivers are off, the voltmeter will read zero volts. Under these conditions, it is not possible to use a voltmeter to determine which line is A and which is B. Having them connected backwards will only be a problem when using theRS-232Data Out Connector.

Some networks will use four wires instead of two. In these networks, one pair will be used to send data out from a master device. The other pair will send data back from one or a multitude of other devices. These networks can be tested using the 485T by treating them as two separate networks. You would first run your tests on one pair of signal lines and then run them again on the other pair.

NOTE: Make sure that you are connected to the correct two lines when using the tester. It is possible for the tester to look like it is working properly when one line is connected and the other is floating or connected to a wire in another pair.

Ground Terminal

The GND terminal on the 485T Tester should be connected to Signal Ground for the device under test. The Signal Ground is usually the same as the power supply ground for your unit. Some systems will also have a Frame Ground that is usually connected to the “Green Wire Ground” of your power supply AC plug. Frame Ground can also be connected to the wire shield if your pair of wires is shielded. You want Signal Ground not Frame Ground. In some systems, Frame Ground will be connected to Signal Ground, usually through a 100 ohm one-halfwatt resistor. You still want to be connected to Signal Ground to make proper measurements, since they may be at different voltages. There is a 100 ohmone-halfwatt resistor connected internally between the GND terminal and the internal Signal Ground (power supply ground) of the 485T. This is the optional grounding method from both theRS-422and the RS485 Standard.

6 Documentation Number 485T1995 Manual

B&B Electronics --PO Box 1040--Ottawa, IL 61350

PH (815) 433-5100--FAX (815)433-5105

Threshold Pot

The large pot in the center of the 485T is the threshold pot. In order for RS-422/485to work properly, you should have a differential input voltage of more than .2 volts across the two lines at the RS422/485 receiver. By adjusting this pot you can determine the amplitude of theRS-422/485signals. Start with the knob turned down to zero volts (full clockwise) with data being sent on your network. The Data LED should be blinking with the data providing that the network is not running at too high of a baud rate. At higher baud rates the LED may be on. Turn the threshold pot up (counterclockwise) until the Data LED stops blinking or turns off. The reading of the pot at that point is the differential input voltage. This reading should be .2 volt or more. If the LED does not stop blinking or turn off with the pot all the way up (full counterclockwise) the reading is above 1 volt. Readings above 1 volt are not available since it would make the lower voltage readings difficult to read. Readings above 1 volt indicate that differential voltage is more than adequate.

Positive Threshold Exceeded LED

The RS-422Standard states that a network should not have a positive common mode voltage of more than 7 volts. This means that the difference between Signal Ground and either the A or B input should not be more than positive 7 volts. ForRS-485the maximum is 12 volts. The Positive Threshold Exceeded LED will light if these limits are exceeded. This reading is taken with respect to Signal Ground so the GND terminal must be connected properly for the reading to be correct.

NOTE: TheRS-422/RS-485switch is used to control this threshold. It must be in the proper position for these readings to be correct.

Negative Threshold Exceeded LED

Both the RS-422and theRS-485Standards state that the network should not have a negative common mode voltage of more than 7 volts. This means that the difference between Signal Ground and either the A or B input should not be more than minus 7 volts. The Negative Threshold Exceeded LED will light if this limit is exceeded. Again this reading is taken with respect to Signal Ground so the GND terminal must be connected properly for the reading to be correct.

Documentation Number 485T1995 Manual

7

B&B Electronics --PO Box 1040--Ottawa, IL 61350

PH (815) 433-5100--

FAX (815) 433-5105

RS-422/RS-485Switch

This switch controls the positive threshold voltage for the Positive Threshold LED only. When the switch is in the RS-422position, the positive threshold is +7 volts. When it is in theRS-485position, the positive threshold is +12 volts. This switch has no effect on the Data LED, the Threshold Pot, or on the Negative Threshold LED.

Termination Switch

In some cases RS-422andRS-485pairs are terminated. This termination is usually needed only for long or high speed networks. Terminations usually consist of a 100 ohmone-halfwatt resistor located at each end of the network. In some complicatedmulti-dropnetworks it may be hard to determine exactly were the “ends” are. A 100 ohmone-halfwatt termination resistor is included in the 485T. It may be helpful if you are testing an existing network with no terminations and want to see what happens if you terminate it at one point. Normally, you should leave this switch OFF (down).

Power

The 485T will work from a nine-voltbattery or from 12 VDC through the 2.5 mm jack marked DC Power 12VDC IN. When you plug in the external power supply (available from B&B Electronics as the Model 485PS) the battery is automatically disconnected.

RS-232Data Out Connector

This is a standard DB-25SRS-232connector. When the DATA LED is blinking, anyRS-422/485data on the A & B inputs is converted to theRS-232format and output on pin 2. You can connect this to a device that has anRS-232port and can read your data. Be sure you turn the threshold pot all the way down (clockwise) to zero. For use on standard ASCII data B&B Electronics sells a program called BreakOut II that will turn a PC into a serial data monitor. With it and a PC you can monitor data on yourRS-422/485network.

NOTE: If your data is garbled you may have the A and B lines connected backwards or the threshold pot may not be all the way down to zero. It is also possible that theRS-422/485signal may be too weak.

8 Documentation Number 485T1995 Manual

B&B Electronics --PO Box 1040--Ottawa, IL 61350

PH (815) 433-5100--FAX (815)433-5105