GE MAC 500 User manual

MAC 500 (MicroSmart)

Servicing Instructions

Version 2.xx

227 470 35 Revision D

Caution:

During repairs/service interventions,
observe the protective measures against
damage due to ESD.

* Marquette Hellige GmbH considers itself

responsible for the effects on safety,
reliability , and performance of the equip­ment, only if:

- assembly operations, extensions,
readjustments, modifications, or
repairs are carried out by
Marquette Hellige GmbH or by
persons authorized by Marquette
Hellige GmbH,

- the electrical installation of the
relevant room complies with the
applicable national and local require­ments, and

- the instrument is used in accordance
with the instructions for use.

* This manual contains service informati-

on; operating instructions are provided in
the operator’s manual of the instrument.

* This manual is in conformity with the

instrument at printing date.

* All rights are reserved for instruments,

circuits, techniques, and names appearing
in the manual.

©

Marquette Hellige GmbH

Printed in Germany

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Servicing Instructions 227 470 35 D

Contents

1 Documentation and nomenclature of Marquette Hellige instrument part Nos

1.1 Configuration of instrument part No

1.2 Configuration of the PCB part Nos

1.3 Instrument status documentation (nominal status)

2 Description of the unit

2.1 Block diagram, total unit

2.2 Mechanical structure

3 Description of the function

3.1 Power supply module

3.1.1 System inlet

3.1.2 Extended range power supply

3.2 Battery

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3.3 Printed circuit board (PCB) MAC 500 (MicroSmart)

3.3.1 Voltage supply and monitoring

3.3.2 Computer

3.3.3 ECG recording and pre-processing

3.3.4 Drive electronics and display

3.4 Internal interfaces

3.4.1 Interface, power supply

3.4.2 Interface, display

3.4.3 Interface, thermal array

3.4.4 Interface, motor

3.4.5 Interface, photoelectric barrier

3.4.6 Interface, keyboard

3.4.7 Interface, ECG input

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3.5 External interfaces

3.5.1 Line inlet

3.5.2 Patient input

3.5.3 IR interface

3.6 Delimitations

4 Unit test functions

4.1 General

4.2 Key test and loudspeaker test

4.3 Display test

4.4 Motor test

4.5 Recording test

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4.6 IR test (MAC 500 with measurement and communication only)

4.7 Recording the results

4.7.1 Error codes

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4.8 Additional functions for final test

4.9 Re-locking option

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5 Selecting the type of appliance

5.1 General

5.2 Selecting the type of appliance

6 Repair notes

6.1 Safety notes

6.2 Component replacement

7 Troubleshooting
8 Maintenance and care

8.1 Technical inspection

8.1.1 Visual checks

8.1.2 Function checks

8.2 Safety Analysis Test

8.2.1 General Information

8.2.2 Protective earth resistance test

8.2.3 Measuring of leakage current

8.2.4 Enclosure Leakage Current Test

8.2.5 Patient Leakage Current Test

8.3 Maintenance, cleaning, disinfection

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9 Technical description
10 Spare parts list
11 Appendix

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Revision History

Each page of this manual has the document number followed by a revision letter, located at the top
of the page. This letter identifies the manual update level. The latest letter of the alphabet
corresponds to the most current revision of the document.

The revision history of this manual is summarized below.

Date Revision Remarks

September 1997 A Initial release of Dervicing Instructions
November 1997 B Update type of appliance, changed part number of the PCBs
March 1999 C ECO no.: 061918; new logo/firmname, serial number entry via

keypad

May 2000 D ECO 064 689, Changed name from MicroSmart to MAC 500

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1 Documentation and nomenclature of Marquette Hellige

instrument part Nos

1.1 Configuration of instrument part No

The instrument part No comprises 8 digits, the first 6 digits determining the instrument type, the
last 2 digits the instrument version. The language is determined by configuration, thus having no
influence on the part No.

E.g. Instrument Type Version

MAC 500, intern. 101 134 09
MAC 500, intern. with measurement, IR 101 134 10
MAC 500 USA 101 134 11
MAC 500 USA with measurement, IR 101 134 12

1.2 Configuration of the PCB part Nos

388 xxx yy Spare part numbers for the operative PCBs.

The instrument documentation, e.g., reference diagrams, circuit diagrams and parts lists are listed
under this part No.
The 388 number is located on the barcode label.

Configuration of the barcode labels:

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303 xxx yy Spare part numbers for PCBs tested especially thoroughly

303 numbers are only given to PCBs where the level of testing applied to 388 PCBs is inadequate
for implementation when servicing in the field, or where only a complete set of PCBs can be
replaced in the field.
In addition to a barcode label (388 number) 303 part Nos also have an additional label with a 303
number and are to be found in the spare parts list under this number.

389 xxx yy Replacement numbers for defective PCBs

Where servicing is required 389 PCBs are available for the replacement of some PCBs. When
using a replacement PCB (389 part No) the defective PCB is to be returned to the Freiburg factory.
Replacement PCB part Nos are included in the spare parts list.
389 PCBs have an additional adhesive label.

1.3 Instrument status documentation (nominal status)

Due to the hardware and software combination unambiguous documentation of the instrument
assembly status is necessary , also in the event of repairs.

This documentation comprises the following documents and measures:

Master Record Index (MRI)

This document is a component of this instrument documentation.

This document states the combination of permissible hardware and software for a particular
instrument version. The permissible PCB Index is given in the “Index” column with each update
delivered. Further permissible PCB Indexes are given in the “compatible” column. The PCB Index
can be found in the PCB barcode label.

Product Status Index

This document is created during manufacture. The Product Status Index documents the
hardware/software product status.

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2 Description of the unit

These service instructions for the V2.x version of the unit describe both the MAC 500
(MicroSmart) as well as the MAC 500 with measurement and communication (101 134 10). Unless
a note appears to the contrary, this description applies to both the MAC 500 (MicroSmart), and the
MAC 500 with measurement and communication (101 134 10).

MAC 500 (MicroSmart) is a portable cardiograph with integrated printer drive. It is designed to
record, register and process ECG signals. It is designed both for mains and battery operation,
operation without battery is also possible. A power supply unit and battery are integrated in the
unit.

MAC 500 with measurement and communication (101 134 10) also includes in the "Auto" mode
the measurement of the ECGs and registration of the measurement results.

MAC 500 (MicroSmart) and MAC 500 with measurement and communication are based on the
same hardware platform.

The following versions of MAC 500 (MicroSmart) are available:

101 134 01 MicroSmart (international) 100...240V~
101 134 02 MicroSmart MC (international), measurement + IR 100...240V~
101 134 03 MicroSmart (Asia) 100...240V~
101 134 04 MicroSmart M (USA), measurement 100...240V~
101 134 05 MicroSmart MC (Asia), measurement + IR 100...240V~
101 134 06 MicroSmart (USA) 100...240V~
101 134 07 MicroSmart (inter. 5-pin) 100...240V~
101 134 08 MAC 500 (USA), measurement 100...240 V~

standardization since May 2000

101 134 09 MAC 500 (international) 100...240V~
101 134 10 MAC 500 (international),

measurement + communication 100...240V~
101 134 11 MAC 500 (USA) 100...240V~
101 134 12 MAC 500 (USA), measurement 100...240V~

The hardware consists of the following function blocks:

- MAC 500 (MicroSmart) PCB

- power supply module

- battery

- keyboard

- printer drive

The following function blocks are implemented as PCBs.

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- MAC 500 (MicroSmart) PCB

- power supply

The patient input, which is a component of the MAC 500 (MicroSmart) PCB, is mounted on the
power supply module and connected to the MAC 500 (MicroSmart) PCB via a flexible supply line.

The intended use, the functions available and operation of MAC 500 (MicroSmart) are described in
the instructions for use.

2.1 Block diagram, total unit

2.2 Mechanical structure

The major mechanical components of MAC 500 (MicroSmart) are the top and bottom shell. The
bottom shell is the basic element carrying the following sub-assemblies:

- Power supply module with sy stem inlet, power supply unit and patient input.

- Battery

- Thermal array drive with paper magazine

- PCB MAC 500 (MicroSmart) with display

The top shell holds the keyboard which is linked to the PCB MAC 500 (MicroSmart) via a flexible
cable.

The 15-pin inlet plug for connecting the patient lead is located at the power supply module. It is
linked to the PCB MAC 500 (MicroSmart) via a flexible cable.

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3 Description of the function

The description of the individual function blocks follows the Block diagram of the total unit in
chapter 1.1 and the function blocks of the P plans.

3.1 Power supply module

The power supply module comprises the following functions:

- System inlet with fuse s

- Extended range power supply

- Patient input

These components are mounted on a carrier plate bolted into the bottom shell of the enclosure.

3.1.1 System inlet

The system inlet is defined as a sy stem inlet module. It includes a three-pin IEC plug and two size 5
X 20 fuses accessible from the outside. The module is a component with snap-type function.

The system inlet is designed as "Universal Input", with the effect that no adjustment to the system
voltage ranges 100V~ ... 120V~ or 220V~ ... 240V~ is required.

3.1.2 Extended range power supply

The AC/DC power supply is designed as a universal extended range power supply. The power
supply unit is purchased complete and mounted on the carrier plate. The power supply supplies an
output voltage of 15.6V, from which all required voltages are generated.

- Input voltage range: 90VAC...264VAC

- Frequency range: 49Hz...65Hz

- Output: 40W max.

- Efficiency

- Output voltage: +15.6V ± 2%

- Output current: 2.6A max.

- Short-circuit-proof

- Approvals: IEC601, UL544, CSA22.2-125, VDE750

70%

≥

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The connection between the AC/DC power supply and the PCB MAC 500 (MicroSmart) is
implemented with a 2-pin lead as follows:

- on the AC/DC power supply: plugged

-on the PCB MAC 500 (MicroSmart): plugged

3.2 Battery

The battery is a rechargeable, maintenance-free lead battery. The battery is purchased complete and
mounted on the bottom shell of the enclosure.

- Rated voltage: 12V

- Rated capacity: 1.2Ah

The connection between the battery and the PCB MAC 500 (MicroSmart) is implemented with a 2­pin lead as follows:

- on the battery: plugged

- on the PCB MAC 500 (MicroSmart): plugged

3.3 Printed circuit board (PCB) MAC 500 (MicroSmart)

The PCB MAC 500 (MicroSmart) holds the entire electronics of the unit. The electronics comprise
the following function groups:

- V oltag e supply and monitoring

On-Off electronics
Battery charge
Input voltage monitoring
Voltage supply +5V

- Computer

Controller (Motorola 68332)
EPROM 512KByte
RAM 256KByte (MAC 500) or 512KByte (MAC 500 with measurement and
communication)
Configuration memory (EEPROM) 256Byte
Reset Generation
Alarm signal output
Real-time clock, buffered
Keyboard interface
IR interface (MAC 500 with measurement and communication (101 134 10) only)

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- ECG recording and pre-processing (floating side)

- Protective input circuit

- Pre-amplifier

- AD converter

- PACE identification

- Electrode label

- Conductor label

- Filter and interface module

- Current supply

- Reference edit

- Drive electronics and display

Array control
Temperature monitoring
Motor control
Voltage supply +24V
Photoelectric barrier analysis

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3.3.1 Voltage supply and monitoring

On-Off electronics

The unit is switched on and off via the ON/OFF key (on the membrane keypad). Enabling and
disabling operates via a toggle function: if the unit is switched off, press the ON/OFF key to switch
the unit on. If the unit is switched on, press the ON/OFF key to switch the unit off. The processor
can switch the unit off via the signal lead "G_OFF" if:

- the input voltag e is too low (exhaustive discharge protection for the battery)

- the unit is not oper ated f or a ny length of time (approx. 5 minutes)

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Battery charge

The battery is charged by means of a special charging IC (UC3906) for lead batteries. The circuitry
monitors the charging current and the charge voltage. The charging IC has the same "temperature
coefficient" as a lead battery, with the effect that the battery charge is optimized over the specified
temperature range. The circuit operates as a "DUAL LEVEL FLOAT CHARGER", with three
distinct charging states:

- high current bulk charge state

- over-charge state

- float state

A charging cycle begins with "high current bulk charge state". In this state the battery is charged
with a constant current ( I
in as soon as a certain voltage value ( U

) while the battery voltage is monitored. The "over-charge state" sets

max

) is reached. In this state the battery voltage is kept at a

12

certain value ( UOC ), while the charging current is monitored. If the charging current drops to a
certain value ( I
almost 100%. In the "float state" the battery voltage is regulated to a precise value ( U

), the "float state" sets in. At this point in time the battery capacity has risen to

OC

).

F

The following values for voltage and current are selected when charging the 12V lead battery :

I
U

max

12

= 250mA
= 13.5V

UOC= 14.2V

IOC = 25mA
U

F

= 13.7V

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Input voltage monitoring

The input voltage is monitored. If it drops to 11.3V, the LED_Bat lights up. This indicates that the
battery is in need of recharging. If the input voltage drops further to 10.3V, the signal "Batt_low"
will be activated. This signal is scanned by the processor. If it is active, the processor will
deactivate the unit (exhaustive discharge protection for the battery).

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Voltage supply +5V

A 500KHz step-down switching regulator is used to generate the 5V voltage. The high-rate
switching frequency allows the entire circuit to be built up with SMD components. The switching
regulator is the module type LT1376. All the functions necessary for a step-down regulator are
integrated in this module.

Input voltage 10V...16V
Output voltage +5 V ± 2%
Output current min. 100 mA, max. 700 mA
Efficiency > 80%
Short-circuit-proof

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3.3.2 Computer

Controller

At the actual core of the unit is the Motorola Controller 68332 with the following integrated
components:

- CPU32, computer core, internal 32 bit reg ister, external 16 bit processing

- T PU, independe nt timing proc essor

- QSM with SCI for the implementation of a single RS 232 interface and a serial QSPI port
with up to 16 channels.

- SIM with Chipselect generation, system monitoring, clock synthesizer

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EPROM

ROM comprises one 4MBit EPROM module (= 512KByte). The data bus width is 16Bit.
Chipselect is the CSBOOT of the 68332.

RAM

RAM comprises a maximum of four static RAM modules with 128KByte each. The data bus width
is 16Bit. Each RAM chip receives its own chip-select signal (separate chip-select for High and Low
Byte) from the controller. This means that the RAM address is software-configurable. The basic
MAC 500 (MicroSmart) unit is only equipped with two RAMs, corresponding to a memory area of
256KByte. The memory capacity can be extended to 512KByte by adding two more RAMs. Access
time is 70 ns, this means that access is allowed without Wait States.

EEPROM

A serial EEPROM is used for the non-volatile memory. This is connected to the QSPI interface of
the 68332. The EEPROM has a memory area of 2048 Bit. (= 256Byte)

Reset Generation

Reset Generation is implemented with an integrated monitor module. It includes the voltage
monitoring with Reset Generation.

Acoustic signal output

The MAC 500 (MicroSmart) has a sound output for acoustic status/alarm signals. The pitch is
selected via a TPU channel of the 68332 (signal name: Beep). In addition, the volume can be varied
in 3 stages. Volume is set via the 3 signals LAUT1, L AUT2 and LAUT3.

Real-time clock

Provides the time and date. During operation it is supplied by the Supply logic; when the unit is
turned off, the unit switches over automatically to a 3V lithium cell which preserves the data. The
control signals for the clock (chipselect- read/write signal) are generated directly by the controller
(MC68332).

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Keyboard interface

MAC 500 (MicroSmart)´s keyboard interface comprises a 5x5 matrix, although only a 4x4 matrix
is required and led to the keyboard. This allows 16 keys to be implemented. The keys are polled in
cycles. To do so, bit combinations are written into a buffer module (column) in cycles. An input
module (row) polled in cycles identifies if a key has been pressed (the combination of output
pattern and input pattern allows the pressed key to be determined).
The keyboard interface is located on the top byte of the data bus. The bits D8..D12 are used for the
keyboard (both input and output).
Both the output buffer and the input buffer are selected via a separate chipselect signal (chipselect
signal of the 68332).

The top two bits (D13...D15) of the input buffer are assigned with additional functions:

D13: Hardware configuration bit (function undetermined) for future extensions

Default: 0

D14: Battery monitor bit: D14 = 1 ==> battery voltage < 11.3V

D14 = 0 ==> battery voltage > 11.3V

D15: Battery monitor bit: D15 = 1 ==> battery voltage < 10.3V

D15 = 0 ==> battery voltage > 10.3V

In addition to the 16 keys of the 4x4 matrix, the keyboard includes a key for switching the unit on
and off. Due to its special hardware configuration, this key is not integrated in the matrix.

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The keyboard also includes 4 LEDs which are selected via the keyboard interface.

Line LED: This LED is supplied directly from the 15V of the power supply. It shows

whether the unit is mains-operated (LED on) or supplied from battery (LED
off).

LED LOBAT: This LED is switched on and off by the controller. It shows that the battery is in

need of a recharge. A LOW level at the signal LED_LOBAT_ activates the
LED.

LED START: This LED indicates the status of the unit. It means that the unit is in an active state !

(processing, printing, etc. in progress). A LOW level at the signal
LED_START_ activates the LED.

LED STOP: This LED indicates the status of the unit. It means that the unit is in a

passive state ! (processing, printing, etc. not in progress). A LOW level at
the signal LED_STOP_ activates the LED.

IR interface

The MAC 500 with measurement and communication is equipped with an IRDA interface.
The IRDA interface is selected via the RS232 interface of the 68332.
(Signals TXD and RXD of the '332).
In addition, an output port of the '332 determines if the IRDA module (TOI3232) is in the
configuration or in the communication mode.
Configuration signal: IR_BR_D = 0 ==> communication mode

IR_BR_D = 1 ==> configuration mode

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3.3.3 ECG recording and pre-processing

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The ASIC chipset HECTOR, consisting of 3 Ics, is used for ECG editing on the floating side. The
MAC 500 (MicroSmart) uses 2 ICs type SDM_HEC2 as AD converters and one IC type
DIGI_HEC2 as filter and interface module. Together with the protective input circuit, a floating
power supply and an interface insulated via optical coupler, the ECG editing is the floating section
of MAC 500 (MicroSmart) and is part of the PCB MAC 500 (MicroSmart). Discrete analog
components and a PIC processor are used for PACE detection.

ECG pre-processing comprises the following functional groups:

- Protective input circuit

- Pre-amplifier

- AD-converter

- PACE detection

- Electrode label

- Lead label

- Filter and interface module

- Power supply

- Reference editing

Protective input c irc uit

The protective input circuit is designed for the connection of 9 input electrodes and a push-pull
modulation, and includes 2 surge diverters and 18 high voltage diodes attached directly behind each
patient lead, as well as a hybrid (ECG input) which ensures the safety of the patient and of the
electronic components. Protection is only assured if a patient lead with series resistors of ≥ 8 kΩ is
used.

Overvoltages reaching the input are limited in the first stage to 90 V through surge diverters and
high voltage diodes. The voltage then passes via a 47 KΩ resistor from each input electrode on the
hybrid to 2 silicon diodes which limit the voltage to 1.2 V before it reaches the downstream
operations amplifier via 100 Ω. The overvoltages reaching the push-pull modulation output are also
limited to 1.2 V by two high voltage diodes over 3.3 KΩ and by two more diode line sections, while
the downstream operations amplifier is protected by 6.8 KΩ.

Patient safety is assured by the above two diodes on the hybrid 'ECG Input' and by the serial
resistance of 47 KΩ. In case of a defective input amplifier, the supply voltag e of ± 5 V can reach the
input. The 100 Ω resistor on the hybrid limits the current flow to the diode, preventing damage to
the diode and limiting the supply voltage to 1.2 V. These 1.2 V are transmitted to the patient over
47 KΩ. The current flowing through it is limited to < 50 µA by the 47 KΩ.

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Pre-amplifier

The 9 connectable electrode signals are transmitted to 9 low-noise operations amplifiers behind the
protective input circuit. These operations amplifiers amplify the input signals by the factor 3.8. This
pre-amplification is necessary in order to maintain the maximum noise value of 15 µVpp over the
entire system.

The R electrode is used as reference for the other electrodes, with the effect that the difference to
the R electrode always applies after each input amplifier. This means that the signal L-R is
available at the output of the operations amplifier for the L electrode. This configuration is
necessary in order to obtain a common-mode rejection in addition to the push-pull modulation. The
signal for the push-pull modulation is taken from the R electrode. The lead-offs are computed in
the software from these differe ntial sig na ls, with the R ele ctrode being ignor ed throug h the renewed
differential formation in the appropriate lead-offs. The 8 differential signals which remain from the
original 9 electrode signals are transmitted to the Σ∆ modulators via a first order low pass with
1 kHz cut-off frequency.

AD converter

After the pre-amplifiers the signals are transmitted to analog-digital converters. The AD converters
are the Σ∆ modulator type. Two ICs of type SDM_HEC2 are used, each of which include 5
converters. The components for the internal integrators, used to adapt the modulators to their task,
are connected to the pins IM2x, OUT2x, REFx, OUT1x and IM1x. Each differential signal at the
output of the AD converter is resolved to 18 bit. With reference to the patient input, one LSB
corresponds to 5µV. Conversion is parallel in all channels, i.e. without any time offset. The
scanning frequency is 1kHz. Using the appropriate control words, it can also be set for 500 Hz and
2 kHz.

A square-wave signal is visible at the outputs OUT1 thru OUT5, which occurs synchronous with
the SWITCH signal. The duty factor of this square-wave signal depends on the input signal. This
data stream reaches the IC of type Ty p DIGI_HEC2.

PACE detection

After the pre-amplifiers the 8 electrode signals lead to a multiplexer 1:8. Using the 3 outputs OP1,
OP3 and OP4 of the chip Chips DIGI_HEC2 the multiplexer selects the electrode to be used for
PACE detection. The selected signal is routed via a first order high pass with 23 Hz cut-off
frequency and amplified by the factor 1,000. The signal then reache a window comparator with a
4,5 mV threshold with reference to the input.

The 2 outputs of the window comparator are put to a PIC processor for further PACE analysis. This
processor supplies a PACE bit if the appropriate signals of the window comparators apply and if
the pulse duration is ≤ 2ms. The overshoot of the PACE pulse is suppressed by the PIC processor.

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Electrode labeling

The 18 bit result of the analog-digital conversion shows if one or more differential signals are
overloaded, i.e. if the differential voltage with reference to the patient input is greater than 0.6V. A
hysteresis of 15 mV (0.6 - 0.615 V) is provided for the query. The query takes place simultaneously
for all 8 channels. The information (1 bit/cha nnel) is tra nsfe rre d to the CPU via the se rial por t in the
word Electrode label.
The overload of a channel can be caused by excessive polarization voltage (>600 mV) or by a
detached electrode. In the latter case a voltage of 1 V is transmitted to the amplifier inputs via the
100 MΩ resistors on the protective input circuit.
One more circuit section is provided which handles the electrode error signal for the R and N
electrodes, because these cannot be detected individually with the converter overload. The
information is transmitted via INP1 and INP2 of the input port of ASIC DIGI_HEC2 in the status
word.

Lead labeling

Different leads can be connected to the patient input. The MAC 500 (MicroSmart) is designed for
use with a 5 and 10 wire patient lead. Lead labeling is identified by means of different voltage
values. For this purpose the 10 wire lead holds a 402 Ω resistor which, together with the series
resistor, generates a voltage in the range of 8.66 mV - 9.19 mV. A voltage in the range of 4.76 mV

- 5.05 mV with a resistance of 221 Ω is generated through the 5-wire lead. The voltage is measured

with the ninth Σ∆ modulator of the ASIC SDM_HEC2. This means that the chip set must be
configured for the transmission of 9 channels.

Current supply

A DC-DC converter is used which generates two alternating secondary voltages from the primary
5V with 125 kHz cycle. Two stabilized direct voltages of +5V and -5V are then generated from
both of these alternating voltages. The 125 kHz cycle is delivered by the CPU. To suppress radiated
noise, a reactor is provided in the current supply.

Reference voltage editing

The reference voltage has values between + 2.5 V and - 2.5 V. Special emphasis is placed on low
intrinsic noise because it directly affects the results of the Σ∆ modulator. The low pass immediately
following the reference element with a cut-off frequency of 8 Hz serves the same purpose. A
compromise had to be found between low noise and rapid stability of the reference voltage
immediately af ter en abling .

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Servicing Instructions 227 470 35 D

Filter and interface module

This IC (DIG I_HEC2) essentially incorpora tes the filte r func tions and the ser ial inter fa ce .

Fundamentally, the transfer bandwidth is 0 - 250 Hz for a scanning frequency of 1 kHz and 500 Hz,
with the upper cut-off frequency determined by a sinc filter of the 3rd order. The lower cut-off
frequency can be set within the range of 0.039 - 79.6 Hz (4.08 - 0.002s) by selecting the time
constant. Selecting this time constant also causes the separation of the DC content, which may be
superimposed over the ECG signal as polarization voltage. An IIR filter algorithm is used. The
algorithm only captures the lower 12 bits of the 18 bit result. This means that any sudden changes
at the input are always represented as changes with amplitudes < 20 mV. Limiting the display range
to ± 10 mV and selecting a suitable value query prevents sudden changes over the entire display
range when exceeding the range limits. A saturation value is delivered at about 10 mV, until the
measuring signal returns to within the display range.

For the useful signal transfer (ECG signal) the lower 12 bits are transferred with the selected
scanning frequency. However, there is also the option of using the appropriate control words for
special function tests to transfer the upper 12 bits without DC separation. In this case 1 LSB
corresponds to 320 µV. Possible function checks include testing the signal path, measuring the
polarization voltage, measuring the electrode impedance and testing the serial data transfer, all by
activating these functions by using the appropriate control words.

Measurements of the polarization voltage are allowed by transferring the upper 12 bits of the
converter result (1 LSB = 320 µV).

During the serial data test, a test word transmitted by the CPU will be returned immediately
thereafter by the ASIC DIGI_HEC2.

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Servicing Instructions 227 470 35 D

3.3.4 Drive electronics and display

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Servicing Instructions 227 470 35 D

Thermal array control

As the data output to the printhead is relatively time-consuming, special hardware has been
provided which relieves the processor of this task.
To drive the printhead, the CPU data for a printline are written block by block and at high speed
into a FIFO. A start signal generated by the CPU informs the printhead control TPH_CONTROL
(seated in a CPLD) that output to the thermal printhead can begin. Several "state machines" within
the TPH_CONTROL read 80 bytes from the FIFO and transmit the serialized data to the printhead.
At the end of the transfer the CPLD generates the latch signal for the array and the trigger signal for
the heat duration generation.

The speed-related heat duration is software-selected. The heat duration value is gained via the
pulse-pause ratio of a TPU channel functioning as PWM channel. After the PWM signal has been
routed via a low pass, a DC voltage proportional to the PWM ratio which is used for setting the
heat duration. With each trigger pulse for the heat duration generation, a capacitor charged via a
constant current source is discharged and a heat duration cycle is started. The linear voltage
increase at the capacitor is compared in a com parator with the analog value supplied by the PWM
channel. If the analog value is exceeded, the heat duration pulse is terminated. In addition, the heat
duration is adjusted as a factor of the printhead substrate temperat ure. The temperature-dependent
voltage obtained via the array thermistor is added to the PWM voltage supplied by the TPU
channel in a summing amplifier.

The supply voltage of the thermal array can be switched off via the power switch if:

- array voltage < 19.2V

- reset active

- motor not running

- array overheated

Temperature monitoring

An array excess temperature monitoring device is fitted to protect the thermal array. Using a
comparator, the voltage of the thermistor is compared with a reference value. If the array
temperature of 60°C is exceeded, the comparator signals this to the processor.