GE Corometrics 126, Service Manual

Page: 97 / 440

Revision B 120 Series Maternal/Fetal Monitor 4-39
2015590-001
Theory of Operation: Communications Board
Communications Board
NOTE:
The 15297 surface-mount board, found in newer builds, replaces earlier
13388 versions. Functionally, the boards are the same.
The optional Communications Board performs three major functions:
‹
Central Systems Interface (J102)
‹ Model 2116B Keyboard Interface (J103)
‹
340 Series Telemetry Interface (J101)
Central Systems Interface
This section consists of three digital-to-analog convertors (U1, U2, U5) and
associated amplifiers (U3, U4, U7) to create the proper levels for modes, heart rate
data, and uterine activity data; a reference generator (U8) which creates ±10 V;
audio buffers (U9) for ultrasound channels one and two; and an output latch (U11)
which provides the FMD1, FMD2, CHK PAPER*, and MARKOUT* digital
signals. The mode signal levels are created by dual DAC U1, and one section of
DAC U2. The output of each DAC is converted to a voltage by U3 (pin 1), U3 (pin
16), and U4 (pin 1) op-amps. The voltage range at that point is 0 to +10 V for a
programmed value of 0 to FF respectively. The next stage (output) of op-amps U3
(pin 7), U3 (pin 10), and U4 (pin 7), level shift and invert the signal, providing a +10
V signal for a programmed value of zero, and –10 V signal for a programmed value
of FF. The final output is filtered and protected by series resistors. The HR1 and
HR2 outputs use DAC U5. The output of the first stage, U7 (pin 1) and U7 (pin 16),
range from 0 V to –10 V with a programmed value of zero to FF respectively. From
this point, the signal is inverted with a nominal gain of 0.24 and level shifted by –1.2
V thus biasing the signal to ±1.2 V. The level shifting pots (R18, R27), and gain
adjustment pots (R20, R29), are used to ensure the proper accuracy of ±1 bpm. The
level shifting for Hewlett Packard (HP) Central Systems is accomplished by opening
the +10 reference to the offset pots R18 and R27. This takes out the +1.2 V offset
thus creating a 0 to 2.4 V signal. The final output is filtered and resistor protected.
The UA circuit operates similar to the HR circuit in Corometrics mode. For HP
levels, the offset is again removed from the reference circuit using the same digital
switch (S1) as for HR1 and HR2. Gain for HP levels must be changed to
accomplish a 0 to +10 V range. To do this, two switches were added (S2, S3) in the
feedback of U4 (pins 10,11). Switch S2 selects the gain for Corometrics levels and
switch S3 selects the gain for HP levels.