Fluke Biomedical 956A-201-M2 - Manual (Page 62)

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Digital to Analog Converter and Cutout Circuitry (Write Only)
The D/A converter (U82) is an 8-bit buffered multiplying device that responds to address 4004. Data is
written and latched by U82 when /DAC, which is tied to /CS (Clear to Send) and /WR (Write), is active
(low). The converter is configured for unipolar operation with a voltage reference of + 10 Vdc. Below is the
digital to analog conversion table with the voltage measured at U82-1 referenced to ground.
MSB LSB ANALOG OUTPUT
1111 1111 -10 Vdc
1000 0000 -5 Vdc
0000 0001 -0.039 V
0000 0000 0 V
U81 operational amplifier A buffers the converter output to drive the three analog output circuits. This
voltage is also provided to J4, analog output option connector. J4 is provided with the necessary supply
voltages to configure a custom analog output range for special applications (i.e. 10 mV to 50 mV) using
the P/N 942-200-95 Analog Output Option circuit board. U81 operational amplifier C provides the user
with a 0 - 10 volt analog output. VR1 is the zero adjustment and VR2 is the gain adjustment for the 0 - 10
volts output.
The circuitry comprised of U80 operational amplifiers A and B, as well as U81 operational amplifier B,
provides a 4 - 20 mA user output. VR7 is adjusted to obtain a 4 mA reading on the output, and VR4 is
adjusted to obtain a 20 mA reading on the output. U81 operational amplifier B and Q1 are configured as a
constant current source controlled by the output of U80 operational amplifier A. The positive feedback
circuitry, comprised of R41 and R43, ensures that the output current will remain constant regardless of the
output load impedance. The maximum load impedance is 500 ohms.
The circuitry comprised of U80 operational amplifiers C and D as well as U81 operational amplifier D
provides a second 4 - 20 mA user output. Operation is identical to the circuitry previously described in this
section. VR6 is adjusted to obtain a 4 mA reading at the output and VR5 is adjusted to obtain a 20 mA
reading at the output.
Gross Counter (Read Only)
The gross counter is a 16-bit configuration. Octal buffer/drivers U40 (4004) and U41 (4006) interface the
low and high counter bytes to the data bus from U45 and U46 respectively. Dual module 16 counters
(U45, U46) accumulate counts from the signal processing circuitry over a program controlled sample
period and make this data available to the MPU. A high level on U45 pins 2 and 12, and U46 pins 2 and
12 cause the counters to clear in anticipation of a sample being initiated. A low level activates the
counters to accept pulses from the signal processing circuitry. Maximum count for a sample period is
32,768. When this count is achieved, U46 pin 8 goes high which causes the pulse counting to stop. This
condition is recognized by the MPU as an indication of counter overflow.
Option Jumper Select (Read Only)
Octal buffer U42 functions as an interface to provide the status of the option jumpers for operating mode,
anti-jam bit data, and serial data from E
2 PROM to the internal data bus and MPU. U42 responds to
address 4008 and is controlled by /SENSITIVITY SELECT. The option jumper read functions are listed in
table 3-17.