Fluke Biomedical 945A - Manual (Page 59)

Digital to Analog Converter and Output Circuitry (Write Only)
The D/A converter (U32) is an 8 bit buffered multiplying device which responds to address 4004. Data is
written and latched by U82 when CS and WR are 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 1 buffers the converter output to drive the three analog output circuits. This
voltage is also provided to J4, analog 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). U81
operational amplifier 3 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 1 and 2, as well as U81 operational amplifier 2,
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 2 and Q1 are configured as
a constant current source controlled by the output of U80 operational amplifier 1. 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 3 and 4 as well as U81 operational amplifier 4
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.
Read Register Decoding
Decoding for READ registers within the main circuit board is performed by U34, which is a 1 of 8 decoder.
Control signals for U34 are REG SELECT, and R/W (active high) as well as address A1, A2, and A3.
U34 decodes address per output, starting with 4000 and ending with 400E. These outputs are active low.
Table 4-19 lists the READ register, the assigned function and the address.
Table 4-19. Control Signal Address Decoding (U34)
Address State Hex Address Function
A3 A2 A1
0 0 0
4000 Switch Interface
0 0 1 4002 Data Entry
0 1 0 4004 Gross Counter Low
0 1 1 4006 Gross Counter High
1 0 0 4008 Sensitivity Select
1 0 1 400A (Spare)
1 1 0 400C (Spare)
1 1 1 400E (Spare)
4-35