Fluke Biomedical 945A - Manual (Page 43)

FUNCTIONAL DESCRIPTION
Functional Description
Detector and Preamplifier
The wide range ion chamber detector block diagram is shown in figure 4-1. Detector electronics are
contained within two circuit board assemblies located in the preamplifier enclosure: the 977-210-10 High
Voltage/Communications Interface circuit board assembly and the 977-200-15 Electrometer circuit board
assembly. Schematic diagrams are located in Appendix B.
The 977-200-15 Electrometer circuit board contains the microprocessor, the EPROM, the integrating
electrometer, the programmable gain amplifier, the analog to digital (A/D) converter, and the auto zero
DAC circuit.
The 977-210-10 High Voltage/Communications Interface circuit board contains the dc voltage regulator,
the high voltage power supply, the BDC/DC converter, the asynchronous communication interface
adapter (ACIA), the Victoreen communication loop driver/receiver, the RS232 communications
driver/receiver, the local 0 - 10 Vdc analog output, the local 0 / - 12 Vdc alarm output, and the
communication loop fuses.
Refer to the electrometer schematic 977-200-18 located in Appendix B. Z7 is a Texas Instruments TMS
70C00 microprocessor, which is utilized to control detector operation. Z8 latches port C of the
microprocessor to form the lower order bits of memory address. Z10 is a 27C64 EPROM containing the
operational firmware for the detector. Z18 is an optional RAM socket which is not utilized for this model
detector. Z9 is a decoder for the upper order address bits. Z11 is a latch used to store control bits to
select the gain of the programmable gain amplifier formed by the combination of Z5, Z4, and RN2.
Available gains are 1 through 128 in binary increments 1, 2, 4, . . . 128. Z3 is an eight bit analog to digital
converter used to digitized the output of the programmable gain amplifier. Analog switch Z17 pin 11
selects either the integrating electrometer or the high voltage sense line as the input to the programmable
gain amplifier. Refer to Table 4-9 for preamplifier register addresses and Table 4-10 for electrometer
control bit assignments.
Table 4-9. Preamplifier Register Addresses
Function Hex Address
Electrometer Control 2000
ADC Convert CMD 4000
ADC Read Data 6000
ACIA T/R Data A000
ACIA Status Register A001
ACIA Command Register A002
ACIA Control Register A003
Input SW3 A008
RAM (not used) C000-DFFF
EPROM E000-FFFF
4-19