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Theory of Operation
Functional Description
2
2-1
Section 2
Theory of Operation
2.1 Functional Description
The Model 943-227-15 Digital Preamplifier is an eight-decade, linear reading radiation. The preamplifier
is sensitive to moisture and is mounted in a NEMA 4 sealed enclosure. The preamplifier box contains a
desiccant package to remove moisture from the interior of the preamplifier. The desiccant has a color
indicator to show when it should be replaced when the color changes to blue.
The detector electrometer converts the current from the detector to a voltage which is measured by the
analog to digital converter of the preamplifier. Under program control, this measurement is made every
50 milliseconds on the first five decades and every 100 milliseconds on the upper three decades.
The electrometer is auto-ranging and auto-zeroing. An internal coarse zero control (factory adjusted) is
provided. The microprocessor in the preamplifier performs data collection, integration and multiplication
by a stored calibration factor, range changing and communication with the Model 942A-200C Digital
Ratemeter or Model 960 Digital Process Radiation monitoring system.
2.2 Main Circuit Boards
The wide range ion chamber detector block diagram is shown in figure 2-1. 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 977-200-15M 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 977-210-10M Communications Interface circuit board contains the dc voltage regulator, the BDC/DC
converter, the asynchronous communication interface adapter (ACIA), the Victoreen communication loop
driver/receiver, the RS232 communications driver/receiver, and the communication loop fuses.
2.3 977-200-15M Electrometer
Refer to the electrometer schematic 977-200-18M 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 digitize 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 is not active in this application. Refer to Table 2-1 for preamplifier register addresses and
Table 2-2 for electrometer control bit assignments.