Fluke Biomedical 942A-200L-M4 - Manual (Page 31)

Theory of Operation
Main Circuit Board
2
2-17
Switch Inputs
Octal inverting buffer U62 functions as an interface to provide the status of the front panel control
switches to the internal data bus and MPU. U62 responds to address 4000. The switch input bit
assignments are listed in Table 2-16.
Table 2-16. Switch Input Bit Assignments (U62)
Data Bit Function
D0 Alarm Acknowledge (Latched)
D1 Check Source (Latched)
D2 (Spare)
D3 (Spare)
D4 Rate-of-Rise/ Trip Test
D5 Warn Alarm
D6 High Alarm
D7 High Voltage
When a switch is pressed, the appropriate input to U62 is pulled low. When U62 is read by the MPU, a
high (1) is available on the data bus. When no switches have been pressed, all outputs (U62) will be low
when read. U63 is a latch which latches switch data from the check source and alarm acknowledge
switches. The MPU controls the /CLEAR SWITCH LATCH signal to reset U63. The circuit comprised by
S1, D1, and R11 is a remote alarm acknowledge. R11 is selected to allow a 20 mA signal to flow through
the S1 infrared diode when a given voltage is present on the J9 remote acknowledge input. When this
voltage is present, the S1 infrared diode is forward biased, causing the S1 phototransistor to conduct.
This effectively forces a low (0) to U63-10, setting the alarm acknowledge bit.
Option Board Bus
The option board bus is available on connector J3. All address, data, and control signals are provided to
allow various digital/analog circuit boards to directly interface to the main circuit board. Decoded signals
for the single channel analyzer option (two channels), asynchronous communications interlace adapter
option, and the general-purpose interface bus option are available on the option board interface
connector.
Signal lnput Circuitry
Detector input pulse circuitry consists of an input buffer, high/low discriminators, signal detection, anti-jam
and signal multiplexer circuits. The following paragraphs describe circuit operation in detail.