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Function Description
Readout Module876A-1
4
4-5
There is a yellow ALERT ALARM jumper connected to the collector (pin 1) of the alert alarm relay driver
stage of U503. The following options are available with the presence or absence of the jumper when the
radiation level exceeds the threshold and triggers the alarm:
Manual Reset - The alarm will continue to be activated even after the radiation level recedes below
threshold until the SAE/RESET indicator push button on the panel is depressed. This action occurs with
the jumper in place.
Automatic Reset - The alarm will continue to be activated only as long as the radiation exceeds the
threshold level. This action occurs with the jumper removed.
The yellow warning light acts similarly to the alarms. It lights when the alarm relay is de-energized.
Each relay contains four Form C contacts, but only two of them are accessible from the rear connector.
The connection to the yellow panel warning light is from terminal 2 of the printed circuit board, which is
connected to the collector (pin 14) of U503 through R526.
Fail/Safe Circuit
Input to the fail/safe circuit comes from pin 1 of U2 on the power supply board, entering the relay driver
board on pin H of the relay driver printed circuit board. The input is a DC voltage that is either high (15 V)
or low (0 V); if it is low, relay K3 is energized (non-alarm condition). If the signal is high, the green
SAFE/RESET lamp will go off and the relay is de-energized, indicating a fault somewhere in the system.
The circuit consists basically of two inverters in tandem so the voltage on the collector of Q502 is
approximately the same as the input voltage, the diode CR508 preventing current flow through the
solenoid if the collector is slightly higher than the 14 V at the other end of the solenoid coil.
Recorders and Computer Buffers
Provisions have been made for delivering DC output voltages to a recorder and computer for further
processing. The buffers are located on the relay driver board. If a commercial device is to be connected, a
signal isolator must be installed between the device and the 876A-1 Readout Module output. Inputs to
both computer and recorder buffers arrive at pin R of the relay driver printed circuit board, coming from
terminal 3 on the ECS printed circuit board. The computer buffer consists of and OP AMP using pins 1, 2
and 3 of U501, the output being taken off the relay driver board at pins 4 (+) and 2 (-) of P502. Resistors
R532 and R533 act as voltage dividers for the output. Their values for standard usage and available
options are shown in Table 4-1.
The recorder buffer consists of an OP AMP using pins 5, 6 and 7 of U501, the output being taken off the
relay driver board at pins 1 (+) and 3 (-) of P502. Resistors R530 and R531 act as voltage dividers for the
output. Their value for standard usage and available options are shown in Table 4-1. Both buffers are
disabled during the ECS test by a switching circuit on the ECS board, so that the ECS current is not
recorded. This circuit consists of Q201 and Q202 on the ECS board and is activated by a muting voltage
during the ECS test.
Table 4-1. Resistance Options for Voltage Dividers
Resistor Std. Option 1 Option 2 Option 3
Recorder
Voltage
0 – 1 V 0 – 10 mV 0 – 5 V 0 – 5V
R530 1 kilohm 119.8 kilohms 200 ohms 200 ohms
R531 200 ohms 200 ohms 1 kilohm 1 kilohm
Computer
Voltage
0 – 5 V 1 – 100 mV 0 – 50 mV 0 – 5 V
R532 200 ohms 11.8 kilohms 23.8 kilohms 200 ohms
R533 1 kilohm 200 ohms 200 ohms 1 kilohm
Pen Clamp