120
Andover Controls
Normally Closed Supervisory Inputs
NC Parallel
With this type of input wiring, a resistor is placed in parallel (across) with the input. When
the door switch is closed (normal position) the door is assumed to be closed. The circuit
presents a reading of zero ohms resistance at the input.
This circuit is not recommended!
Circuit Action Input Senses Condition
Switch Closed Zero Ohms Door Closed
Switch Opened External Resistance Door Open
Wire Cut Infinite Resistance Violation or error
Input Shorted Zero Ohms Door Closed
When the door is opened the switch opens and the external resistance is measured. If
the wires to the door are cut it appears as an infinite resistance resulting in a door
violation or error condition.
Shorting the input causes it to look like a normal “door closed” condition. In this case the
door could be forced open and the system would not detect the change in condition.
AUX
or
DOOR
or
REX
RET
Contact
Closure
External
Resistor
10 K
Ω
typical
AC-1
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