Continuum I/O System Reference
121
Normally Closed Supervisory Inputs
NC Series Parallel
With this type of input wiring, two external resistors are added to the input, one in series
and the other in parallel. When the door switch is closed (normal position) the door is
assumed to be closed. The circuit presents a reading composed of both resistances in
parallel that works out to be half the value of one of them (5K).
Circuit Action Input Senses Condition
Switch Closed External Resistance
÷
2 Door Closed
Switch Opened External Parallel Resistance Door Open
Wire Cut Infinite Resistance Violation or error
Input Shorted Zero Ohms Violation or error
When the door is opened the switch opens and the value of the parallel resistor is
measured. If the wires to the door are cut it appears as an infinite resistance. Shorting the
input causes a zero ohm resistance. Both a short and an infinite resistance (open) should
result in an error condition.
AUX
or
DOOR
or
REX
RET
Contact
Closure
External
Resistors
10 K
Ω
typical
AC-1
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