CCST question

A sinking PLC digital input is suitable for PNP field sensor devices and looks for ________ on the DI channel coming from the sensor’s switched lead to indicate a logical true condition.

A. +5 VDC
B. +24 VDC
C. -24V DC
D. 0 V
CCST Answer

The answer is B, “+24 VDC.” A sinking digital input on a PLC I/O card is looking for positive 24 VDC at the input terminal. This configuration is most commonly used because +24 VDC at the digital input terminal indicates an ON or TRUE condition.

Reference: Goettsche, L. D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

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