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Question 793 of 1,386

Concerning figure "A" of the illustration, if the illustrated PLC ladder diagram is for a simple motor controller, what does "Input A" represent? EL-0236

USCG illustration EL-0236 for Engine Electricity question 793
Illustration EL-0236
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CM CMArower05 · Mar 17, 2015
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fo fowen · Oct 10, 2018
This is incorrect. This is also a common error committed by those who do not understand ladder-logic programming for a PLC. In the context of PLC programming Input A is a normally open or normally closed pushbutton or switch--it's not specified which. What is shown next to "Input A" above is a PLC "make" instruction. If the thing referenced by "Input A" delivers a high signal to the PLC pin to which the Input A device is connected, then the make is "made", and logic flows on through on the rung from the left ladder rail onto to Input B. What is referenced by a make instruction can be a NO pushbutton or switch as well as a normally closed pushbutton or switch. That is, it is not necessary to connect a make only to a NO input device. Likewise, what is illustrated by Input B above is a "break" instruction. It operates exactly opposite the make, and it too can be connected to either a NO input device or a NC input device.