What instruction would you select for each of the following discrete input field

Question

What instruction would you select for each of the following discrete input field devices to accomplish the desired task. (NOTE: they mean PLC instruction (examine if closed or examine if open should be answer for each of these.)

A. turn on a light when a conveyor motor is running in reverse. the input field device is a set of contacts on the conveyor start relay that close when the motor is running in reverse

B. when a pushbutton is pressed, it operates a solenoid. the input field device is a normally open pushbutton.

C. stop a motor from running when a pushbutton is pressed. the input field device is a normally closed button.

D. when a limit switch is closed, it triggers an instruction ON. the input field device is a limit switch that stores a 1 in a data table bit when closed.

Explanation / Answer

a)Turn on a light when a conveyor motor is running in reverse. The input field device is a set of contacts on the conveyor start relay that close when the motor is running forward and open when it is running in reverse."
If "Run_In_Forward" is OFF then turn ON "Reverse_Light"
The "OFF" implies XIO.

B), if I want to "see" the same o/p as the i/p I should use "XIC" and if I want the opposite then "XIO"

XIO Examine If Open will return a "0" or FALSE for a closed switch because XIO always produces the "Opposite" of the bit it is looking at. So in this case, the XIO would be looking at a "1" and will produce the Opposite or "0", which would mean an Open Circuit with the STOP NOT PRESSED. You want a "1", so use a XIC, which always produces the SAME as the Input being examined. "What you C (see at the input) is what you get with an XIC"!

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