Programmable Logic Controller (PLC) Questions and Answers – 11

Programmable Logic Controller (PLC) Questions 

Suppose we have a Siemens S7-200 PLC connected to a pair of pushbutton switches and light bulbs as shown in this illustration:

Siemens PLC Questions

Examine the following relay ladder logic (RLL) program for this Siemens PLC, determining the statuses of the two lamps provided neither switch is pressed by a human operator:

Siemens S7-200 PLC Ladder Logic for Switch - 2

Furthermore, determine the necessary switch actuation statuses (i.e. pressed versus unpressed) to turn lamp Z on.

Answer :

Output Q0.1 will activate to energize lamp Y, but the other output (and lamp) will remain off.

To energize lamp Z, you must release (unpress) both switches.

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Credits :  Tony R. Kuphaldt – under CC BY 1.0

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2 thoughts on “Programmable Logic Controller (PLC) Questions and Answers – 11”

  1. I think that there is a mistake in your answer:

    If neither switches are pressed, it means both of theme are conducting (they are normally closed switches) it means I.11 and I.07 are both at state “1”, but in the program in the first rung, the contacts are negated it means that Q0.1 will be de-energized = lamp Y is “off”.


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