How does a safety PLC work

hardwareplc

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I have read the manual, but I am not sure how the omron safety PLC works.

I guess this safety PLC use Output Power Supply Output (21,22) to drive the output circuit, but why does it need (1,2) the power supply for input device and test output.

Also what is test output? Why is it useful?

Best Answer

Most likely the outputs are galvanically isolated from the inputs, necessitating multiple power supplies.

Below that image you posted is a reference to the DeviceNet Safety DST1-series Safety I/O Terminals Operation Manual. An explanation for the test outputs is provided there:

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The test outputs are most likely not galvanically isolated from the inputs since they share a common power supply. This could limit their usefulness.

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