Yes, the purpose is to allow using fewer PLC Inputs or Outputs to do a the same job. It is usually the Inputs that are the problem, as most PLC programs need about 2 or 3 times more inputs than outputs.I would then turn the plc outputs on accordingly to input the desired signal to the multiplexer which in turn, turn low-voltage output signals that will serve as inputs to electromagnetic relays. Is my undestanding correct?
As for multiplexing analog I/O, I have only multiplexed analog inputs. The way I did that was to use 9 PLC digital relay outputs to consecutively switch one PLC analog input terminal (only 2 total available) to 9 different sets of physical analog input wires. So in my case, no "analog multiplexer" was required, only digital outputs.
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