In this article we will discuss about PLC circuit diagram of input and output module of PLC. PLC input cards rarely supply power, it means that needs external power supply for the inputs and sensors. Below is an Input card and Ladder Logic diagram that shows how to connect an AC input card.
In the diagram above there are two inputs, one is a normally open push button, and the second is temperature switch or thermal relay.
The PLC inputs must convert a variety of logic levels to the 5VDC logic level used on the data bus. This can be done with circuits similar as the picture below. Basically the circuits condition the input to drive an optocoupler. This electrically isolates the external electrical circuitry from the internal circuitry. Other circuit components are used to guard against excess or reversed voltage polarity.
As with the input modules, output modules rarely supply any power, but instead act as switch. External power supplies are connected to the output card and it will switch the power on or off for each output.
PLC outputs must convert the 5VDC logic level on the PLC data bus to external voltage levels. The circuits will show on the picture below. Basically the circuits use an optocoupler to switch external circuitry. This electrically isolates the external electrical circuitry from the internal circuitry. Other circuit components are used to guard against excess or reversed voltage polarity.