A little more about Inputs and Outputs

All of the field devices connected to a PLC can be classified in one of two categories:
• inputs
• outputs

Inputs are devices that supply a signal/data to a PLC. Typical examples of inputs are push buttons, switches, and measurement devices.
Basically, an input device tells the PLC, “Hey, something’s happening out here…you need to check this out to see how it affects the control program.”

Outputs are devices that await a signal/data from the PLC to perform their control functions. Lights, horns, motors, and valves are all good examples of output devices. These devices stay put, minding their own business, until the PLC says, “You need to turn on now” or “You’d better open up your valve a little more,” etc.


An overhead light fixture and its corresponding wall switch are good examples of everyday inputs and outputs. The wall switch is an input-it provides a signal for the light to turn on. The overhead light is an output -it waits until the switch sends a signal before it turns on.
Let’s pretend that you have a souped-up overhead light/switch circuit that contains a PLC. In this situation, both the switch and the light will be wired to the PLC instead of to each other. Thus, when you turn on the switch, the switch will send its “turn on” signal to the PLC instead of
to the light. The PLC will then relay this signal to the light, which will then turn on.

There are two basic types of input and output devices:
• discrete
• analog

Discrete devices are inputs and outputs that have only two states: on and off. As a result, they send/receive simple signals to/from a PLC. These signals consist of only 1s and 0s. A 1 means that the device is on and a 0 means that the device is off.

Analog devices are inputs and outputs that can have an infinite number of states. These devices can not only be on and off, but they can also be barely on, almost totally on, not quite off, etc. These devices send/receive complex signals to/from a PLC. Their communications consist of a variety of signals, not just 1s and 0s.


The overhead light and switch we just discussed are both examples of discrete devices. The switch can only be either totally on or totally off at any given time. The same is true for the light.
A thermometer and a control valve are examples of the other type of I/O devices-analog. A thermometer is an analog input device because it provides data that can have an infinite number of states. Temperature isn’t just hot or cold. It can have a variety of states, including warm, cool, moderate, etc. A control valve is an analog output for the same reason. It can be totally on or totally off, but it can also have an infinite number of settings between these two states.

Because different input and output devices send different kinds of signals, they sometimes have a hard time communicating with the PLC. While PLCs are powerful devices, they can’t always speak the “language” of every device connected to them. That’s where the I/O modules we talked about earlier come in. The modules act as “translators” between the field devices and the PLC. They ensure that the PLC and the field devices all get the information they need in a language that they can understand.

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