Common PLC input/output (I/O) can be analog and/or digital.
An analog signal is one whose value continually varies over time. They are best represented as a continuous signal like a sine wave. Each point on the wave has a precisely defined value. For example, analog inputs for PLCs can include signals from pressure transducers and temperature from thermocouples. Common examples of standard analog I/O are 4 to 20 mA for current with voltages ranging from 0 to 120 Vac or 0 to 48 Vdc. By contrast, a digital (or discrete) signal is one with a value at a specified interval of time. Think of an “on/off” status of a switch or relay, a motor or an indicator light.
For small, limited tasks, only one type of I/O may be required (analog or digital only, for instance.) However, if the task or machine is more complex and has both analog and digital I/O, then the PLC should be able to handle both types.
You should also consider the ability to expand I/O as needed. So designing a PLC system that is expandable means that as the needs change I/O modules can be added to the PLC to accommodate changing I/O needs.
In the end, the key point about PLC I/O is to know the application requirements. This will determine whether analog or digital or both types of I/O are needed and how many I/O points are required.