Early PLCs were limited to digital, or discrete, I/O interfaces.

As we discussed earlier, these types of interfaces only allow on/off devices (0s and 1s) to be connected.

limited the PLC and meant that the PLC only had partial control over the operations and applications.

Now, there is a large range of discrete AND analog interfaces available. This allows controllers to be applied to almost any time of process.

Analog devices represent physical quantities that could have an infinite number of values. Most analog inputs and outputs vary from 0-20 milliamps, 4-20 milliamps or 0-10 volts.

Analog sensors measure a physical quantity over a specific range. They then generate a corresponding voltage or current signal.

Analog input modules usually have many input channels that allow multiple devices to interface with the PLC.

The two main types of analog input modules are voltage and current sensing.

Common quantities measured by a PLC analog module are:

• temperature
• speed
• level
• flow
• weight
• pressure
• position

Read Next:

• Terminal Assembly
• PLC Status LED
• DC Discrete Input Modules
• AC Discrete Output Modules