a computer network to insert external plc IO modules.

 The main difference from other computers is that harsh conditions (such as dust, moisture, heat, cold), and a facility for extensive plc IO module arrangements PLC's armor. These PLC connected to sensors and actuators. PLC read limit switches, analog process variables (such as temperature and pressure), and a complex positioning system location. Some use machine vision. In the side of the actuator, PLC operation of the electric motor, pneumatic or hydraulic cylinders, magnetic relays, solenoids, or analog outputs. Input / output configuration can be built a simple programmable logic controller, or the PLC can be attached to a computer network to insert external PLC I / O modules.
 plc digital input output io module or discrete signal performance of a binary switch, resulting in a simple on or off signal (1 or 0, True or False, respectively). Buttons, limit switches, photoelectric sensors, and a discrete signal device provides an example. Discrete signal by a voltage or current in a specific range to another are assigned to the sending off. For example, a PLC can use 24 volt DC I / O, with values ​​above 22 volts DC on behalf of the following values ​​represent 2VDC off, the middle value is undefined.