I/O stands for Input/Output., and is an electrical interface between a PLC and field instrumentation.

There are several different types of physical I/O:

• Analog Input 

  • 4-20mA
  • 0-10V
  • Pulse Width Modulation

• Analog Output

  • Same subtypes as Analog Input

• Discrete Input

  • Dry Contact
  • DC Input

• Discrete Output

  • Relay
  • DC Output

Network communications can also be considered I/O. Network I/O can be serial or communications or Ethernet.** (See another section for industrial networking.)**