Many real-world signals are either on or off, open or closed, true or not-true. The most common type of PLC control system input is discrete (digital). To a discrete input module, discrete devices are essentially switches that are either on or off.  However, to convert the real-world situation (gate open, switch on) into a binary digit that can be understood by the PLC – several things need to occur.  The collection of steps and devices that accomplish this is the PLC’s discrete input system.

The input device must first convert the real world situation into an electrical signal (e.g. switch is open or closed).  The electrical signal that the device sends to the discrete input module can be of different magnitudes and types. However, most manufacturers provide a finite list of standard electrical types and ratings of input modules.

Common Discrete Input Types

  • 24 volts AC/DC
  • 48 volts AC/DC
  • 120 volts AC/DC
  • 240 volts AC/DC
  • TTL
  • 5 to 50 volts DC

The discrete input module usually provides some isolation and debouncing of the signal, converts the real world electrical signal to a level that can be used by the plc, adn writes the current value (1/0) to the proper memory location during each input scan.