Different types of PLC I/O circuit


The input/output (I/O) section of a PLC is the section to which all fi eld devices are connected and provides the interface between them and the CPU. Input/output arrangements are built into a fixed PLC while modular types use external I/O modules that plug into the PLC.


Pilot Duty Outputs

Outputs of this type typically are used to drive high-current electromagnetic loads such as solenoids, relays, valves, and motor starters. These loads are highly inductive and exhibit a large inrush current.

Pilot duty outputs should be capable of withstanding an inrush current of 10 times the rated load for a short period of time without failure.

General - Purpose Outputs

These are usually low- voltage and low-current and are used to drive indicating lights and other non-inductive loads. Noise suppression may or may not be included on this types of modules.

Discrete Inputs

Circuits of this type are used to sense the status of limit switches, push buttons, and other discrete sensors. Noise suppression is of great importance in preventing false indication of inputs turning on or off because of noise.

Likewise, output control is limited to devices such as lights, relays, solenoids, and motor starters that require simple ON/OFF switching.

Analog I/O

Circuits of this type sense or drive analog signals.

Analog inputs come from devices, such as thermocouples, strain gages, or pressure sensors, that provide a signal voltage or current that is derived from the process variable. Standard Analog Input signals: 4-20mA; 0-10V.

Analog outputs can be used to drive devices such as voltmeters, X-Y recorders, servo motor drives, and valves through the use of transducers. Standard Analog Output signals: 4-20mA; 0-5V; 0-10V.

The analog input interface module contains the circuitry necessary to accept an analog voltage or current signal from the level transmitter field device. This input is converted from an analog to a digital value for use by the processor.

Special - Purpose I/O

Circuits of this type are used to interface PLCs to very specific types of circuits such as servomotors, stepping motors PID (proportional plus integral plus derivative) loops, high-speed pulse counting, resolver and decoder inputs, multiplexed displays, and keyboards.

This module allows for limited access to timer and counter presets and other PLC variables without requiring a program loader.