Mechatronics, Industrial Control & Instrumentation

PLC Input & Output Modules Circuits

The PLC Input/output modules are control ports either built into the PLC unit or, more commonly, are packaged as separate plug-in modules where each module contains a set of ports. These I/O modules act as interfaces to the outside world. I/O Modules can be broadly classified into two groups: Those that utilize discrete I/O levels and those with analog signal levels.

Discrete I/O Modules

This is the most common type and deals with ON-OFF devices. Discrete input modules connect the real-world switches to the PLC and are available for either AC or DC voltages (typically 240 V ac, 120 V ac, 24 V dc and 5 V dc). As illustrated in Figure 1.0 below, circuitry within the module converts the switched voltage into a logic voltage for the processor. Note that an AC switch voltage must first be converted into a DC voltage with a rectifier as shown in Figure 1.0. Furthermore, an input module will typically include an opto-isolator to prevent possible high-voltage spikes from entering the PLC. A typical discrete input module would have 4, 8 or 16 inputs.

Figure 1.0: PLC Discrete Input Module – AC input module circuit

Figure 1.1: Discrete input module – DC input module circuit

Discrete output modules provide ON-OFF signals to drive relays, lamps, small motors, motor starters and other devices. Several types of output ports are available: Triac outputs control AC devices, transistor switches control DC devices, and relays control AC or DC devices, and they too provide isolation. A typical discrete output module would have 4, 8, 16 outputs.

Figure 1.2: PLC output module circuits – Triac, Transistor and Relay control devices.

Analog I/O Modules

This type of modules allows the PLC to handle analog signals. An analog input module like the one shown in Figure 1.4 has one or more ADCs i.e. analog-to-digital converters, allowing analog sensors, such as temperature to be connected directly to the PLC. Depending on the module, the analog voltage or current is converted into an 8, 12, 16 bit digital word.

An analog output module contains one or more digital-to-analog converters (DACs), allowing the PLC to provide an analog output, for instance, to drive a DC motor at various voltage levels.

There are specialized modules that perform particular functions that are available for many PLCs. Some of them include:

Motion control module – runs independently to control multi-axis motion in a device such as a robot.
PID module – an independent running PID self-contained controller (PID control can also be implemented with software).
Thermocouple module – interfaces a thermocouple to the PLC.
Communication module – connects the PLC to a network.
High-speed counter module – counts the number of input pulses for a fixed period of time.

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John Mulindi

John Mulindi is an Industrial Instrumentation and Control Professional with a wide range of experience in electrical and electronics, process measurement, control systems and automation. In free time he spends time reading, taking adventure walks and watching football.