Input & Output Modules of a PLC – Types & Applications

The input/output modules act as the signal interface between the monitoring sensors and actuators, and the controller. Additional they provide electrical isolation, if needed, to convert the input signals into an electronic format appropriate for evaluation by the controller; the I/O modules provide the memory storage and format the output signals for displays and control functions. From these functional capabilities of the input/output modules, it is clear that they form an important component of a PLC.

Modules can be classified into three categories:

1. Those used with discrete I/O levels.
2. Those used with analog signal levels.
3. Those that have intelligence to evaluate and modify the input signals before they can be used by the controller/also referred to as smart modules.

Modules for example, can be configured for local signals say upto 500 ft and some can be configured for remote signals from 500 ft to 10, 000 ft. Typically, the I/O modules have 16 inputs or outputs, but can be as high as 32, or as low as 4. We also have modules that have both the input and output ports.

Discrete Input Modules

Discrete input modules function as ON/OFF signal receivers for the processor. The basic function of the input module is to establish the presence or absence of a signal. The inputs from peripheral devices to the input modules can be ac or dc signals. The voltage ratings for input modules can vary from 24 V or 240 V, ac or dc, as well as 5 V and 12 V TTL levels.

The different types of applications that can be used with the discrete input modules are shown in the table below:

Table 1.0 Discrete Input Modules’ Applications

Type of Input	Application
Discrete input	Push buttons, switch, relay contacts, starter contacts, proximity switch, photo-electric device, float switch.
TTL input	CMOS logic or TTL level.
DC or AC inputs	General purpose high, medium, or low-level inputs.
Discrete parallel inputs	Thumbwheel switches, weigh scales, bar code readers, position encoders, analog-to-digital converter (ADC), BCD/parallel data devices.

A discrete module normally will have 16 inputs, which can be segmented into groups of 4, 8, or 16. An illustration of how wiring can be implemented in the various modules is shown in the figure below:

In Figure 1 (a), switches are connected in blocks of 4 with ac or dc power supplies. An open switch gives a ‘0’ level input and a closed switch gives a ‘1’ level input. In Figure 1(b), the inputs are transistor logic levels, and the logic output transistor with load is illustrated. If the transistor is ON, the input is ‘0’ level and if OFF, the input is a ‘1’ level.

The input stage of a dc or ac module is used to detect presence or absence of a voltage and to convert the input voltage to a logic 5 V level. The figure below shows a block diagram of a discrete input module with ac or dc input:

The front ends of both the dc and ac modules are shown. With a high dc input voltage, the voltage is stepped down to a low voltage, which then goes through a de-bounce circuit with a noise filter and threshold detector for ‘1’ or ‘0’ detection, followed by optical isolation, so that the signal can referenced to the signal ground of the processor. The ac module input utilizes a bridge rectifier to convert the ac to dc, and then uses the same circuit blocks as in the dc module. The LED is used to indicate the input logic level of the input signal. The input level LED indicators are usually located above the tag strip.

Analog Input Modules

Normally analog input modules are used to convert analog signals to digital values or words. Analog signals are derived from pressure, temperature, flow, position, or rate measurements. Analog signals can either be single ended or differential. An illustration of a differential analog receiver is shown below:

The analog input signal is converted into a digital word. Since the inputs are floating, the output of the ADC employs an optoisolator to reference the digital signal to the ground level of the controller.

Analog input applications include: pressure, temperature, flow rate, humidity, thermocouple, RTD, magnetic, acceleration sensors, and so forth. The typical input voltage levels are from 50 mV to 500 mV, up to ±10 V and current ranges from 20 to 50 mA.

Special Functions Input Modules

Various types of discrete modules are available to process or interface special signals. Special functions are given in the table below:

Table 1.1 Special Function Input/output Modules

Special Function Input Module Type	Application
Latching input	Detection of short duration signals.
Interrupt input	Immediate response to signal changes.
Voltage comparator input	Analog set-point comparison.
Fast input	Fast response to dc level changes.
Rapid response I/O	Offers fast input/output response.
Relay contact output	High current switching and signal multiplexing.
Wire fault input	Wire break and short circuit detection.

To detect fast transients of a few microseconds that would typically be missed by standard input modules, the latching input module is used to detect transients and set a latch.

The interrupt function module is used to interrupt the processor’s scan sequence, in order to perform a task that requires immediate attention.

The voltage comparator module is used to compare the amplitude of the input to an internally generated voltage or an externally derived voltage.

The fast input module performs a similar function to the latching module, but does not latch the transient. It only holds the information for a scan cycle, so that it can be detected and recorded.

The rapid response module is similar to the latching input module, but can immediately enable an output without having to wait for a scan cycle.

The relay output module has isolated relay contacts to handle high currents and to multiplex signals.

The fault input module is used to interface wire fault detection circuits to the processor.

Discrete Output Modules

Discrete output modules are used to interface output information from the controller to the peripheral units, to provide electrical isolation, and to provide the data in an appropriate format for utilization by the external units.

The output from the modules can be discrete ac or dc outputs or relay contacts. The output voltage can be from 12 V to 230 V, ac or dc, and TTL levels with multiple or isolated contacts.

A list of discrete output applications is given in the table below:

Table 1.2 Discrete Output Applications

Type of Output	Application
Discrete outputs	Solenoids, alarms, motor starters, horns, buzzers, fans.
TTL output levels	TTL and CMOS logic devices.
AC, DC outputs	General purpose high, medium, or low load, ac or dc.
Parallel outputs	Seven segment displays, BCD controlled message displays, DAC, BCD/parallel data input devices.

The block diagram below, illustrates a solid state discrete output drivers using triacs. Only two drivers are shown, but usually the drivers would be in groups of four or eight drivers in a module. The outputs have filtering and surge suppression to protect the drivers against transients and inductive spikes and are fused for protection against overloads. The LED is located above the tag strip and is used to indicate the logic state of the output.

Analog Output Modules

Analog outputs from the PLC are used to drive analog meters, proportional valves, chart recorders, and variable speed drive controllers. They are also used for current and voltage for pneumatic transducers. The voltage and current ranges are the same as the input ranges.

The digital output from the controller is fed via an optical isolator to a digital-to-analog converter (DAC) to reference the signal to the ground of the peripheral device. The analog output of the converter is amplified and fed to a voltage or current driver, which can have single-ended output or a differential output. The output signal also will meet the standard voltage or current control ranges.

Recommended Resource: Beginner’s guide to PLC programming – A Transition from Relay systems to PLCs

Intelligent/Smart Input/output Modules

These are specialized modules developed to interface to the controller. They typically comprise of their own processor and memory, and can be programmed to perform operation independent of the central processor.

Generally, intelligent/smart modules can be classified as shown in the table below:

Table 1.3 Intelligent Input/output module categories

Intelligent Category	Intelligent I/O Module Type
Closed loop	PID control module. Temperature control module.
Position and motion	High-speed counter module. Encoder input module. Stepper positioning module. Servo positioning module.
Process specific modules	Press controller module. Injection molding module.
Artificial intelligence (AI) module	Voice output module. Vision input module.
Serial and network communications	ASCII communication module. Serial communication module. Loop Controller interface module. Proprietary LAN network module. MAP network module.
Computer coprocessor	PC/AT computer module. Basic language module.

Smart modules performing closed loop control algorithms are needed for PID functions, such as maintaining pressure, temperature, flow and level at set values. This introduction of smart sensors, reduces the load on the processor and communication to the processor can be done via Fieldbus. The PID module can employ either analog or digital techniques to implement the control function.

The temperature control module usually controls 8 to 16 temperature zones. The module is configured for two-position control (heat on/heat off), or three-position control (heat on/heat off/cool). The set points are stored in the processor. This can be applied in a large building HAVC, or controlling the zone temperatures in injection molding machines.

The position and motion modules enable PLCs to control stepper and servo motors in feedback loops, to measure and control rotation speeds and acceleration, and to control precision tools. These modules employ high-speed counting, rotational and linear position decoders and open and closed loop control techniques, in order to measure axis rotation and linear speed and position.

The examples of industrial applications of position and motion modules are indicated in the table below:

Table 1.4 Applications of Position and Motion modules

Module	Applications
High-speed counter module	Up/down counting. Generate interrupt for set count. Generate gating. Generate delays.
Encoder input module	Absolute position tracker. Incremental position tracker.
Servo-positioning module	Transfer and assembly lines. Material handling. Machine tool setting. Table positioning Precision parts placement. Automatic component insertion.
Stepper positioning module	Open loop position. Setting dwell times. Define motion speed. Motion acceleration.

Process specific modules are intelligent modules designed to perform certain control functions or a specific series of operations. The operations they perform are usually repetitive, requiring precise measurements and complex numerical algorithms. Examples include: profiling and controlling plastic molding.

Artificial intelligence modules are used for example in voice recognition, synthesized speech and visual inspection. The sound module can give alarm announcements, voice recognition and echo evaluation when using sound waves for flaw detection. The video module can provide dimension gauging, visual inspection, flaw and defect detection, position analysis and product sorting.

Serial and network modules are used for data communication. The serial modules communicate between other PLCs, message displays, operator terminals and intelligent devices. The network modules are used for LAN and WAN. The MAP protocol is used for communication to robotic devices and various computers of different manufacturers they can support.

The coprocessor modules provide functions such as math functions, algorithms, data manipulation, outputs of reports, outputs to printer, displays and mass data storage using basic programming language functions that would be otherwise impossible to perform with ladder logic.

Related articles:

• The Basics of Ladder Diagrams for Programming PLCs
• Basic Features of Distributed Control Systems (DCS)
• Digital PID Controllers
• Basic Features of Programmable Logic Controllers (PLCs)
• Programmable Automation Controllers (PACs) – Features & Applications
• How PLC Systems are Protected from Plant Voltages
• A Basic Introduction to Ladder Diagrams for PLC Programming