PLCs (programmable logic controllers) are the control hubs for a wide variety of automated systems and processes. They contain multiple inputs and outputs that use transistors and other circuitry to simulate switches and relays to control equipment. They are programmable via software interfaced via standard computer interfaces and proprietary languages and network options.

I/O Channel Specifications

Programmable logic controllers I/O channel specifications include:

• Total number of points
• Number of inputs
• Number of outputs
• Ability to expand
• Maximum number of channels

Number of points is the sum of the inputs and the outputs. PLCs may be specified by any possible combination of these values. Expandable units may be stacked or linked together to increase total control capacity.

Maximum number of channels refers to the maximum total number of input and output channels in an expanded system.

PLC system specifications to consider include:

• Scan time
• Number of instructions
• Data memory
• Program memory

Scan time is the time required by the PLC to check the states of its inputs and outputs. Instructions are standard operations (such as math functions) available to PLC software. Data memory is the capacity for data storage. Program memory is the capacity for control software.

Available Inputs

Available inputs for programmable logic controllers include DC, AC, analog, thermocouple, RTD, frequency or pulse, transistor, and interrupt inputs. Outputs for PLCs include DC, AC, relay, analog, frequency or pulse, transistor, and triac. Programming options for PLCs include front panel, hand held, and computer.

Software Programming Languages

Programmable logic controllers use a variety of software programming languages for control. These include:

• IEC 61131-3
• Sequential function chart (SFC)
• Function block diagram (FBD)
• Ladder diagram (LD)
• Structured text (ST)
• Instruction list (IL)
• Relay ladder logic (RLL)
• Flow Chart
• C
• BASIC

The IEC 61131-3 programming environment provides support for five languages specified by the global standard: Sequential Function Chart, Function Block Diagram, Ladder Diagram, Structured Text, and Instruction List. This allows for multi-vendor compatibility and multi-language programming.

SFC is a graphical language that provides coordination of program sequences, supporting alternative sequence selections and parallel sequences.

FBD uses a broad function library to build complex procedures in a graphical format. Standard math and logic functions may be coordinated with customizable communication and interface functions.

LD is a graphic language for discrete control and interlocking logic. It is completely compatible with FBD for discrete function control.

ST is a text language used for complex mathematical procedures and calculations less well suited to graphical languages.

IL is a low-level language similar to assembly code. It is used in relatively simple logic instructions.

Relay Ladder Logic (RLL), or ladder diagrams, is the primary programming language for programmable logic controllers (PLCs). Ladder logic programming is a graphical representation of the program designed to look like relay logic.

Flow Chart is a graphical language that describes sequential operations in a controller sequence or application. It is used to build modular, reusable function libraries.

C is a high level programming language suited to handle the most complex computation, sequential, and datalogging tasks. It is typically developed and debugged on a PC.

BASIC is a high level language used to handle mathematical, sequential, data capturing and interface functions.

Programmable logic controllers can also be specified with a number of computer interface options, network specifications and features. PLC power options, mounting options and environmental operating conditions are all also important to consider.

Image credit:

Allen-Bradley / Rockwell Automation