PLC Ladder Logic Programs

PLC Ladder Logic Programs For ease of understanding, ladder logic programs can be compared to relay schematics. In this article, I will explain how traditional relay schematics are converted into PLC ladder logic programs. Latching Relays: Electromagnetic latching relays are designed to hold the relay closed after power has been removed from the coil. Latching […]

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Entering Ladder Diagram

Entering Ladder Diagram Most of today’s PLC programming packages operate in the Windows environment. For example, Allen-Bradley’s RSLogix software packages are Windows programming packages used to develop ladder logic programs. This software, in various versions, can be used to program the PLC-5, SLC 500, ControlLogix, and MicroLogic family of processors. An added feature is that

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PLC Programming Languages

PLC Programming Languages The term PLC programming language refers to the method by which the user communicates information to the PLC. The standard IEC 61131 ( Figure 1 ) was established to standardize the multiple languages associated with PLC programming by defining the following five standard languages: Ladder diagram language is the most commonly used PLC

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PLC Scan Cycle

PLC Scan Cycle When a PLC executes a program, it must know, in real time, when external devices controlling a process are changing. During each operating cycle, the processor reads all the inputs, takes these values, and energizes or de-energizes the outputs according to the user program. This process is known as a program scan

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Memory Map for PLC Processor

Memory Map for PLC Processor While the fundamental concepts of PLC programming are common to all manufacturers, differences in memory organization, I/O addressing, and instruction set mean that PLC programs are never perfectly interchangeable among different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible. The

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Central Processing Unit of PLC

Central Processing Unit of PLC The central processing unit (CPU) is built into single-unit fixed PLCs while modular rack types typically use a plug-in module. CPU, controller, and processor are all terms used by different manufacturers to denote the same module that performs basically the same functions. Processors vary in processing speed and memory options.

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I/O Module Specifications of PLC

I/O Module Specifications of PLC Manufacturers’ specifications provide information about how an interface device is correctly and safely used. These specifications place certain limitations not only on the I/O module but also on the field equipment that it can operate. Some PLC systems support hot swappable I/O modules designed to be changed with the power

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Analog I/O Modules of PLC

Analog I/O Modules of PLC Earlier PLCs were limited to discrete or digital I/O interfaces, which allowed only on/off-type devices to be connected. This limitation meant that the PLC could have only partial control of many process applications. Today, however, a complete range of both discrete and analog interfaces are available that will allow controllers

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Discrete I/O Modules of PLC

Discrete I/O Modules of PLC The most common type of I/O interface module is the discrete type ( Figure 1). This type of interface connects field input devices of the ON/OFF nature such as selector switches, pushbuttons, and limit switches. Likewise, output control is limited to devices such as lights, relays, solenoids, and motor starters that

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