All PLCs include both up-counters and down-counters. Counter instructions and their function in ladder logic are explained in this article. Typical examples of PLC counters include the following: straight counting in a process, two counters used to give the sum of two counts, and two counters used to give the difference between two counts. Counter …
The most commonly used PLC instruction, after coils and contacts, is the timer. This article deals with how timers time intervals and the way in which they can control outputs. We discuss the basic PLC on-delay timer function, as well as other timing functions derived from it, and typical industrial timing tasks. Timer Instructions in …
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 …
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 …
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 …
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 …
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 …
Boolean Equation for Logic Gate Circuits An understanding of the technique of writing simplified Boolean equations for complex logical statements is a useful tool when creating PLC control programs. Some laws of Boolean algebra are different from those of ordinary algebra. These three basic laws illustrate the close comparison between Boolean algebra and ordinary algebra, …
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. …
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 …
