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
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
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
Input Output Section of PLC The input output section of PLC is the section to which all field devices are connected and provides the interface between them and the CPU. Input/output arrangements are built into a fixed PLC while modular types use external I/O modules that plug into the PLC. Figure 1 illustrates a rack-based