Most PLCs have arithmetic function capabilities. Basic PLC math instructions include add, subtract, multiply and divide to calculate the sum, difference, product, and quotient of the content of word registers. The PLC is capable of doing many arithmetic operations per scan period for fast updating of data. This article covers the basic mathematical instructions performed […]
PLC Data Manipulation Programs Data manipulation instructions give new dimension and flexibility to the programming of control circuits. For example, consider the hardwired relay-operated, time-delay circuit in Figure 1 . This circuit uses three electro-mechanical time-delay relays to control four solenoid valves. The operation of the hardwired circuit can be summarized as follows: The hardwired time-delay
Data manipulation involves transferring data and operating on data with math functions, data conversions, data comparison, and logical operations. This article covers both data manipulation instructions that operate on word data and those that operate on file data, which involve multiple words. Data manipulations are performed internally in a manner similar to that used in
The program control instructions covered in this article are used to alter the program scan from its normal sequence. The use of program control instructions can shorten the time required to complete a program scan. Portions of the program not being utilized at any particular time can be jumped over, and outputs in specific zones
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