The sequential function charts (SFC) language is based on an old logic language called Grafcet, which was based on binary Petri nets. It uses iteration loops such as Do-While or For-Next, conditional statements such as If-Then-Else or Case, and other math functions such as Sqrt() or Sin(). Structured text (ST) is a high-level language similar to computer languages such as Pascal. Inputs are fed into a block from devices or other blocks’ outputs outputs carry data to the next block or to a variable. It describes the function between input and output variables using different types of blocks, such as AND, OR, comparisons, math, timers and counters. As mentioned previously, ladder evolved from circuit diagrams for relay logic, which was familiar to maintenance electricians.įunction block diagrams (FBD) was another graphical programming language used in some controllers, including distributed control system (DCS) controllers. Ladder logic was defined further, along with subroutines and other aspects of PLCs.
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