established to standardize the multiple languages associated with PLC programming by defining the following five standard languages: • Ladder Diagram (LD)—a symbolic depiction of instructions arranged in rungs similar to ladderformatted schematic diagrams. • Function Block Diagram (FBD)—a graphical depiction of process flow using simple and complex interconnecting blocks. • Sequential Function Chart (SFC)—a graphical depiction of interconnecting steps, actions, and transitions. • Instruction List (IL)—a low-level, text-based language that uses mnemonic instructions. • Structured Text (ST)—a high-level, text-based language such as BASIC, C, or PASCAL specifically developed for industrial control applications. Ladder diagram language is the most commonly used PLC language and is designed to mimic relay logic. The ladder diagram is popular for those who prefer to define control actions in terms of relay contacts and coils, and other functions as block instructions. Figure 5-15 shows a comparison of ladder diagram programming and instruction list programming. Figure 5-15a shows the original relay hardwired control circuit. Figure 5-15b shows the equivalent logic ladder diagram programmed into a controller. Note how closely the ladder diagram program resembles the hardwired relay circuit. The input/ output addressing is generally different for each PLC Horizontal scanning order End of ladder Vertical scanning order Return for next scan Figure 5-13 Scanning can be vertical or horizontal. (Figure 5-13). Allen-Bradley PLCs use the horizontal scan by rung method. In this system, the processor examines input and output instructions from the first command, top left in the program, horizontally, rung by rung. Modicon PLCs use the vertical scan by column method. In this system, the processor examines input and output instructions from the top left command entered in the ladder diagram, vertically, column by column and page by page. Pages are executed in sequence. Both methods are appropriate; however, misunderstanding the way the PLC scans a program can cause programming bugs. 5.3 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 5-14) was PLC programming languages Textural language Instruction list Structured text Sequential function chart Graphical language Functional block diagram Ladder diagram Figure 5-14 Standard IEC 61131 languages associated with PLC programming. PB1 CR CR SOL LS 1 1 2 (a) Hardwired relay control circuit (PB1) (CR1) (CR2) A B D Y C (LS1) (b) Equivalent ladder diagram (LD) program (c) Equivalent instruction Figure 5-15 Comparison of ladder diagram and instruction list programming. SOL PB 1 CR 1 LS1 CR 2 SOL START AND OR AND NOT OUT list (IL) program Basics of PLC Programming Chapter 5 81
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