PROGRAMMING

Each PLC manufacturer offers a different set of instructions within its PLC family. Many of these instruction sets are not applicable to other PLCs, and there is no easy way to translate an already written PLC program to another brand of PLC’s programming format.

The International Electrotechnical Commission (IEC) SC65B-WG7 committee developed the IEC 1131 standard in an effort to standardize programmable controllers. One of the committee’s objectives was to create a common set of PLC instructions that could be used in all PLCs.

PLC Programming Method • Ladder Diagram (LD) —a graphical depiction of a process with rungs of logic, similar to the relay ladder logic schemes that were replaced by PLCs. • Function Block Diagram (FBD) —a graphical depiction of process fl ow 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 specifi cally developed for industrial control applications

Ladder Diagram & Mnemonic Code

Ladder Diagram & Mnemonic Code

Ladder Diagram & Function Block Diagram (FBD)

logic instruction set LOAD LOAD NOT AND AND NOT OR OR NOT OUTPUT END OR LD AND LD

LOAD LOAD NOT

AND AND NOT

OR OR NOT

OUT END

OR LD

AND LD