Programmable Logic Controllers 15 1.4.2 The IEC Standard The full IEC 61131 standard covers the complete life cycle of PLCs: Part 1: General definition of basic terminology and concepts. Part 2: Electronic and mechanical equipment requirements and verification tests for PLCs and associated equipment. Part 3: Programming languages. Five languages are defined: ladder diagram (LAD), sequential function charts (SFC), function block diagram (FBD), structured text (ST), and instruction list (IL). Part 4: Guidance on selection, installation, and maintenance of PLCs. Part 5: Software facilities needed for communication with other devices based on the Manufacturing Messaging Specification (MMS). Part 6: Communications via field bus software facilities. Part 7: Fuzzy control programming. Part 8: Guidelines for the implementation of PLC programming languages defined in Part 3.