6.1 Instruction Lists

A programming method, which can be considered to be the entering of a ladder program using text, is instruction lists (IL). Instruction list gives programs which consist of a series of instructions, each instruction being on a new line. An instruction consists of an operator followed by one of more operands, i.e. the subjects of the operator. In terms of ladder diagrams an operator may be regarded as a ladder element. Each instruction may either use or change the value stored in a memory register. For this, mnemonic codes are used, each code corresponding to an operator/ladder element. The codes used differ to some extent from manufacturer to manufacturer, though a standard IEC 1131 3 has been proposed and is being widely adopted. Table 6.1 shows some of the codes used by manufacturers, and the proposed standard, for instructions used in this chapter (see later chapters for codes for other functions).