The statement list, usually abbreviated IL, is one of the IEC 61131-3 standard in methods for the programming of programmable logic controllers ( PLC). Many PLC vendors refer to the language used by them even as STL, if they do not strictly adhere to the IEC 61131-3, so that, existing STL programs hardly transmitted to controllers from other manufacturers.
STL is mainly used for logical linking of control inputs and outputs. Typically, a (digital ) input to the working register (also called accumulator ) loaded (load digital input 0, "LD % IX0.0 " ), with other inputs, constants or memory values linked (exclusive - or memory- bit 3, " XOR % MX0.3 " ) and written to an output (store digital output 1, " % QX0.1 ST ").
The main features of STL are that operators have only one operand and the syntax of the language is based on the assembly language. Thus, it offers only very complicated structuring opportunities through jump instructions. Advantages, however, if due to a shortage of memory the CPU being used, the program code is to be kept small. On older controls IL programs are still relatively common. IL programs are but compared to programs in higher level languages in larger projects very confusing and difficult to maintain. Today, therefore, be for the programming of control normally the higher the IEC 61131-3 languages , in particular structured text (ST ) or Sequential Function Chart, Continuous Function Chart or often also "C" used.
STL Sample Programs
Example 1: And operation on two binary inputs to a single output
LD INPUT 1 AND INPUT 2 ST OUTPUT Example 2: Addition of two integers
LD VALUE1 ADD VALUE2 ST OUTPUT Example 3: RS flip-flop ( reset dominant )
LD S- INPUT S OUTPUT LD - R INPUT R OUTPUT In comparison, the examples of Siemens S7, (non-standard compliant ) German mnemonics: