Comparison of Programming Modes: SIMATIC, IEC (GS 2.6)

SIMATIC Instruction Set

Most PLCs offer the same types of basic instructions, but there are usually small differences in their appearance and operation from one vendor to another. The SIMATIC language is simply the instruction set offered by Siemens. That is, many of these instructions usually look and operate slightly differently when compared to similar instructions in another brand of PLC. The main points to consider when you select the SIMATIC instruction set are:

·SIMATIC instructions usually have the shortest execution times.

·All three editors (LAD, FBD, STL) work with the SIMATIC instruction set.

IEC 1131-3 Instruction Set

The International Electrotechnical Commission is a worldwide organization that develops global standards for all fields of electrotechnology. Over the last several years, they have developed an emerging standard that specifically relates to many aspects of PLC programming. Basically, this standard encourages different PLC manufacturers to offer instructions that are similar in both appearance and operation. There are a few key differences between the SIMATIC instruction set and the IEC 1131-3 instruction set.

·There are fewer instructions available in the IEC 1131-3 set. Some instructions that are normally included in the SIMATIC set are simply not standard instructions in the IEC 1131-3 specification. (Instructions comparable to the SIMATIC instructions are provided as a non-standard extension of the IEC 1131-1 set. They are shown with a red mark ( ) in the Instruction Tree.)

·Some box instructions in the IEC 1131-3 set accept multiple data formats. This concept is often referred to as overloading. For example, rather than have separate ADD_I (Add Integer) and ADD_R (Add Real) math boxes, the IEC 1131-3 ADD instruction examines the format of the data being added, and automatically chooses the correct instruction in the CPU. This can save valuable program design time.

·When you use the IEC 1131-3 instructions, the instruction parameters are automatically checked for the proper data format. For example, if you tried to enter an integer value for an instruction that expected a bit value (on/off), an error would be indicated. This feature helps to minimize programming syntax errors.

The main points to consider about IEC programming are:

·IEC 1131-3 specifies that variables must be declared with a type and supports system checking of data type.

·It is usually easier to learn how to create programs for different brands of PLCs when you use IEC 1131-3 standard instructions.

·Fewer instructions are available (as specified by the standard) but you can use many of the SIMATIC instructions as well.

·Some IEC 1131-3 instructions operate differently than their SIMATIC counterparts (timers, counters, multiply, and divide).

·The IEC 1131-3 instructions may have longer execution times.

·The IEC 1131-3 instructions can only be used within the LAD and FBD editors.

See Also:

Comparison of Editors: LAD, FBD, STL (GS 2.5)