Choosing Between The Simatic And Iec 1131--3 Instruction Sets - Siemens SIMATIC S7-200 System Manual

Choosing Between the SIMATIC and IEC 1131- -3 Instruction Sets
Most PLCs offer similar basic instructions, but there are usually small differences from vendor to
vendor in appearance, operation, and so forth. Over the last several years, the International
Electrotechnical Commission (IEC) has developed an emerging global standard that specifically
relates to many aspects of PLC programming. This standard encourages different PLC
manufacturers to offer instructions that are the same in both appearance and operation.
Your S7-200 offers two instruction sets that allow you to solve a wide variety of automation tasks.
The IEC instruction set complies with the IEC 1131--3 standard for PLC programming, and the
SIMATIC instruction set is designed specifically for the S7-200.
Tip
When STEP 7--Micro/WIN is set to the IEC mode, it displays a red diamond 〈♦) in the Instruction
Tree beside the instructions that are not defined by the IEC 1131--3 standard.
There are a few key differences between the SIMATIC instruction set and the IEC instruction set:
The IEC instruction set is restricted to those instructions that are standard among PLC
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vendors. Some instructions that are normally included in the SIMATIC set are not standard
instructions in the IEC 1131--3 specification. These are still available for use as
non-standard instructions, but if you use them, the program is no longer strictly IEC 1131--3
compatible.
Some IEC box instructions accept multiple data formats. This practice is often referred to as
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overloading. For example, rather than have separate ADD_I (Add Integer) and ADD_R (Add
Real), math boxes, the IEC ADD instruction examines the format of the data being added
and automatically chooses the correct instruction in the S7-200. This can save valuable
program design time.
When you use the IEC instructions, the instruction parameters are automatically checked
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for the proper data format, such as a signed integer versus an unsigned integer. For
example, an error results if you try to enter an integer value for an instruction that expected
a bit value (on/off). This feature helps to minimize programming syntax errors.
Consider these points when you select either the SIMATIC or the IEC instruction set:
SIMATIC instructions usually have the shortest execution times. Some IEC instructions
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might have longer execution times.
Some IEC instructions, such as timers, counters, multiply, and divide, operate differently
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than their SIMATIC counterparts.
You can use all three program editors (LAD, STL, FBD) with the SIMATIC instruction set.
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You can use only the LAD and FBD program editors for IEC instructions.
The operation of the IEC instructions is standard for different brands of PLCs, and the
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knowledge about creating an IEC-compliant program can be leveraged across PLC
platforms.
While the IEC standard defines fewer instructions than are available in the SIMATIC
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instruction set, you can always include SIMATIC instructions in your IEC program.
IEC 1131--3 specifies that variables must be declared with a type, and supports system
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checking of data type.
Programming Concepts, Conventions, and Features Chapter 5
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