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Publication LOGIX-AP008B-EN-P - June 2008 57

Logix Features that May Not be Familiar to S7 Users Chapter 2

Add-On Instructions

Add-On Instruction Summary

Add-on Instructions are the equivalent of STEP 7 Function Blocks, with

private data and advanced parameter choices. In particular, the INOUT

parameter type or “pass by reference” makes it possible to efficiently pass data

structures to the code.

Because the Add-On Instruction is so similar to the STEP 7 Function Block, it

is likely that the S7 programmer who is converting to Logix will make use of it

quite readily.

Comparison between FBs and Add-On Instructions:

• Both can be called as named functions from anywhere in the program.

• Both contain a private data area of static data, although it is not truly

private in the case of STEP 7.

• A STEP 7 function block also has a temporary data area.

• In the Add-On Instruction, local static data will do the same.

Both have three types of parameters – input (pass by value), output (pass by

value) and in-out (pass by reference). The pass by reference parameter is a

considerable benefit, since it allows large data structures to be passed

efficiently.

The Add-On Instruction will automatically maintain a change history by

recording a timestamp and the Windows user name at the time of the change.

This is not available with STEP 7 Function Blocks

With the Add-On Instruction a pre-scan routine can be configured to run

when the controller goes from Program mode to Run mode, or powers up in

Run mode. Under these conditions, the pre-scan routine will run once, and can

typically be used to initialise data. In STEP 7 the Organization block OB100

does the same, but the pre-scan code cannot be specifically attached to a FB.

If the Add-On Instruction is called from a SFC step and the SFC is configured

for Automatic Reset, a post-scan routine defined in the Add-On Instruction

will execute once when the SFC exits that step. It could be used for resetting

data. A STEP 7 FB has no built-in equivalent (although it is easy to program).

An Add-On Instruction can have an EnableInFalse routine, which will be

called (if present) when the rung condition at the Add-On Instruction call is

false. In this case, the input and output parameters will pass values. A STEP 7

FB has no equivalent.

Add-on Instructions are explored further in Chapter 4.

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Siemens 7 Manuel d'utilisateur Page 57