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Function blocks (FB/FX)
Data blocks (DB/DX)
Formal structure
of the blocks
Block header
Block body
CPU 948 Programming Guide
C79000-G8576-C848-04
You use function blocks to program frequently recurring and/or
complex functions (e.g. digital functions, sequence control systems,
closed loop controls and signalling functions).
A function block can be called several times by higher order blocks
and supplied with new operands (assigned parameters) at each call.
Using block type FX doubles the maximum number of possible function
blocks.
Data blocks contain the (fixed or variable) data with which the user
program works. This type of block contains no STEP 5 statements and
has a distinctly different function from the other blocks. Using block
type DX doubles the number of possible data blocks.
All blocks consist of the following two parts:
• • a block header
and
• • a block body
The block header is always 5 words long and contains information for
block management in the PG and data for the system program.
Depending on the block type, the block body contains the following:
• • STEP-5 operations (in OB, PB, SB, FB, FX),
• • variable or constant data (in DB, DX)
and
• • a formal operand list (in FB, FX).
STEP 5 Programming Language
2 - 13
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