Siemens CPU 948 User Manual

2.3.1
Structure of Function Blocks The block header (five words) of a function block has the same
structure as the headers of the other STEP 5 block types.
The block body on the other hand, has a different structure from the
bodies of the other block types. The block body contains the function
to be executed in the form of a statement list in the STEP 5
programming language. Between the block header and the STEP 5
statements, the function block needs additional memory space for its
name and for a list of formal operands. Since this list contains no
statements for the CPU, it is skipped with an unconditional jump that
the programmer generates automatically. This jump statement is not
displayed when the function block is displayed on the PG!
When a function block is called, only the block body is processed.
Absolute or symbolic
operands
You can enter operands in a function block in absolute form
(e.g. F 2.5) or symbolically (e.g. MOTOR1). You must store the
assignment of the symbolic operands in an assignment list before you
enter the operands in a function block (see /3/).
Fig. 2-4 shows the structure of a function block in the memory of a
programmable controller.
JU
Name of the FB/FX
Formal operand 1
Formal operand 2
Formal operand n
5 words
1word
4 words
3 words
Block
header
Block
body
BE
3 words
Skip formal
operand
list
1st STEP 5 user operation
3 words
List of
formal
operands
STEP 5
user
program
Fig. 2-4 Structure of a function block (FB/FX)
Function Blocks
CPU 948 Programming Guide
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Siemens CPU 948 Specifications

General

Brand	Siemens
Model	CPU 948
Category	Controller
Language	English