Siemens Simatic S7-200 System Manual page 217

Address parameters such as IN4 (&VB100) are passed into a subroutine as a DWORD (unsigned double
word) value. The type of a constant parameter must be specified for the parameter in the calling routine
with a constant descriptor in front of the constant value. For example, to pass an unsigned double word
constant with a value of 12,345 as a parameter, the constant parameter must be specified as DW#12345.
If the constant describer is omitted from the parameter, the constant can be assumed to be a different
type.
There are no automatic data type conversions performed on the input or output parameters. For example,
if the local variable table specifies that a parameter has the data type REAL, and in the calling routine a
double word (DWORD) is specified for that parameter, the value in the subroutine will be a double word.
When values are passed to a subroutine, they are placed into the local memory of the subroutine. The
left-most column of the local variable table shows the local memory address for each passed parameter.
Input parameter values are copied to the subroutine's local memory when the subroutine is called. Output
parameter values are copied from the subroutine's local memory to the specified output parameter
addresses when the subroutine execution is complete.
The data element size and type are represented in the coding of the parameters. Assignment of
parameter values to local memory in the subroutine is as follows:
-
Parameter values are assigned to local memory in the order specified by the call subroutine
instruction with parameters starting at L.0.
-
One to eight consecutive bit parameter values are assigned to a single byte starting with Lx.0 and
continuing to Lx.7.
-
Byte, word, and double word values are assigned to local memory on byte boundaries (LBx, LWx,
or LDx).
In the Call Subroutine instruction with parameters, parameters must be arranged in order with input
parameters first, followed by input/output parameters, and then followed by output parameters.
If you are programming in STL, the format of the CALL instruction is:
CALL subroutine number, parameter 1, parameter 2, ... , parameter
Example: Subroutine and Return from Subroutine Instructions
M
A
I
N
S
B
R
0
Network 1 //On the first scan, call subroutine 0 for initialization.
LD SM0.1
CALL SBR_0
Network 1 //You can use a conditional return to leave
//the subroutine before the last network.
LD M14.3
CRET
Network 2 //This network will be skipped if M14.3 is on.
LD SM0.0
MOVB 10, VB0
S7-200 Instruction Set Chapter 6
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