Rockwell Automation 1775-S4B,D17756.5.3 User Manual SCANNER/MSG HND - Manual (Page 74)

Using Symbols and Expressions
in Report Generation
Chapter 8
8Ć6
An assignment statement can look very much like an algebraic equation,
but there is one important difference. In algebraic equations, there is no
difference between the statements:
A=B
and
B=A
But in an assignment statement, the two statements are very different.
Suppose, for example, that before the 1775-S4B scanner executes the
statement, A contains the value 5 and B contains the value 10. If you now
execute the first statement (A = B), both A and B would contain the value
10. However, if you execute the second statement (B = A) instead, A and B
would contain the value 5. Therefore, instead of thinking about an
assignment statement as A equals B, it may be useful to think of it as the
value in A is replaced by the value in B.
Area 7 of PLC-3 memory is the system scratchpad area. This area stores a
user symbol(s) and the value or string assigned to that symbol. When the
1775-S4B scanner executes a procedure which contains user symbols, the
1775-S4B scanner stores the numeric or string value that is generated by
the assignment command. The 1775-S4B scanner makes this area large
enough to contain all the user symbols generated in a procedure:
If the user symbol is procedural, the section(s) of the scratchpad area
that store the symbol are deleted after the procedure executes.
If the user symbol is interprocedural, the section(s) of the scratchpad
area that store the symbol remain, since other procedures can recognize
interprocedural user symbols.
When you are calculating maximum memory usage for your report
generation procedures, remember that user symbols take up memory while
the procedure is executing.
If you nest procedure execution, the scratchpad area is not deleted until the
calling procedure and all nested procedures have executed. Refer to
chapter 10 for detailed information on nesting procedures.
8.3.2
Memory Storage for User
Symbols