vasso123

I'm reading the Hans Berger book and having problem to figure out what are formal parameters?

1. Difference between formal and actual parameters

Formal parameters are used to define the data type, whereas actual parameters relate to the current value.

Example:

Formal parameters	Actual parameters
CALL FC1
on :=	E0.3
off :=	A4.5

• A formal parameter is a wildcard for the actual parameter. The actual parameters are assigned to the formal parameters during a block call.
• The formal parameters of FCs must always be assigned to actual parameters.
• Actual parameters do not have to be assigned to FBs when FBs are called. If there are no actual parameters assigned to FBs when FBs are called, the previous value written during the last call in the instance DB is applied as the actual parameter.
• The input, output and IN_OUT parameters used by the block are saved as pointers to the actual parameters of the calling code block. These pointers are not visible to the user.
• Output and IN_OUT parameters cannot be assigned constants because constants cannot be written.

Formal parameters may be assigned absolute addresses, symbolic names or constants as actual parameters. However, there are restrictions (see tables below).