Technical data

Basics of Designing a Program Structure
4.2 Blocks in the User Program
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 89
Thus, observe the following points:
• If possible, initialize the OUTPUT parameters.
• Set and reset instructions depend on RLO. When these instructions are used to determine the
value at an OUTPUT parameter, no value is generated if the result of a previous logic
operation (RLO) = 0.
• Always ensure that data are written to the OUTPUT parameters - irrespective of any program
paths in the block. Pay special attention to jump instructions, to the ENO output in LAD and
FBD as well as to BEC (Block End Conditional) and the influence of MCR (Master Control
Relay) instructions.
Note
Although the OUTPUT parameters of an FB or the INOUT parameters of an FC and FB will not output
random values (the old output value - or input value as output value - is going to be maintained even if
no data are written to the parameter) you should still observe the points above in order to avoid
unintentional processing of "old" values.