Statement List (STL) - DCE FEL ČVUT v Praze

C Parameter Transfer
!
The parameters of a block are transferred as a value. With function blocks a copy of
the actual parameter value in the instance data block is used in the called block. With
functions a copy of the actual value lies in the local data stack. Pointers are not
copied. Prior to the call the INPUT values are copied into the instance DB or to the
L stack. After the call the OUTPUT values are copied back into the variables. Within
the called block you can only work on a copy. The STL instructions required for this
are in the calling block and remain hidden from the user.
Note
If memory bits, inputs, outputs or peripheral I/Os are used as actual address of a
function they are treated in a different way than the other addresses. Here, updates
are carried out directly, not via L Stack.
Caution
When programming the called block, ensure that the parameters declared as OUTPUT are
also written. Otherwise the values output are random! With function blocks the value will be
the value from the instance DB noted by the last call, with functions the value will be the
value which happens to be in the L stack.
Note the following points:
• Initialize all OUTPUT parameters if possible.
• Try not to use any Set and Reset instructions. These instructions are dependent on the
RLO. If the RLO has the value 0, the random value will be retained.
• If you jump within the block, ensure that you do not skip any locations where OUTPUT
parameters are written. Do not forget BEC and the effect of the MCR instructions.
Statement List (STL) for S7-300 and S7-400 Programming
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