Why can Heavac or Modem FBoxes only be placed in a COB?

FAQ #100288

 

Since the initialisation of the adjust parameters of the FBoxes is done in the first program cycle, the concerned FBox must be processed while this program cycle.

A further point to be considered it the fact that some of the FBoxes do need several program cycles until their task is processed (e.g. FBoxes for reading analogue input values do only process one channel per program cycle).

As result of those facts it is absolutely necessary that the FBoxes of the HVC and Modem library are processed cyclically in an COB. Also a permanently called FB or PB is possible.
It is not allowed to place FBoxes of the HVC or Modem library in a XOB or in an SB! If they are placed in a XOB the initialisation isn't guaranteed; if they are placed in a SB the code of the FBoxes don't work correctly.