7.1.1 Operating by Means of the FB PID_FM
The operating parameters (for example, setpoint value, manual manipulated value)
of the FM 355 are transferred cyclically from the FB PID_FM to the FM 355.
Operating parameters are all the in/out parameters, which lie in the instance DB of
the function block between the parameters op_par and cont_par.
In order for the data transmission to be possible without a high run time in the
CPU, transmission is normally (when LOAD_OP = FALSE) carried out via direct
I/O accesses. The data are multiplexed since only four bytes per channel are
available in the I/O address area of the module. It can therefore take up to three
cycles of the CPU or of the FM 355 until the operating values have been
transferred to the FM 355 and become effective there - the respectively longer
cycle is decisive.
If you want the operating values to be transferred immediately (within one cycle of
the CPU or of the FM 355) to the FM 355, you can then set the parameter
LOAD_OP to TRUE. The transmission is then carried out via the SFC WR_REC,
the FB then requires more run time (refer to the Technical Specifications Function
Blocks in Section A.2). After the data transmission has been carried out
successfully the LOAD_OP parameter is reset to FALSE by the FM PID_FM. If the
FM 355 is used in distributed remote I/Os, it can take some call cycles.