System variables in system POUs

Question:
Which aspects have to be observed when using system variables in the system POUs PLC_Restart and PLC_ColdStart?

Response:
The system POUs PLC_Restart and PLC_ColdStart do not create their own process image. The data exchange between the hardware inputs and outputs and the variables of the PLC program is, however, carried out via the process image.
As result, the following consequences occur when the program is processed:

• If the value of a hardware input (e.g. analog input) is assigned to a variable in the system POU PLC_Restart or PLC_ColdStart, the result of this assignment is always 0 (independent of the input signal).
• If hardware outputs (and therefore the corresponding system module) are only used in one of the two system POUs PLC_Restart or PLC_ColdStart, a level change initiated by the system POU is not output by the hardware outputs, e.g. a digital output cannot be switched on in the POU PLC_ColdStart.

Recommendations:

• Do not use system variables (hardware inputs and hardware outputs) in the two system POUs- PLC_Restart and PLC_ColdStart.
• Check in the dialog Process image creation (Register index: Resources>Process image) in which task the process image of the system POU used is created.

URL for linking this AKB article: https://www.lenze.com/en-de/go/akb/200412046/1/