PLC programming tips and tricks

PLC application programming is typically done in step sequences. An unexpected behaviour of the application program may arise if the aforementioned commanding and call types are mixed.

With regard to the commanding method, a PLCopen FB can be distinguished according to:

• Trigger commanding ("Execute" TRUE only for one cycle)
• Level commanding ("execute" is applies at least until" done" = TRUE)

PLC programmers should give thought beforehand to which of the two commanding methods they wish to use and should stick to it within one project.

With regard to the call method, a PLCopen FB can be distinguished according to:

• Call within all FBs in each PLC cycle
• Call only of the relevant FB in each step

Here also, PLC programmers should give thought beforehand to which of the two call methods they wish to use and should stick to it within one project.