Hi guys,

I was wondering why the CPU regardless of the model doesn't indicate a bus fault or any error in the first place when the HMI is disconnected from it whether its connected via profinet or profibus? But when any other device like a VFD or a communication module is disconnected the CPU glows the BF led instantly. Yes, the HMI shows the alarm of disconnection from the PLC but why does the PLC not?

dchartier

Hello aamirawan;

A VFD or communication module acts as a slave (or IO Device) and is integrated in Step 7 through a gsd file. A slave connection is monitored constantly by yhe PLC (as aMaster Class 1) since losing it could affect the ability of the PLC to control the network and the process.

An operator panel is not considered a slave, it is a Master Class 2 and you do not configure it in Step 7 through a GSD file. It is configured through its own software, WinCC for example, and it can interrogate the PLC whenever it has access to the Profibus token. Therefore it's connectivity to the PLC is not monitored through Step 7 mechanisms, but in WinCC. The master class 2 has no requirement to remain constantly connected to the PLC since it is a monitoring device.

Here is a presentation of the different devices available on z Profibus network; Profinet networks have implemented similar distinctions:

https://us.profinet.com/profibus-device-types-explained-master-vs-devices/

Hope this helps, 

Daniel Chartier

 Thanks Daniel. Your answer is all i needed :)