Profibus Switch on/ off

In one of the systems our plant, there is a S7-400PLC. In one of its profibus network, there is a sinamics drive after the plc and after that there are 2 ET-200S remote IO system is present. So normally in the plc and in the last ET-200S,  profibus switch was in on position and in the drive and in one remote io profibus switch was in off position.
Todaay I performed a trial for learning purpose...
Intentionally I kept the profibus conector switch in plc in off position, then I saw bus fault in the last  ET-200S and in the PLC. When I put the switch of the last ET-200S in OFF position also then bus fault from this last node disappears but bus fault comes in the second last node.

Why it happened. Please anyone explain

Hello Swagato;

The termination resistors act on the Profibus (RS-485) network to adapt the transmission line impedance, blocking the appearance of reflections on the line.

When termination is lost on one end of the bus (especially when both terminations are lost) then reflections appear and transmission of frames on the bus is perturbed. It is difficult to predict which nodes will be affected, this will vary with bus speed, segment length, actual line impedance... Even using the diagnostic buffer information you will see multiple node loss, without specific details. When an end node is powered down, it ceases to power the termination resistance and nodes become lost on the bus. If the master node is lost then every station on the bus will send catastrophic diagnostics, since the whole network is managed by the master.

Conclusion: always ensure termination is set as per Profibus-DP standards. If termination is to be removed regularly from one of the end nodes, use an active termination module (one that is self-powered and provides adequate termination to the network).

Information in this forum discussion could be interesting to you:

https://support.industry.siemens.com/tf/ww/en/posts/on-off-terminal-resistor-on-profibus-dp-cable/153620?page=0&pageSize=10

Hope this helps, 

Daniel Chartier