I'm working with a system using four M3 XD26 PLC:s, all four with XN05 Ethernet extentions for ModBUS coummunication between the PLC:s and two NI cRIO:s (9073 and 9074).
The communication usually works fine but now and then the ModBUS masters (cRIO:s) seem to loose the connection to different XN05 causing the system to shut down. The shut down is made when any PLC loose connection (desn't receive any data over the ModBUS link for a few seconds) and is a essential security part of the control system to avoid accidents caused by a failed control system or broken Ethernet cable.
Now to my problem: If th XN05 module for a short while (say ~1 second) looses it's connection (unplugged - replugged Ethernet cable) to the Ethernet switch it's connected to, it will take 5~7 seconds before the PLC software will start to receive ModBUS data again. It doesn't matter if th Ethernet connection is lost for 1 or 30 seconds, it will allways take these 5~7 seconds before the connection is reetablished again. It doesn't matter if I let my ModBUS master try to close the connection and reconnect again after 500ms or after say 10 seconds. The NX05 doesn't seem to respond to the masters attempts to reconnect (checked this with WireShark).
The very interesting thing is that this only happens if the connection between the NX05 and the switch is broken. If the connection between the switch and the master is broken for 0.5, 1, 13 or 20 seconds, the PLC software will receive the ModBUS communication as soon as the cable is reconnected. So, the problem seems to appear when the XN05 looses it's connection to SOMETHING, but not when the ModBUS data stream is interrupted.
It doesn't matter if the time out in the XN05 settings is set to the default 10 minutes or if its set to 0 minutes, I'm using a fixed IP for all units on the network and the netwirk isn't connected to anything else but the four PLC, two cRIO:s, a PC with SCADA and a PC for network analysis.
I have tried different switches and end up with exactly the same behaviour...
I suspect that the XN05 module have a internal time out of approximatly 5 seconds...by som reason... I can't find any information about this so I'd be very thankful if someone could help me with this. Is it perhaps possible to make the NX05 or the communication a bit more "stupid" so it will do what it's told do in stead of this?
With best regards
M3 XN05 Ethernet extention
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