Intermittent comms to ENBT/A

[radfahrer]

I have an iFix SCADA talking to a Controllogix via an ENBT/A card on the Logix and I'm having intermittent comm. loss issues. The intermittency is on the order of days between occurrences so it's difficult to troubleshoot.

I have noticed that the ENBT card appears to change from 100MBps to 10MBps when the comm issue happens. When I unplug and re-connect it it comes back at 100MBps. The card is set to autonegotiate.

Has anyone seen this type of issue before? Are there recommended settings for an ENBT card (speed and duplex) or is auto-negotiate the best?

Another observation I've made is that I can occassionally trigger the comm alarm when I remote-desktop in to the SCADA server which is communicating to that PLC (using mstsc)...which seems to indicate some type of network overload. The SCADA and PLC are on the same Enet hub and I'm terminalling in from a remote network connected to the hub as well. The PLC, SCADA and remote network connection are the only devices on the hub.

The hub is an older model (got it cheap from the IT group), a 3Com Superstack II Dual Speed Hub.

Is the fact that this is a hub and not a switch a potential cause for this issue, or is that not a major factor with so few devices connected?

Any suggestions appreciated,

Greg Krueger

 

[Eddie Willers]

I think it is very likely that the hub is the reason that you're seeing this behavior on the part of the 1756-ENBT. Maybe the -ENBT is detecting the large amount of traffic from the remote desktop session and re-initiating the autonegotiation function, or maybe the hub is changing its port behavior.

SuperStack II was a broad product family for 3COM... no way to tell for sure which features (full duplex ? Autonegotiate?) the hub supports. Do your IT guys like pranks ?

Very likely this problem will not recur if you replace the hub with even an inexpensive switch.