So this was admittedly not very smart but I thought I'd share anyhow. I've run some fiber to bring in a Smart Wireless gateway and chose to utilize a little DIN rail appliance that for all appearances was a media converter, with two ports for RJ-45 (copper) jacks and four connections for fiber. Connecting the copper individually had no adverse impact on the network, in fact I was able to commission the WiHART gateway on DeltaV. The first time I landed the secondary Ethernet cable communications on the entir DeltaV network went away - as in all the controller nodes (at least) went to "no communication" on both primary and secondary, and all the dynamic data on the operator screens went pink and @@@@. Fortunately we have little if any control happening over the PCN (controller-to-controller) and once the "converter" was removed from the network, everything came back on line.
So I found a bad ethernet cable up in the reactor structure, a short one between the WiHART gateway and the far-end media converters, which were ones I purcahsed "for purpose" from a different supplier. They seemed to work fine otherwise with the device in the house and I thought I had demonstrated as much when I replaced the short cable up in the structure. I could ping the gateway from the house and connect using a laptop / browser interface. So everything on the media side seemed to be fine.
When I hooked up the secondary again, I was watching diagnostics "live". All controllers immediately went to "not connected" and once again all the operator screens went to pink and @@@@.
So I was starting to compose a ticket on Guardian when I thought I should pull a model number off the surplus / spare device I was using. When I looked it up on the internet, I learned I wasn't using a media converter at all - what I had was a "no longer available" i.e. obsolete REDUNDANT industrilal ethernet SWITCH . . . which probably started hammering DeltaV's PCN looking for one of its bretheren as soon as both cables were connected. It had been in a box in the rack room for ten years or more - now I wish I had donated it to a school a long time ago.
The "converter" that was really a redundant industrial ethernet switch was from a reputable, industrial strength manufacturer (not from NewEgg or Office Depot) whose products many of us use in our networks. It just wasn't what it appeared to be. Needless to say I cancelled the Guardian ticket. The plant survived but I definitely won't be attaching random hardware to the PCN any more.
In reply to AdrianOffield:
Nope it has two fiber connections (four SC connectors) and two RG45 (copper) connectors. On the other end, the fiber goes to two individual media converters, whose copper side is connected to each port of a DeltaV-ready Smart Wireless Gateway.
I can post a picture / link when I get back to the office but my mis-application of course shouldn't reflect on the quality or competence of the device supplier . . .