Healthy serial communications


#1

Hi
Just wondering if anyone does tests on their serial communications lines to determine the ‘health’ of their serial comms?

At our workplace here we have several patient call systems (not Opto22) that use RS485 communications to gather information from remote I/O. We used to have a lot of trouble with some of the I/O boards failing with errors on the main system until we were asked to check the communications line to determine its health. It would involve unplugging the master controller from the 485 line and measuring a dc voltage between each tx/rx line to dc ground. Around 2.5Vdc was considered healthy but under 2Vdc was considered problematic. The solution is easy, just replace the line driver chips.

We are currently experiencing some funny serial communications issues between an S1, some B3000 serial and G4 mistic brains, and was wondering if there was a similar technique that can be used on Opto22 hardware to determine if the serial line has an issue? i know i cannot replace the serial chips as per above, but it could give a good indication of which serial line to follow (if your controller is in the middle)
I have measured a couple of 485 lines with the controller unplugged and they measure around .2Vdc WRT gnd.

Nick


#2

Hi Nick. The only health checks I do on comms. is to verify that everything is talking as it should, so I don’t have any specific testing recommendations for you. But I can share my experience with a recent 485 issue. A failing UPS took out a couple of G4 regulators and Mistic brains. No problem Opto still carries these : ) Initially all was well but we began to have intermittent comm issues where only the new G4 Mistic Brains would get kicked offline. Sometimes the brains would reconnect no problem, while other times it took a couple of power cycles to get them chatting again. Looking at the new G4 Mistic Brain boards it was obvious that there had been some hardware revisions. I checked with Opto22 for any know caveats for the new boards in an old system and they knew of none, but they did send me down the path for correcting the problem. Cabling. As it turned out, six feet of cable was to blame for the intermittent comm issues. The original contractor who installed the system did not use twisted pair wire for the panel to panel jumpers. While the old hardware revisions did not seem to mind the new ones did. After installation of the correct cable for 485 all of our comm issues have cleared up completely.

Seeing that you have a mix of Mistic and newer brains I thought I would share my experience. Older hardware may be more tolerant to improper cabling.


#3

Hi Nick,

I tried replying over a week ago but I guess the post is held in moderation…

Long story short, I don’t have any specific tests for healthy comms but I did recently have an intermittent 485 problem after adding new hardware revision G4 Mistic Brains. The new G4 brains were occasionally getting kicked offline. Turned out the problem was cabling from the original installation. The short two foot sections of cable that jumpered the mistic racks was not twisted pair. Replaced 6 feet of wire and the comm issues have disappeared. Maybe you are having a similar issue…


#4

hi chris
thanks for the advice. all of the cabling between the brains has been run in cat5/6. the only issue i could have seen is that there is no shielding on the cable.

as it turned out it was a wiring issue that caused this issue. there was a loose wire on one of the brains at the end of the line which, after correcting, fixed the problems straight away. i think the wire was making contact enough to not pose a problem until the wiring in the cabinet was disrupted and problems started from there.

damn cables!!


#5

Yes Nick, been there more than once, done that more than once. I had that same problem last year, and it wasn’t until I got to pulling on and messing with all the serial cabling that I found it. In fact, it seems the most of the hardest to solve hardware problems come down to improper wiring deisgn. Now to be honest, I use Ethernet cable for serial cable all the time because it is cheap, and it has essentially the best characteristics for serial. The down side is that is uses solid 24 ga wire which is notorious for breaking (versus using stranded) when used for terminal block connections (hence I guess one of the other reasons they came up with RJ45 connectors). The problem is that it is so hard to see a broken (or not quite broken) 24 ga wire. I think the biggest reason is in the process of stripping the wire, it is very hard not to nick the wire. The nick then manages to break later on after you do the checkout. BTW, I use a cable by a manfiacturer called Shireen. They have cat 5 shielded with a outdoor rated sheath that is reaasonably tuff.


#6

Hi Chris -
Re:

…I guess the post is held in moderation…

Sorry your message got stuck in “moderation,” (this whole “Products” forum had an incorrect setting to moderate replies).
That’s been corrected now. Hooray for free speech and thanks for the helpful replies!
-OptoMary