RS - 485 on wrong cable

[Noway2]

I was visiting a customer's installation that uses a building managment system to monitor the real time status of our equipment via modbus.

During my investigation, I discovered that the installation is wired with a combination of 45 Ohm cable interspersed with aproximately 6' sections of non twisted pair, alarm wire. Being rs-485 modbus it was designed to work with 100/120 Ohm cable.

While the technicians were able to cherry pick parts to make the system effectively "work", I am curious as to what effects this would have on the system. I would expect to see the usual effects of t-line mistatch (ringing, stair stepping, runt pulses, etc), but what else?

Anyone have experience with this type of arrangement that you could shed some light on what to look for when analyzing it?

 

[MacGyverS2000]

The downside would obviously be reduced noise immunity... it may work now, but when a new piece of equipment is installed it could make the entire system worthless.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

You will find that almost no one uses 120 ohm cable for 485. They use what's cheap or available. I would not expect any problems if you're talking less than a few hundred feet and a logical baud rate like 34.5k or less.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[controlsdude]

its a bomb waiting to go off if the install was done this way. A year from now or two you will be in there finding these splices again and replacing them to get the network back up again.

My slant on this is the equipment works because it has enough power to overcome these connections. later when you have lighting strikes and power outages, the equipment that communicates will become weaker than before and then you will start having comm problems.

So just wait until lightning strikes and then you can blame it on the lightning and charge them for it.

 

[Noway2]

I am in agreement with y'all, it is a failure waiting to happen. Fortunately in this installation / facility, lightning is not really a concern. Believe me, if there is a power outage here, it will be because nothing matters anymore.

We have setup a string similar to the installation in our own lab and are running some experiments and measurements on it. We are rapidly concluding that the system is very marginal and that it doesn't take much to put it over or under the margin. What is unfortunate is that the metrics used are like receiving a satelite TV signal - it there or it isn't and the preceived result is the same whether the signal integrity is 45% or 95%.

 

[itsmoked]

So what's the data rate?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Oh, and what's the approximate length of this kludge?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Noway2]

The data rate is 19,200 bps. This is higher than we recommend, but typical of most of the BMS systems that we interface to.

As far as the length goes, in our lab we are running several units tied in the string. Between some of the strings, there is say 50' of cable. Between others, there is a whole 1000' spool of cable. Of course, cable spooled up doesn't respond quite the same as if it were layed out flat, but should be well enough for our application.

We also ordered a spool of 'alarm wire', like is used in the customer's application, but it hasn't arrived yet.