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Is anyone an expert on RS485?


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I'm hoping one of our members may have a lot of experience on this data protocol! Apologies, but this is a work related issue and I decided it would be best in the members section as it involves one of my major clients.

Over the past ten years, I have installed several LED external signboards which are simply used to indicate to lorry drivers where to go to get their load of tarmac. All of them use a standalone PC with specific software to run the sign and all the user has to do is type in the message and click "update sign"

I've had no problems at all with previous installs but the latest sign I fitted works fine on the test bench but will only run when the tarmac plant is shut down ie when there is no vibration and less power being drawn. The sign has its own independant electrical supply and the RS485 data signal in a two wire circuit which runs via a protocol converter from a USB port.

In short, I've been stuck on this job for the past 2 weeks and each time I manage to get the thing working, it fails within a short time and the PC chucks out an error message "communication fail" To me it seems the signal to the display is either corrupted by something or possibly the data cable is damaged somewhere and when the plant vibrates, the signal cable is getting disconnected.

So far I've tried everything except running a whole new data cable to the sign but their old unit had run on the same cable for years without a fault!

Any ideas would be very welcome as if this carries on, my small business is going to be severely affected :cry

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My first thought is either port on PC or as you say data cable

Couple of questions:

Are you using the pc that runs the sign when on the test bench?

Have you tried using a laptop connected to the sign on site with a different cable ? (Might help diagnose if cable fault)

Will also have a word with a couple of techie friends


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Thanks Andy,

In the workshop I used my own laptop but I will try it instead of their PC...I never thought of that. Port settings on the PC they are using are correct and set for 9600 baud which is also right.

Looks like a spot of nightwork for me tomorrow as it's theonly time I can get access under the hot storage tarmac bins to run in the new data cable. I only wifh I knew exactly what the cause was as the blooming thing always works when the place is shut!

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Thanks Andy!!

I'm back there tomorrow (weather depending) at 5am to re-fit their old sign which also runs on 485. Somehow, last week, they had a power situation which corrupted all the letter fonts on their old sign and left our new one blank but otherwise ok.

Basically, my hope is that I can get there early tomorrow, test their old sign locally on the PC that has ran it for years and refit it to get the pressure off me. On Friday, between 4 pm and about 2 am I will run in two new Cat5 shielded cables from the control point to the new signboard which will allow a new cable connection to both. That way they can use their older unit and also verify that the new version stays working as it should.

Lord knows, I've fitted a fair few of these devices in the last ten years and never had a problem like this one...in fact no probs at all :cry

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