RS-485 Communications
RS-485 Communications
(OP)
Because my current application uses 485 comms for an industrial application I figured I'd start here and move the thread if I have to. My current setup uses a standard 485 communications network through a multiplexer (refereed to as a repeater) to the PLC to save comms channels. At one of the plants where this system is setup they were adding new equipment and arc welding way to far from their ground point and ended up killing the system. As it turns out they blew a comms channel, the repeater, and several nodes. So as me and my crew are putting the pieces back together we started having some very strange 485 problems. After replacing all the equipment on the damaged runs including the cabling we started having problems talking with nodes in the middle of the new run. Figuring it had to be a faulty repeater I replaced that again with the same results nodes at the beginning of the chain would communicate while the ones in the middle would not. After spending countless hours slogging through boards and other possible problems we got exasperated and started trying all sorts of weird stuff to get the network back up. As it turns out we tied all the node strings together and ran a single pair of wires back (as in a star topology) and IT FREAKING WORKED?! I was always told that you could not operate a 485 network in that configuration or without a terminating resistor, but somehow it's working. Now for the second oddball part, after taking all the bad equipment back to the shop to test it I find that apparently the number of the node makes a difference as to weather or not it works. I have no idea what is going on but if anyone has some information about why a star topology would work for RS485 or why a node number would cause comms problems I would appreciate the insight.





RE: RS-485 Communications
Attempted baud rates, spur length, total link length, grounding, isolation, common mode, biasing and terminating or lack thereof all contribute to whether a system works or not.
Your spur topology might work at 9600 but fail miserably at 19.2k, or the first 5 nodes work OK until the 6th node is attached, and then none work, due to biasing or loading. Or what works on the bench fails when the much higher common mode in the field saturates the drivers.
The biggest fallacy is 2 wire RS-485, where ground reference is needed somewhere, but many systems ignore 3rd wire grouding altogether. Heck, the 2 nodes worked on the bench as a Beta prototype, didn't it? Then ship it ! !
Out of curiosity, do you know if the repeater isolated the damage from one segment to the other segment side of the repeater? Most commercial repeaters are isolated, side-to-side, but I'm sure someone sells a non-isolated repeater, there's always a market for cheap.
The best working reference I know of is here:
http://sss-mag.com/pdf/485appno.pdf
National Instruments' web note on biasing is pretty good, too:
http://www.ni.com/support/serial/resinfo.htm
RE: RS-485 Communications
RE: RS-485 Communications
http://www.robustdc.com/library/an006.pdf