Losing Control Signal Over 800 Foot Distance with RS-485


Thread Starter


I'm new to the MODBUS world and am trying to test various installation scenarios for a touchscreen fan speed control system my company is developing. In this case, the system works properly when I run a 2 foot twisted pair length from a cat5 cable. But when I connect an 800 foot spool, I get no control. DC Voltage still reads adequate on the distal end, but apparently the digital signal is getting lost in the mix. Could it be just signal noise from all of the cable being coiled up in a box?

Any insight is much appreciated, Thanks!
The title of your post seems contradictory with the body, but here are a couple possible scenarios:

1) Your title (RS485) is a typo and you're really running Modbus/TCP. If so, there's probably a bad spot in your 800ft spool of CAT5 cable.

2) You actually ARE running 485 and you're using a pair or two out of a CAT5 cable. CAT5 cable is not for RS485. It's not shielded, and may not have appropriate capacitance values and so on. At 2-20ft, it will probably work. If you need to run 800ft, get the correct cable for the signal. If you're using CAT5 to try to run RS485, I assume you're probably also not terminating. At short lengths, say under 100 feet or so, my experience has been that you can get away with not terminating, in fact it sometimes works better not to. At 800ft, you need to terminate.
Did you put a termination resistor at each end of the cable. The following link may be of assistance:


William (Bill) L. Mostia, Jr. PE
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FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

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RS485 is a balanced differential communications network. By balanced I mean the line impedance and capacitance has to be at a certain max level. You can find information about this on Google by typing in RS485. You are using Cat5 cable that is not suitable for distances of more than 300 ft. You need to use proper shielded cabling such as Belden 3105A or equivalent cable. In addition to this you need to have two 120 Ohm resistor installed in parallel, one on each end of the 2 wire trunk line. This enables impedance balancing.

Good luck,

The first thing I would try is connecting termination resistors on both ends, between A and B. The resistors should match the impedance of the cable, usually 120 Ohms...

My 2 cents
> what baud rate are you attempting?

The baud rate is 115200

> and have you attempted several baud rates?

No, 115200 is the baud rate that the programmer told me to run.
Twisted pair is not just any twisted pair. Use the proper cabling--and add the termination resistors.

And you just might find--it works.

Oh, and in the field, ground (earth) the shield drain wire properly--at one end only. Or you'll be thinking you weren't given good recommendations here. Improper shield drain wire grounding on twisted, shielded pair cabling is nearly as bad as using the wrong, unshielded, twisted pair cabling.

Seriously--use the proper cable, properly ground the shield drain wire, and add the termination resistors.
> No, 115200 is the baud rate that the programmer told me to run.

Sometimes people choose the highest baud rate available because they think faster is better. I try to use a baud rate that is fast enough to get the amount of data I need with the least amount of errors. For instance, If I'm moving 10 or 20 words once a second I use 9600 baud.

Curt Wuollet

Yes, but I wouldn't count on it out of the box either, wet string would probably work for 2 feet. You might consider the proper impedance shielded cable if you would like to use RS485 at any appreciable speed at that distance. Wire isn't just wire in this case, it's a balanced transmission line. With the proper cable, 800 feet should be no problem.

Higher the Baud rate, higher the attenuation.
So signal is affected in a short range itself.
If the issue is noise, try reducing the baud rate.
It wont drastically affect the process speed in non critical systems.

At lower baud rate your signal should travel in longer wires with less distortion.

Type of cable, Termination, Baud Rate, Distance etc are the factors affecting here.

If you detect connection at the other end, then Noise is the Villain.