Long distance control - RS485?

[Cmdr_Vimes]

I'm working on a project controlling a simple robotic crawler. Since the crawler is going underground through varying different materials I am opting for a wired control rather than wireless.

The cable would end up being a couple of hundred metres long. Does anyone know of a good method of control over that distance? I am considering using RS485 but I am worried about the voltage drop over such a distance. Does anyone know of a level shifter that could raise the data signal to, say, 20V?

 

[marcbarker]

EIA485 or LVDS is simple. Voltage drop is expected, or you're paying too much for cable. You can budget for something crazy like 90 % signal loss! As long as there's about 50 - 100 mV at the remote end itll work.

If you're worried about the distance, just get a big drum of the said cable and try it, I think you'll be suprised. If the command data rate is low (very likely), you don't need fancy low loss cable, you could probably use doorbell wire and it work just as well. After all telephone cabling is something like 22 AWG and can carry mental ADSL data rates OK.

 

[dougy83]

RS485 is suitable for that distance. Maximum data rate decreases as the distance increases, but RS485 is good for over 1km. Have a look at: RS485, specifications and in depth tutorial

 

[Mr RB]

I would choose a cable with high flexibility (fine strands) and multicore, and also parallel say 2 or 3 separate wires to make each of your signal lines in case the cable gets nicked and a wire gets cut or breaks at the connector etc.

 

[Cmdr_Vimes]

Thanks guys. The cable spec only really allows space for one RS485 twisted pair. The data rate will be far below the 100kpbs limit at 1200m (thanks for the link dougy83).