How long can the RS 485 cable be between 2 devices?

 

By using RS-485, a maximum of 32 devices can be connected in daisy chain network. And the maximum distance allowable is 1200 meters.

This communication cable should be laid separately from electrical cables and electrical trenches.

 

It's dependent on the speed.
Can't remember off the top of my head.

 

Hi there,

The cable length is depended on what baud rate that is used. For more info:
http://en.wikipedia.org/wiki/EIA-485

 

The longest distance supported by the serial communications is 100ft, normally that value applies on RS232. If you are using 485 distances between 10 & 20 ft, it's your answer.

 

"No maximum bus length is given for RS485, but ends up around 1200 meters at 200kbps or 50 meters at 10Mbps"

Reference:
http://www.interfacebus.com/Design_Connector_RS485.html

Issac

 

I see several posters give 1200 meters. I am not disputing that, but is it really practical?

I don't think it's reasonable to have a separate cable trench or tray just for the communication cable.

At what distance would it be more cost effective to use fiber?

Roy

 

Roy,

If some place already has 1200m twisted pair in the ground, why not (besides noise, degraded cable, common mode)?

If not, then I'd certainly look at wireless before laying either FO or copper.

Issac

 

i had work with mod bus rtu protocol using rs 485. i didn't calculate the distance, but the system is working perfectly with seven individual panels connected to a remote scada via modbus..may be the distance between the panels will be around 1200 meters..

 

Hello Roy and all,

I am also not so sure about this 1200 Meters distance, but what could be maximum possible distance for RS-485 2-Wire Communication PRACTICALLY?

Do you have any idea as per any practical experience of some previous cases or projects.

Please let me know.

Regards:
Prateek

 

I suspect RS-485 will work at longer distances than the usually cited maximum 1200m (at low baud rates) from 2 different farther-than-the-spec non-485 experiences.

One experience involved a half duplex, no handshake 3 wire (Rx,Tx, Gnd) RS-232 link with the usual 50ft (15m) distance limitation. The plant "electrician" had run 200ft (60m) of 14g Romex (parallel, not twisted pair) and connected the devices. I got called when it didn't work. After changing the wiring on one end so Tx was connected to Rx instead of Tx, the link came alive at 4800 baud, if I remember correctly. I was astounded it worked -non-twisted pair, no shielding, longer than the max-spec. That was turn-of-the-century era and it's still running today.

The other situation involved a long weekend (3 day) community fair at the local park that needed a single point internet connection in the early 10Mb internet days, before WiFi was a gleam in anyone's eye, sometime in the late 1990's/early 2000's(?)

The intended service was to update the community's new web site with a web cam photo once a minute to showcase the fair. The nearest household with a "broadband" internet connection (that they could 'borrow') was through the woods on the backside of the park. The organizing committee paid for a big spool of CAT5 and had a couple of teenagers run it through the woods, loosely stapling the cable to a tree every so often. My recollection is that it was about a 1,500 ft (500m give or take) run. The link worked great for that level of service. The committee sent emails, got their once-a-minute web cam updates where attendees oohed and aahed that they could see nearly instant cam updates "over-the-internet".

My recollection is that IEEE's Ethernet spec is 100m. But light duty service will work at longer distances.

At lower baud rates, you might well push RS-485 longer distances than 1200m.

I suspect the distance limitation is the rounding effect capacitance has on the leading edge of a bit. Lower baud rates extend the mid-bit UART "read" so are lower baud rates are more "tolerant" of capacitive distortion.