3
\$\begingroup\$

I am trying to find a standard (hopefully with a chip set) which can communicate over copper (twisted pair) for at least 2km, maybe up to 10km, using a baud rate somewhere between 10kbps and 100kbps.

I have looked at rs485, but this seems to max out at 1.2km even though I imagine dropping the baud rate might improve the range.

\$\endgroup\$
  • 1
    \$\begingroup\$ Is optic fiber communication an option? \$\endgroup\$ – Anindo Ghosh Jul 16 '13 at 9:25
  • 2
    \$\begingroup\$ It's certainly possible using a line driver - "Send RS-232/V.24 data over 24 AWG wire up to 6.4 km at 19.2 kbps or up to 24.1 km at 1200 bps!". \$\endgroup\$ – RedGrittyBrick Jul 16 '13 at 9:36
  • 2
    \$\begingroup\$ Are you allowed to have phantom powered data repeaters? Twisted pair is OK but screened twisted pair will work better over longer distances. \$\endgroup\$ – Andy aka Jul 16 '13 at 9:46
  • 1
    \$\begingroup\$ SHDSL could be an option. \$\endgroup\$ – Thorn Jul 16 '13 at 10:06
  • 1
    \$\begingroup\$ ... or ISDN BRI chips would give you 144 kbps over that distance (and more). \$\endgroup\$ – Dave Tweed Jul 16 '13 at 11:06
3
\$\begingroup\$

For the 10km distance you want I think you'll be lucky to avoid repeaters (phantom powered or otherwise).

Cable type is a big consideration. Standard RS485 cable appears to attenuate at 2dB per 100 metres and here's what Linear technology believe the max data rate to be with respect to cable length: -

enter image description here

At 10kbaud, 4,000 ft (1.22km) is the limit but that is for Belden 9842 cable (designed for RS485 communications). It has 2dB attenuation every 100 metres at 1MHz.

If instead, you chose Belden 9860 you'd get only 0.66dB per 100 m at 1MHz.

A simple calculation will tell you that 9860 (not designed specifically for RS485) will give a distance of 1.22km x 3 = 3.6km for the same attenuation.

I'd also consider using FSK for transmitting the data.

EDIT You'd probably get double that range and if you could drive FSK with 20Vp-p you might get 10km. More volts means it goes further for the same receive signal to noise.

The graph provided by Linear tech is interesting because of it being curtailed squarely at 1.22km and 10MHz - these is not related to actual physical characteristics of the cable and, I suspect, you could reasonably predict that at 10kbaud, the transmit distance is more approaching 40,000 feet or 12.2km.

\$\endgroup\$
1
\$\begingroup\$

10km is not outlandish, especially if you can use more than one pair. In telecomms, equipment known (to me, in the UK) as CWSS or HDSL (equipment made by ECI Telecom and Fujitsu respectively) was used as the standard solution in the national network to provide standard 2Mbit links (EG for ISDN30 phone systems) or 1Mbit data over short distances on single-pair. It was originally designed as a quick-fix whilst waiting for fibre to be installed but proved good/cheap enough (probably more the latter) to become a permanent fixture.

The kit uses 2 pairs as standard for 2Mbit, 3 pairs for long lines or poor/noisy cables, or 1 pair for short runs at reduced speeds of 1Mbit.

For slower speeds (<1Mbit), very old (1970's - 1980's) kit called Kilostream was used, although I doubt you'd find anything much about that of any use.

Base-rate (64/128k) single pair ISDN is very robust and the kit is not complicated, I don't know how hard it would be to find the "serving" end though. The customer end (NTE) are common and disposable, I've probably got an old one laying round you could have for the price of postage.

All this is constant rate bi-directional, so not subject to speed fluctuation or asymmetric data rates like DSL.

Unfortunately all these technologies are designed to be run from a serving exchange, so the customer end equipment is very available but the other end really isn't, and is usually designed to slot into a 19" rack shelf serving multiple of customers.

There will be international standards for all this kit as nothing goes into the telecomms network without it. Whether you can find them, and sustain the will to live whilst reading through them, is another matter.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.