Connecting two computers over a distance of 500 meters (RS-422 or RS-485)

I'd like to connect two laptops over a distance of 500 meters. I've a copper cable with 4 wires. I've no idea if I should use RS-422 or RS-485.

Because I'd like to have the connection power independent I guess I should use an USB to RS-4xx converter.

Can someone tell me if I should use RS-422 or RS-485 for a stable, full-duplex and fast connection between two laptops over a distance of 500 meters? Does someone even know a good product?

Stefan

Reply to
Stefan Mueller
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Either one. Electrically, they're almost identical. If you do RS-485, make sure you're adapter supports "4-wire" mode so you can do full-duplex.

I haven't done that stuff for a while, but RS-232RS-4xx converters and RS-4xx adapters from Opto22 were always completely bullet-proof. Stuff from BlackBox was usually OK as well, but quality varied.

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Reply to
Grant Edwards

B&B Electronics at

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have a few RS232-RS485 models with isolated power and ground generated internally. No power is drawn from the host RS-232 port.

I've used their products in the past with great results, and their tech support folks can probably advise on which unit would best suite your application.

Regards,

Bruce

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Reply to
Bruce

If it's twisted pair, you can use VDSL modems, e.g. Allnet ALL126M/ALL126S. You won't get 10 MBit/s with RS422 or RS485.

Mit freundlichen Grüßen

Frank-Christian Krügel

Reply to
Frank-Christian Kruegel
[...]

If you need full duplex, RS-422. RS-485 is half-duplex.

Regards,

-=Dave

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Reply to
Dave Hansen

Hopefully these wires are arranged as two twisted pairs.

Since you need full duplex and have 4 wires, RS-422 would be the natural selection.

If you only have those 4 wires, please note that the "fail safe" termination must be used to get a proper ground potential reference. If the cable also contains a shield, it could be used as a ground potential reference and thus, the "fail safe" termination would not be mandatory.

If your PC (or PCMCIA card) does not have the RS-4xx interface, you definitely need either the USB or Ethernet to RS-4xx converter.

While I would normally specify optoisolation for such long distances (500 m) to avoid any ground potential problems, however, if at least one system is a laptop powered by own batteries or powered through a portable power supply with a 2 terminal "Euro" mains connector and the laptop is not grounded (through other peripherals), the laptops are freely floating, so it would be OK to have a galvanic connection between the two PCs.

Lightning protection would be an issue, but on the other hand, optoisolated cards with only 500-2500 V isolation might not survive much better.

Paul

Reply to
Paul Keinanen

The PC UART won't do it, but there are drivers that achieve

3MBit @ 500m.

Rene

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Reply to
Rene Tschaggelar

Unless the environment is controlled, eg a shipyard or something else made as one piece, I'd care about ground problems. The RS4xx drivers have common mode voltage immunities of just a few volts. Possibly a pair of fibres is a better choice.

Rene

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Reply to
Rene Tschaggelar

That's why for a couple dollars more, you buy the galvanically isolated adapter. All the ones I've ever seen were optically isolated, but in theory they could use pulse transformers or something more exotic.

He's already got a 4-wire cable installed, so I doubt that a pair of optical fibres is a choice at all. Certainly the incremental cost of galvanic osolation in a pair of 4xx adapters is going to be a tiny, tiny fraction of the cost of pulling half a km of fibre.

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Reply to
Grant Edwards

You mean that I just need to buy two galvanically isolated 'USB to RS-422' adapters and I shouldn't have any problems? What network speed will I have over a distance of 500 meters?

Is RS-422 better than the 4-wire RS-485?

Stefan

Reply to
Stefan Mueller

What do you mean by "fail safe" termination? Optoisolation? Or biasing high when all drivers are off?

Ross

Reply to
Ross Marchant

Should work.

It depends on the cable. With decent twisted pairs you should be able to run in the 100Kbps range. With individually shielded 18AWG twisted pair you can probably get to twice that.

RS-422 is virtually identical to 4-wire RS-485. IIRC the differences in the specs had something to do with receiver impedances and the max tx->rx fanout being slightly different. Since you're wiring them up 1:1, it doesn't matter.

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Reply to
Grant Edwards

The latter. You need to terminate both ends with 100-150 Ohm termination resisters (pick a value that matches the characteristic impedance of your cable) and also pull one line up (+5) and one line down (Gnd) with something like 4K resistors (opinions vary on the best value of the pullup/down resistors).

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Grant Edwards                   grante             Yow!  Did you move a lot
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Reply to
Grant Edwards

You didn't mention what bitrate you need, or if cost was a factor or if you have control over the programs running on either laptop, or if bit-level timing is an issue.

I once connected two laptops through about 50m of wire by using the built-in modems. I set one modem to "answer" and the other one to "blind dial" with a fake telephone number. I got a very reliable full duplex connection at 28800 bps (with error correction) and I didn't have to buy any extra hardware. However not all modems will allow you to do this, some modems will simply refuse to do anything if there's no DC voltage on the line. Also I have no idea if this would actually work over 10 times the distance that I used. But if you're looking for a quick and dirty solution this might be worth a try.

--Tom.

Reply to
Tom

"Fail safe termination" is the expression used in the RS-422 and RS-485 standards for biasing the receiver into the Mark space even when there are no active transmitters on the bus (or when the cable is disconnected).

Paul

Reply to
Paul Keinanen

That sounds like the ideal case for 500m of cable, but probably achievable. You should also check the maxium speed limitation of the opto-isolation device.

best regards, Johnny.

Reply to
Johnny

I don't need a specific bitrate. I just use 'net use' on my two Windows 2000 laptops to map a couple of network drives. With my old RS-232 to RS-485 converters I've a bitrate of 56kbps. But they are not power independent. Therefore I'm looking for an USB solution. But I've no idea which product to buy.

Stefan

PS: If the new bitrate is higher I'm not unhappy ;-)

Reply to
Stefan Mueller

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Look at the Analog Devices iCoupler Series of Isolated 485 and 422 buffers, as well as at

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. Both companies use Giant Magnetoresistive materials to transmit the data across an isolation barrier. These buffers can handle up to 40Mbps.

Regards Anton Erasmus

Reply to
Anton Erasmus

AD the coil of a PO relay in series with a 50V DC supply and connect the modem pair in parallel across that. This will satisfy the

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Paul E. Bennett ....................
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Reply to
Paul E. Bennett

What I was trying to say before either finger trouble or an un-expected glitch posted this before I had finished composing was:-

Connect the two modems back to back with a single pair of wires (paralelled)and use a telecommunications relay coil in series with the 50V DC supply to provide the right line conditions. You may even use the relay contacts to do something about providing ring current (or indication) to the called end. There are circuits in telephony books that might give you all the information you need.

Others with telephony knowledge may possibly post an ASCII art circuit (nudge).

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Paul E. Bennett ....................
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Paul E. Bennett

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