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

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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



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

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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.

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I haven't done that stuff for a while, but RS-232<->RS-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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Re: Connecting two computers over a distance of 500 meters (RS-422 or RS-485)
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B&B Electronics at http://www.bb-elec.com/welcome.asp 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
http://www.rentron.com


Re: Connecting two computers over a distance of 500 meters (RS-422 or RS-485)
On Wed, 7 Sep 2005 18:34:11 +0200, "Stefan Mueller"

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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

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

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The PC UART won't do it, but there are drivers that achieve
3MBit @ 500m.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Re: Connecting two computers over a distance of 500 meters (RS-422 or RS-485)
On Wed, 7 Sep 2005 18:34:11 +0200, "Stefan Mueller"

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

Regards,

                               -=Dave
--
Change is inevitable, progress is not.

Re: Connecting two computers over a distance of 500 meters (RS-422 or RS-485)
On Wed, 7 Sep 2005 18:34:11 +0200, "Stefan Mueller"

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Hopefully these wires are arranged as two twisted pairs.

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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.

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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.


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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


Re: Connecting two computers over a distance of 500 meters (RS-422 or RS-485)
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What do you mean by "fail safe" termination? Optoisolation? Or biasing high
when all drivers are off?

Ross



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

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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).

--
Grant Edwards                   grante             Yow!  Did you move a lot
                                  at               of KOREAN STEAK KNIVES this
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Re: Connecting two computers over a distance of 500 meters (RS-422 or RS-485)
On Thu, 8 Sep 2005 08:46:12 +1000, "Ross Marchant"

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"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
 

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

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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
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

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

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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.

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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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                                  at               BIO-NEBULATION!
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Re: Connecting two computers over a distance of 500 meters (RS-422 or RS-485)
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



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

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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.

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

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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.


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

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[Snipped]

Look at the Analog Devices iCoupler Series of Isolated 485 and 422
buffers, as well as at http://www.nve.com . Both companies use
Giant Magnetoresistive materials to transmit the data across an
isolation barrier. These buffers can handle up to 40Mbps.

Regards
  Anton Erasmus


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

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




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

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

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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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