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From: ftpam1@acad3.alaska.edu (MUNTS PHILLIP A)
Newsgroups: comp.sys.tandy
Subject: Re: Modem to Modem minus telephone network
Message-ID: <1991Feb26.181823.17434@ims.alaska.edu>
Date: 26 Feb 91 18:18:23 GMT
References: <1991Feb26.094452@snoc01.enet.dec.com>
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In article <1991Feb26.094452@snoc01.enet.dec.com>, cameronjames@snoc01.enet.dec.com (James Cameron) writes...
>For a charity canoe race checkpoint, I want to connect two Tandy 102 laptops
>over a distance of around 500 metres, with the least wiring possible.  The
>machines have integral direct-connect Bell type modems as well as an RS-232
>connector.  [fyi; the Bell modems are otherwise useless in Australia]
> 
>My options appear to be
> 
>	1)  RS-232 cabling (two core plus shield earth),
>	2)  Standard telephone cabling (two core, twisted pair),
>	3)  Field telephone cabling (one core, with earth return),
>	4)  Radio-modems.
> 
>I want to avoid option 1 due to the high cable cost and the risk of damage due
>to electrical storm activity and other forms of induced EMF.  Option 4 looks a
>little complicated unless I can find prebuilt radio modems that I can just plug
>in and run.  So down to options 2 and 3...
> 
>I guess for these I'd use a telephone power supply, but I don't know how to
>build one nor how it is connected.  Can anyone help me with this?  How would I
>achieve single core operation?
> 
>I've seen the recent discussions in sci.electronics about FM transmitter chips
>for radio modems, but does anyone have any information on prebuilt units?
> 
>Regards...
>James Cameron
>Digital Equipment Corporation (Australia) P/L
>(cameronjames@snoc01.enet.dec.com)

     Telephone modems often operate fine without the -48V supply from the 
telco line.  The trick is to get them to dial without waiting for the dial
tone; this will depend on the modem and perhaps how it is programmed.

     Another option would be "short haul modems", which convert RS232 to
RS422 and are good for a kilometer or so.

Philip Munts N7AHL
NRA Extremist, etc.
University of Alaska, Fairbanks