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Trying to use dormant telephone wire through house walls for null modem/terminal stuff

Bill-kun

Bill-kun

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Nov 2, 2020
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686
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Michigan
I had this idea some months ago. My house came with landline telephone wire, with ports in 3 different rooms. I haven't had a landline telephone since I moved in, so the line is dormant. I'd like to see if I can use it for null modem transfers. Is it as simple as plugging the wires in, then using a breakout block on each end to map them to the right serial port pins? The lines have 4 contacts, and 3 contacts is the minimum for serial null modem, so the hardware is there.
 
Make sure to disconnect it from the distribution block. Dont wanna throw 48v into a serial cable!
 
How fancy do you want to get? I'd recommend 4-wire RS-422 if you want noise immunity, or, alternatively RS-485.
2-wire full-duplex is possible with more sophisticated gear.(e.g. CANbus).
 
RS-485 would be good as you could use a multidrop configuration and have several machines working.

I suppose you could also find some gandalf style modems (like a LDS 120) if you have four wires.
 
I had a similar situation when I moved in here. My main office is detached from the house quite a bit, in the garage (HUGE garage). I thought about using the existing pots wiring for temporary internet till I could run a line, ended up just using powerline networking.

Perhaps thats an option here as well. Wasnt expensive at all. By time you buy everything and fiddle around, its 20 dollars or so for a powerline networking starter kit. I ended up pulling around 120mbps off it. And thats through a second power meter.
 
You could use two ethernet to VDSL2 devices

www.saratota-direct.co.uk

VC-231 - Ethernet over VDSL2 Converter (Profile 30a)

*For the Latest / New / Upgraded Version please see the Planet VC-231G* Overview The VC-231 also defines symmetric band plan for the transmission of upstream and downstream signals, the band plan performs higher transmission quality in short range for central side (CO) in symmetric mode. In all
www.saratota-direct.co.uk www.saratota-direct.co.uk
 

30 bux out the door, Ive seen peeps get 300mbps off em, Ive done over 200 in this house made in the 30s. Point is for the price you spend in time , effort, and materials unless you have gear to recycle, this is far better. And best part, internet wherever the is an outlet, not just 3 jacks.
 
He says serial null-modem and 3-wires minimum. So he's speaking about RS-232, which should be obvious anyway. This means that pretty much all answers so far were off. ;)

Whatever, RS-232 with 3 wires means software flow control. While possible, it limits speed and cable length. Just try and see if it works stable. With lots of luck, you may get 19,200 bps, but I would not expect more that 9,600. As twolazy said, make sure the cable is no longer connected to the distribution block, as you will otherwise destroy any serial port you connect the wires to.
 
twolazy said:

30 bux out the door, Ive seen peeps get 300mbps off em, Ive done over 200 in this house made in the 30s. Point is for the price you spend in time , effort, and materials unless you have gear to recycle, this is far better. And best part, internet wherever the is an outlet, not just 3 jacks.
Click to expand...

Powerline Ethernet adapters are nice, but they aren't a silver bullet. I've installed dozens of these things and they can be extremely temperamental.

There are two scenarios that will stop these devices from working well or at all.

1) If the house has a jack panel or split feeds for different parts of the house. This is common on large houses or really old houses that may have had improperly built additions added later on.

2) If there's a problem with the plugs or romex in the walls. Bad connections on plugs will cause intermittent connectivity or extremely poor throughput.
 
Timo W. said:
He says serial null-modem and 3-wires minimum. So he's speaking about RS-232, which should be obvious anyway. This means that pretty much all answers so far were off. ;)

Whatever, RS-232 with 3 wires means software flow control. While possible, it limits speed and cable length. Just try and see if it works stable. With lots of luck, you may get 19,200 bps, but I would not expect more that 9,600. As twolazy said, make sure the cable is no longer connected to the distribution block, as you will otherwise destroy any serial port you connect the wires to.
Click to expand...

Not to be too picky (and in no way offended), but to clarify: I did realize he was speaking of RS-232 and I only recommended the link for its discussion of using the wiring inside the wall. On that point, the 2 projects are similar. I figured the rest of the link's project regarding installing the PBX, was irrelevant to Bill-kun's project.

Also, this is definitely "your mileage may vary" as I've connected 2 computers together using a 50ft (~15m) 4-wire phone line extension cord and successfully transferred large files at 38,400bps and on occasion 56,700bps. I found that the resources of the computers involved were the real limiting factor. The question to my mind is how long is the wiring between each room in the house? Also there is the interesting question of what would happen if more than 2 computers are hooked-up to the phone lines at the same time? (Regardless of whether or not they are trying to access the phone lines, of course.)

As far as twolazy's good recommendation is concerned, accomplishing this in the US may mean having a phone company technician come out and check to be sure nothing is connected. But, depending on how recent the outside utility box for the phone lines was upgraded, you should be able to easily do this yourself by opening the subscriber access panel and unplugging the line-in jacks.

@Bill-kun: I think the procedure will be just as simple as you suggest. I'm very curious myself to see how well this works and look forward to hearing more about it from you.
 
Timo W. said:
He says serial null-modem and 3-wires minimum. So he's speaking about RS-232, which should be obvious anyway. This means that pretty much all answers so far were off. ;)

Whatever, RS-232 with 3 wires means software flow control. While possible, it limits speed and cable length. Just try and see if it works stable. With lots of luck, you may get 19,200 bps, but I would not expect more that 9,600. As twolazy said, make sure the cable is no longer connected to the distribution block, as you will otherwise destroy any serial port you connect the wires to.
Click to expand...

He also says he has 4 wires.
 
Is it as simple as plugging the wires in, then using a breakout block on each end to map them to the right serial port pins?
Click to expand...
Yes.
RxD, TxD and a signal ground are the basic pins you need to do null modem. Since you only have four wires in the wall your breakout blocks will have to force either RTS/CTS or DSR/DTR.

Two things of note however:
-RS-232 begins to get funky if cable length exceeds 50 feet. Expect issues if it's longer.
-You mentioned you have three different ports in the house. If you were only planning to use two that is completely fine, however RS-232 doesn't work correctly if the cabling has to go three different directions. Actually I don't even know if you can wire it three way, but keep whatever is on the third leg powered off when it's not needed. You can test that but if it doesn't work there's not a lot else you can do.
 
If you were trying to make old Macintoshes chat with each other in this situation then it'd be real simple: PhoneNet to the rescue. There are stories of people running Localtalk over barbed wire fences thanks to the magic of RS-422.

Honestly I probably wouldn't bother with this. I've lived in old houses (including the one I'm in) where the in-wall wiring is so bad it acts like a crystal radio... you can literally hear AM on it. Basically unless you go to the junction box and re-jigger it into separate point-to-point connections you're going to essentially have giant messy antennas hanging off your serial wiring. Worst case you might get enough noise and static to blow out the transceivers on your serial cards. You can get those Wifi-to-Serial adapters for around $60 each, maybe that's the way to go if you really want "null modem" connectivity.
 
Well, the dormat telephone wire is more complicated than I thought. I wish I knew the history of the use by previous owners. The dormant telephone box on the back of the house has 3 lines in it, each with different landline telephone number written on it. There are 4 wall plates in the house (not 3; I miscounted), and one of them has 2 ports in it. When I tried using by telephone wire tester in several combinations of where the "master" module was and where the "slave" module was, the maximum number of lights I got was 1 on one of the numbered lines, and 1 on another of the numbered lines. The third line didn't light any lights.

What the heck? The wires in the house wouldn't just disintegrate. Other than calling a telephone repairman and explaining my lovely eccentric idea to him on a house call, what can I do to find out more about the infrastructure I have, or thought I had? Is it possible an animal in the crawlspace chewed through the wires at some point in the past? Actually, that does sound like the most likely explanation, now that I think of it.
 
The best person who can map out your phone's wiring is yourself. It took me an afternoon but I know where every wire runs from the closet.
Also yes I have opened panels in the floor and found mouse chewed cabling. It's a thing.
 
I guess I’m lucky. The person that owned the place before me did a complete remodel, and all phone lines run to a box that was hooked up to cable to provide phone service, so all end in a known place. Maybe I should see what I can do with them one day…
 
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