Comment by pjdesno | Hacker News Reader

Comment by pjdesno | Hacker News Reader

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I'm surprised that you could get a modem pair to connect with a plain wire. An old dial telephone (e.g. a 500 set) was an entirely passive device, relying on power from the central office end of the wire. The carbon mic didn't need an amplifier - it's just a resistor, with resistance varying according to sound pressure; a transformer subtracted out the mic from the combined signal and sent the difference to the speaker.

I did a quick search on 1670 schematics and came up with very little. Note however that several sources identify the USR101 chip as a ROM - that's clearly not the case, as a schematic shows no address or data bus, and you can see a line from the coupling transformer to an RC network and several pins on the device. It's almost certainly the analog front end and the modulator/demodulator.

If you could find a datasheet for that device you'd probably be able to figure out how the whole thing works, but I'm guessing those were never available to the public and have been lost to the mists of time.

Battery power on common battery phones (in a common battery phone there isn't actually a battery, the terms here are mostly historic) is required for a carbon transmitter to work and, for DTMF dialing, it's used by some DTMF keypads. But battery isn't actually that important to most modern phones, which use ADC/DAC instead of an analog audio setup. Modems are similar, they have their own audio electronics that rely on their external power source. That's a good practice in general because battery power on phone lines is pretty variable and not always very clean, so you don't want to run ICs off of it if you can avoid it. The current is also too limited to use it for much, even indicator lights will run into certification problems.

The issue with the USB modems might actually be a bit different... more frequent use of single-pair digital business telephone systems in the '90s lead to problems with people connecting them to conventional modems. Some of these digital systems used unusually high battery voltages (sort of a PoE-esque method of providing meaningful current to the desk phones for their lights and displays), and there was a concern that they could damage the ADC on modems. So a lot of newer modems actually check the voltage on the line before they connect the audio electronics to it, as a safety precaution. There's an AT response that indicates that the modem is unhappy with the line voltage and I tenuously remember that at least some modems have an AT command to defeat this protection and make them connect anyway. Digital Line Detect is what the feature is usually called, but you might more generally call it a line voltage check.

I used to work at an ISO in 1997 and we tested Mototola Sufr 28.8 and USRobotics Sportster 33.6 modems this way, just a short pair between them back-to-back. ATX1D on one side, ATA on the other, connection established.

However, recently a friend of mine tried to use the same trick with two USB modems from Trendnet and it failed. It seems they either don't have oomph enough or they rely on 9vdc off-hook carrier voltage to work.

Cool! I was an intern with the modem folks at Motorola Codex back in the 80s; I probably worked with some of the folks who did the data path on that one.

(hmm, actually by that time it's likely that all the Codex tech had been killed, and your modem came from the Motorola UDS folks in Huntsville. Makes sense - Motorola Codex couldn't build a $500 modem to save their lives, never mind a $100 one, and a lot of the folks interested in building consumer modems left to found Zoom Telephonics)

> 9vdc off-hook carrier voltage to work

In the US it's nominally 48v, although it drops to a small fraction of that with any significant (or even fairly insignificant) load.

I'm sure I found the same sources you just did. USR101 is surely not a ROM by this [0] schematic because the speaker audio goes directly to one of the pins on it - that doesn't seem right.

I guess enough signal makes it through T1 to let it work on dry lines. That sort of thing isn't unheard of [1]. I used to mess around with obsolete dial up modems as a kid and I could definitely get them to connect with just wires - sometimes. It didn't always work, and some modems would not work with other ones, etc.

The weird thing here is the 1670's second long pulses. Maybe it would do something else if connected to a real phone line? I'd surely try it myself but I only have a 1660 modem on hand.

0 - https://www.zimmers.net/anonftp/pub/cbm/schematics/misc/1670...

1 - https://en.wikipedia.org/wiki/Sound-powered_telephone

People used to use barbed wire fence for phone lines.

https://www.atlasobscura.com/articles/barbed-wire-telephone-...

As of 2019, there were places in Arizona and New Mexico where telephone line was still run along the top of fenceposts, alongside the barbed wire.

Here's 1gbit running over 8 strands of barbed wire: https://www.sigcon.com/Pubs/edn/SoGoodBarbedWire.htm

Minor nit, the provided link shows 100Mbps, not 1Gbps. That being said, if the article is taken at face value, running 1Gbps would likely also be possible with a similar setup.

It doesn't even need a wire - wet string is enough: https://www.revk.uk/2017/12/its-official-adsl-works-over-wet...

ADSL works completely differently than audio as described by pjdesno.

Kids these days with their fancy copper wire, back in MY day we used CANS and STRING.

See https://hackaday.com/2024/11/19/dial-up-internet-using-the-v... for a description of a telephone line simulator.

Some modems want to detect the line before they will work (voltage and/or dialtone), but if you can get past that with settings or a battery/etc, I'd think you can just listen and send audio on a plain wire and it should work.

Yeah, getting a direct-wire modem connection always seemed like one of those things that shouldn't work but somehow does

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