ASCII by Jason Scott

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Ring Detect —

People sometimes hit me up with simple questions that you’d think I could just dash off, but a lack-of-pithiness reaction stops me from doing so. Sure, I could whip up some off-the-cuff answer, but it wouldn’t be accurate, and that always stumps me. A classic one is “What’s your favorite arcade game?” There’s about five of them, for different reasons, but I feel a pressure to answer with the favorite and I still don’t know what that is.

But another question, more rarely asked, is “What was something cool you learned making the documentary?” This one, after I’ve had time to mull it and decide, is easily answered:

Dial-Up Bulletin Board Systems predate auto-answer modems.

Isn’t that crazy? The logical way people might assume, looking back, would be “auto-answer modems and home computers show up, someone puts them together, and tah-dah, BBS”. I know I certainly did.

But in fact, when the first BBS is assembled in 1978, auto-answer modems had not yet hit the market. Instead, hackers trying to piece together such an item utilized ring detection circuits.

The fun part is that now people are starting (just starting) to have a situation where ring detection won’t mean the thing it used to, especially if your telephone is IP-based (like a Vonage device) and you carry a cell phone. But back when it was copper wires connected to your home from a centrally located facility owned by the phone company (called a Central Office), the central office would send various voltages and signals down the line, which your phone would deal with or respond to as needed.

By the way, when you picked up the phone, you were connected to a tone generator at the central office that would generate the dial tone while waiting for your commands. When it was pulse/dial dialing, you would be breaking and restoring the circuit with each “click” of the dial; 9 times meant “9″, 4 times meant “4″ and so on.

(This was also why you could, if you were talented, click the reciever button on the phone in sets of numbers and dial that way. This sounds like a worthless talent until you realize that they used to put locks on dials for “incoming calls only” phones in places, and so this way you could make free calls. Also, the dial-tone was sometimes just another phone number, so people who were screwing around could call that number and get the “dial tone”.)

With the addition of “Touch-Tone”, you could just send down pairs of signals down the line to replace the “pulse” signals you’d sent before. But the idea was the same; you were connected to a machine a few miles away that was sending you a “I’m listening” sound, take your signals, and then route your call.

ALL of this is faked up and replaced on cellphones and especially IP phones. When you pick up an IP phone, you get a dial-tone, but it’s being generated by the telephone or routing box itself; there’s no “central office” at play. There’s certainly nothing like “Touch-Tone” having any meaning either; after you play with your box or your cell phone, a request is sent down to the switching equipment in toto. After the connection is done, your phone or your cell phone can generate “Touch-Tone” for the purposes of communicating with items connected down the line, but the tones themselves played no part in your call connection.

But back to the POTS (Plain Old Telephone Service) copper-loop days. Besides the signals sent down the line (including on-off/pulse and Touch-Tone), you had specific voltages that indicated the state of the line. One of these was “ringing”: 40 Vrms. When your telephone got this voltage on this copper-line connection, it would start ringing. It was years and years ago, sometime the middle of last century, that the ringing you heard on the line when you called someone was ever actually their phone. In fact, you were being given a “ringing” recording by equipment. This is also why you might pick up on 2 rings but the person calling you would hear 3 or 4.

So it was inevitable that electronics people would be able to build a circuit that “listened” on the telephone line for a voltage surge from the 40 Vrms change on the line, and know that the phone was “ringing”. (When you pick up the phone, the voltage drops, by the way.)

What was explained to me by Ward Christensen (and later Randy Suess) was that Randy built a ring detection circuit, and hooked it to the S-100 machine. Now here’s where we get into more of a time warp.

What it would do, upon detection of ring, is RESET THE MACHINE. The S-100 machine would then boot, start the BBS program, and then pick up the modem and send a carrier.

In other words, it could boot the machine and run the “application” in the time between the first and third rings of a telephone.

This level of jiggery-pokery involving interacting directly with hardware is a hallmark of this era. The S-100 was a “home” computer in the vaguest sense; nothing like the pre-made plastic boxes most people associate when they hear that term. It was a hand-built project, and you knew, very deeply, what it was and how it functioned. It was a small but important jump for people like Randy to want it to “do” something with phones and to just add another circuit to it.

But this whole little world of voltages, circuits and telephones is shifting. Within a generation there will be acknowledgement but vague skepticism that we ever wired every single house and building, everywhere, with a circuit that was always kind of on and ready to go. And that to make a computer respond to a ringing phone on this always-on circuit, you had to whip out your soldering iron and get cranking.


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One Comment

  1. re: circuit voltage –

    Thus, the “black box” was invented. Black boxes were devices that, if I remember correctly, kept the voltage on your phone line from dropping. Apparently when your phone rings it is at one voltage, but when you pick it up the voltage drops. This is how the phone company (at least used to) was able to detect that you had answered the phone (which was also when billing began). Since the black box kept the voltage from dropping, the phone company could not detect that you answered the call, and billing was never started. Phree calls!

    By the time I began reading about black boxes (the early 80′s) they were already being referred to as “something that *used* to work.” The story I heard back then was that they were often used by bookies so that people could place toll free bets.

    Apparently black boxes officially quit working when digital switching was introduced, although I suspect the phone company caught on long before that (“Why are people letting a phone number ring for an hour?”). You are right, when everything goes 100% digital all these old tricks and techniques will quit working, or at least move aside for all new tricks and techniques. :)