50-year-old modem: an inside look

Original author: Brian Benchoff
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A few years ago, the author visited a flea market organized by W6TRW at the Northrop Grumman car park in Redondo Beach, California. Between the televisions resembling polar bears, and many chargers for telephones and power supplies, there was a wooden box with a lock, a wooden handle and a DB-25 connector on its side. Next to the connector is a switch: half duplex - full duplex. The author understood what it is. Modem. Wooden modem. Namely, an acoustic pairing modem, released by Livermore Data Systems around 1965.

The modem is still at the flea market. Immediately after photographing, the author bought it for $ 20.

Since not everyone knows what a modem with acoustic pairing is a short digression into history. The problem was that once the telephone companies were not the only lines. They also had to rent telephones. Those readers who caught dailap connected modems directly to telephone lines. And then, when this modem was made, it was forbidden to do so. According to American law of 1934, it was impossible to connect anything to a home telephone at all by any means. In 1956, after the Hush-A-Phone Corp v. The United States relaxed the rule: it became possible to connect mechanically . Hush-A-Phone - it 's what .

Formally connecting various devices to a telephone line electrically in the United States was allowed in 1968 (Carterphone solution ). But until 1978, this opportunity could not be used, since tariffs, specifications and certification procedures had not been developed. Therefore, from 1956 to 1978, it made sense to use acoustically coupled modems and answering machines. In practice, they were released longer - by inertia.

This modem, which is now on the author’s desk, is an integral but unusual page in the story. It was made before the Carterphone solution, and therefore cannot be connected directly to the telephone network. It was designed before the development of many microcircuits that are considered classic today. The first version of this modem was released just a year later than Bell 103, the first commercially successful modem. Here is a great example of how many possibilities you can squeeze out of just thirteen transistors. Then this modem was forgotten for a long time, until two videos were shot about it, one in 2009, the other in 2011:

Videoblogger phreakmonkey got an early instance of the modem with a serial number a little over 200. Such modems differ in walnut wood cases, the details of which are connected by “swallow tails”. According to phreakmonkey, on this basis, you can determine how old the modem is, because “dovetails” are labor intensive. Starting with serial number 850, modems began to be placed in teak cases with box connections. Then the details of the case began to be connected with tongues. Livermore Data Systems needed to make modems faster and faster.

In 2007, blogger Brent Hilpert looked into such a modem and described its device.. Especially interesting is its scheme. All thirteen transistors in the modem are standard and at that time widespread. One germanium PNP transistor was used there for a reason incomprehensible to the author. Transistors of all these types are still easy to find in old stocks today. Only about twenty dollars - and in your hands a complete set of transistors needed to repeat exactly the same modem. True, other details will be needed, including miniature transformers.

Actually someone pulled out the acoustic interface from the modem, the rest is fully consistent with the documentation. On the cross-board there are three boards. On the first - all the power supply components except the transformer, on the second - the modulator, on the third - the demodulator. On transistors 2N5138, the date is stamped: 37 week of 1969. More precisely, it was not possible to establish the release date of the modem itself, but most likely it was manufactured and shipped before 1970.

A tongue-and-groove connection means a late release modem.

The author bought this modem just to keep it at home. This is a wooden modem, but hardly any of the author’s acquaintances can imagine how cool he is. This is an art object with so many unusual things. The author wanted to fix it, but realized that it was impractical.

Firstly, for this you will need to find an original acoustic interface device. Due to its absence, visitors to the flea market did not understand what the device was in front of them. The Livermore Data Systems logo and serial number were originally located on this device, and now their absence only made it difficult for other visitors to recognize the product as a modem, because they are not employees of computer museums. It is tempting, of course, to print the details of the acoustic interface device, but will hands reach this?

Secondly, the parameters of many capacitors have precisely “floated away” in it. Of course, it’s interesting to take and sort through all the boards, but if the author wants to get a working modem with acoustic coupling, there is a better option.

It's about a witty design called a data toilet", developed by Chaos Computer Club in 1985 in response to a similar ban, which then continued to operate in Germany. Such a modem is simpler and has more options. It is based on the AM7910 chip, which is still on sale sometimes and works at speeds up to 1200 baud. it build a modem from the ground as soon as possible, than discrete transistors.

In general, the recovery of a wooden modem does not make sense, but it was very interesting to analyze, arrange a photo shoot and collect everything as it was. so look inside about almost all the elements Tronic, while it had no chips, but if you suddenly will fall to the author the right to this modem device acoustic coupling, he is, of course, think again:. maybe it is worth to take up the repair?

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