Several years ago, the author visited a flea market hosted by W6TRW at the Northrop Grumman car park in Redondo Beach, California. Between the polar bear-shaped TVs and the plethora of phone chargers and power supplies was a wooden box with a lock, a wooden handle, and a DB-25 connector on the side. Next to the connector is a switch: half duplex - full duplex. The author understands what it is. Modem. wooden modem. Namely, an acoustically coupled 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 an acoustically coupled modem is, a small digression into history. The problem was that once upon a time, not only the lines were the property of the telephone companies. They also had to rent telephone sets. Those readers who found the daylap connected the modems directly to the telephone lines. And then, when this modem was made, it was forbidden to do so. According to the American law of 1934, it was impossible to connect anything to the home telephone in any way at all. In 1956, after Hush-A-Phone Corp v. United States rule relaxed: mechanically it became possible to connect. Hush-A-Phone is .
It was formally allowed to connect various devices to a telephone line electrically in the USA in 1968 (). But until 1978, this opportunity could not be used, since tariffs, specifications and certification methods were not developed. Therefore, from 1956 to 1978, it made sense to use acoustically interfaced modems and answering machines. In practice, they were released longer - by inertia.
This modem, now standing on the author's desk, is an integral but unusual page in history. It predates the Carterphone solution and therefore cannot connect directly to the telephone network. It was designed before the development of many of the chips that are considered classics today. The first version of this modem was released just a year after Bell 103, the first commercially successful modem. Here is a great example of how many possibilities can be squeezed 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:
Video blogger phreakmonkey got an early copy of the modem with a serial number of just over 200. Such modems are distinguished by walnut cases, the parts of which are connected by dovetails. According to phreakmonkey, this feature can be used to determine how old a modem is, because dovetails are labor-intensive. Starting with serial number 850, modems began to be placed in teak wood cases with box connections. Then the body parts 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 . His scheme is especially interesting. All thirteen transistors in the modem were standard and widespread at the time. One germanium PNP transistor was used there for a reason not clear to the author. Transistors of all these types are still easy to find in old stock today. Only about twenty dollars - and in your hands is a complete set of transistors necessary to repeat exactly the same modem. True, other details will be needed, including miniature transformers.
Actually, someone pulled out the acoustic interface device from the modem, the rest is fully consistent with the documentation. There are three boards on the backplane. On the first - all the details of the PSU, except for the transformer, on the second - the modulator, on the third - the demodulator. The 2N5138 transistors are dated: Week 37, 1969. It was not possible to more precisely 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 imagine how cool he is. This is an art object, in which there are so many unusual things. The author wanted to fix it, but realized that it was impractical.
Firstly, for this you need to find the original acoustic interface device. Due to its absence, visitors to the flea market did not understand what kind of device was in front of them. The Livermore Data Systems logo and serial number were originally on this device, and now their absence only made it difficult for other visitors to recognize the goods 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 the hands reach this point?
Secondly, the parameters of many capacitors definitely βfloatedβ in it. Of course, it is interesting to take and sort through all the boards, but if the author wants to get a working modem with acoustic pairing, there is a better option.
This is an ingenious design called "β, developed by the Chaos Computer Club in 1985 in response to a similar ban, which then continued to operate in Germany. Such a modem is simpler, and it has more possibilities. It is made on the AM7910 chip, still occasionally found on sale, and operates at speeds up to 1200 baud. It is possible to build a modem from scratch on it faster than on discrete transistors.
In general, there is no point in restoring this wooden modem, but it turned out to be very interesting to disassemble it, arrange a photo shoot and put everything back together as it was. Almost all electronics looked like this from the inside, until there were microcircuits in it. But if suddenly the author comes across an acoustic interface device suitable for this modem, he, of course, will think again: maybe itβs still worth taking on the repair?
Source: habr.com