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I'm currently working on a project with an old device that exposes at least one V.35 interface. From my understanding and reading. V.35 commonly used a huge, bulky connector that looked like this:

V.35 connector

However, this is huge and likely wasn't a good connector for things that needed a smaller form factor. I know that many companies used all sorts of custom connectors for V.35, in fact my device uses a DB44 connector. However, I got curious. This website notes (emphasis mine):

The V.35 plug is standard. It is a black plastic plug about 20mm by 70mm, often with gold-plated contacts and built-in hold down and mating screws.

I've looked at the V.35 standard available at: https://www.itu.int/rec/T-REC-V.35/en and can't find a single mention of this plug. In fact, there seems to be very little defined about the cable or plug itself. Am I missing it? Was this bulky connector actually ever "standard" for V.35? If not, where did this design come from?

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This is the Winchester connector. The form factor is specified in ISO 2593. ISO standards are typically not available for free, which may be why you had trouble finding information on it. I'm not aware of any standard actually specifying it for v.35. The first link above (to the O'Reilly book) suggests it was simply a de facto industry standard.

This connector was commonly used on external T1/E1 CSU/DSUs or channel banks, back when those were a thing. They provided a synchronous serial link to a router or PBX. You're right that their large size was not very practical, and hardware vendors often used a smaller connector on the other end. Some standardized on DB-25, which had enough pins to support v.35 signalling. Cisco had their own custom DB-60, which was roughly the size of a standard DB-15 or DVI connector, but with more and smaller pins.

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    My experience of these connectors is in retro lab equipment – the shells are well defined and the connectors can carry a variety of pin-type connections -- coax carrying RF, for example, as well as a fairly chunky amount of current. As a result, I've seen them on old (late ~1980s) NMR instruments, spectrophotometers, and similar. Their the ruggedness is useful, as well as the human-accessible nature of the shell. However, their undoubted heft and bulk ultimately made them highly non-competitive in both resources and space. – Landak Sep 20 at 15:26

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