Jay Maynard (jmaynard) wrote,
Jay Maynard

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The connector conspiracy: IBM's a member

When IBM designed System/360 in 1964, one of the things they defined was the interface to peripheral equipment from their I/O channels. This interface achieved 1.5 megabyte/second speed over distances of a few hundred feet. Not bad for 1964.

In the process, they defined two connectors (annoyingly interchangeable with each other), called bus (which carries the actual data) and tag (which carries the control signaling). The connectors are big blocks with hermaphroditic pins that are fairly easy to bend, but also fairly easy to repair if you've got the parts.

When they introduced the P/370 and P/390 personal mainframes, they needed to interface channel-attached devices to the host PC. They created a board to do this, called the S/370 Channel Emulator/A board. (They hung /A on the end of all of their Micro Channel boards, for some esoteric reason.) Since the channel connectors are far too large to fit on a PC's back panel, they put a connector on the back of the board. A cable with two standard channel interface connectors plugged into that.

Later on, they improved performance by adding some intelligence, in the form of an Intel 960 embedded processor. The ARTIC board, as it was called, came in both Micro Channel and PCI versions, and had daughtercards that provided the actual electrical interface. This let them use the same ARTIC base board to do lots of different things, including acting as a channel interface. It, too, had a connector and a pigtail cable that provided the standard channel connectors.

You guessed it: the two pigtail cables are different. The connector on the back of the S/370 Channel Emulator/A is 62 pins; the one on the back of the ARTIC channel emulator is 78 pins. I have no idea why IBM did that, but it's a real nuisance, since the cables are rarer than the boards.

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