|Re: HP 9100B magnetic card|
Message #2 Posted by Tony Duell on 27 Feb 2008, 2:13 p.m.,
in response to message #1 by Robert Bennett
What exactly do you want to know? I've worked on the 9100B, and have a fair idea of how the card reader works.
I assume you've got the unofficial schematics (for example from the Australian Site).
The card reader has a 4 track head, and stores 6-bit keycodes. One track is a clock track, handled by the reader electronics. The other tracks store the keycode as 2 3-bit words, the odd bits and even bits are separated. It talks to register 2 (main accumulator) in the 9100, the interface is not _quite_ the signals on the external connector. And yes, it does make use of the fact that the register is made of JK flip-flops to load the 2 sets of bits.
The main problem with the reader is that the drive roller decays with time (what a suprise!). The original roller, I believe, had a flat on it, but a circular one will work, it's just harder to push the cards in. A 1" (outside diameter) O-ring will work, but the problem is fixing it to the hub. I ended up pulling the old hub off the motor using a special home-made puller tool, and machining a new hub from 7/8" brass rod.