|Re: 82161A casette drive|
Message #9 Posted by Tony Duell (UK) on 7 May 2002, 10:07 p.m.,
in response to message #5 by Johnny
To take your points in order : Yes, we do seem to share several interests -- like PDP11s, HP calculators, etc :-).
The RAM is an 1818-0643, which my equivalents list gives as P2114A-6. So it's not just pinned out like a 2114. It _is_ a 2114 :-). Do be warned that if you attempt to trace out the circuitry of the 82161, it's a bit odd. The microcontroller has no external memory bus, so the HPIL chip and the RAM are linked up usign a couple of the controller's I/O ports. And since the RAM has 10 address lines and 4 data lines but the HPIL chip has 8 data lines and 3 address lines, one port is used for the HPIL data _and_ (most of) the RAM address lines while another set of port lines are used for the HPIL chip address lines and the RAM data lines. It sounds crazy, but as everything is software driven, it can work.
I have no idea how the checksum is done. It might be done on the data in the RAM (in which case you might have a microocontroller failure), or it might be done 'on the fly' as I suspect. However, as the RAM is a standard part, and as I've had a lot of trouble with deffective 2114s, it would make sense to check this first before attempting to find another dead drive to raid a microcontroller from.
To demagnetise the heads you use a demagnetiser (aka 'degausser' or 'defluxer'). This is basically an electromagnet that's conencted to an AC supply (normally the mains). You turn it on, bring the pole piece up to the head face, move it around a bit, draw it away slowly (to at least arm's length) and turn it off.
At one time every shop that sold tape recorders sold demgnetisers (along with other useful things like splicing tape and jigs). I have one from that era, so I have no idea where you'd get one now. But surely they're still being made.