[BBC-Micro] Disc interface problem [was Manufacturing fault]

[Alex Taylor]

On 10/11/2007, Mike Tomlinson <mike at jasper.org.uk> wrote:
> In article <d8834d20711081214se812a6buefa3cd0643068b16 at mail.gmail.com>,
> Alex Taylor <zeem.uk at googlemail.com> writes
> >I bridged the bent pin to the copper track with a blob of solder, and
> >the machine boots up fine now...
> >
> >Except that disc access causes the machine to hang.
>
> Check the NMI interrupt link - S9, IIRC.

The machine's at work at the moment, but from memory the link has been
snipped open. Is that right?

I've been probing around the disc connector while a drive is plugged
in, and while I type commands. For example, the DS0 line is high until
I type *., at which point it drops low. Typing *.1, it stays high.
However the DS1 line seems to be permanently low, regardless of what I
type. The motor on line ('load head' in the service manual) seemed to
be doing something, but now seems to be stuck low too. I've tried
swapping out IC79 that drives these lines, but to no avail.

Next thing I want to look at is what the pins on the 8271 itself are
doing. One of the things I'm struggling with at the moment is which
direction the disc interface signals work, relative to the motherboard
connector. I.e., does the drive itself put 5v on to the wires, which
the Beeb then pulls low? (I'm learning this as I go along, and some of
the theory still escapes me!)

-- 
Alex Taylor