Quick journey to the basement

Tony Duell ard.p850ug1 at gmail.com
Tue Jan 19 09:17:01 CST 2021

On Mon, Jan 18, 2021 at 10:22 PM Chris Zach via cctalk
<cctalk at classiccmp.org> wrote:
> That's probably it. The drive is not stuck, so I'll put it on the MFM
> tester later this week and see what's up. Does the adapter board have
> its' own power supply?

Yes it does. It's the little white 3-pin connector on the board. From
memory it's +5V, ground, -5V but do check that before powering up.

A word of warning. These boards (known as 'DIB's (Disk Interface
Board)) use totem-pole devices to drive the disk drive signals. If you
power up the drive with this board connected but unpowered it has the
annoying trick of driving said lines low. Which means write gate is
asserted. Which in turn means the boot track gets corrupted. Do not
ask how I found that out,

> I'm guessing it's a Shugart SA1000 to MFM interface card. On the bottom

Alas not. The interface back to the PERQ EIO board (the 50 pin cable)
is sort-of the same signals as the SA4000, but not quite. As some of
the signals were controlled by software (microcode actually) they
could be re-assigned. The SA4000 has a step and direction pair of
signals to move the heads. IIRC with this interface board you can load
a value into a register on the board and it'll move the heads that
many cylinders.

It also does some of the very low level MFM encoding and decoding,
clock separation, etc.

The 5.25" drive DIB is covered in the PERQ 2 schematics book which I
assume you've looked at.


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