Resurrecting RK05

[tony duell]

> 
> The standard RK8E do a select using just one pin, meaning you can have
> at most 4 drives on the system.
> The RK11 have a binary unit number encoding, meaning you can have 8 drives.

The RK11-D (and I guess RKV11) use binary encoding on the select lines, so up to
8 drives on the cable. The RK11-C uses 1-of-n encoding (like the RK8E) and allows
for 4 drives per cable. But the RK11-C has 2 cable connectors, so you can have
8 drives on it.

The older RK02 and RK03 drives (Diablo model 30) used 1-of-n encoding only,
and will not directly work with the RK11-D. I seem to remember a Plessey clone
of the RK11-D which had an interface board in the disk cable slot which had a 
decoder IC on it to allow for 1-of-n selects.

> More fun is that the same pins were used for both binary and 1ofN
> selection. I don't remember how on earth this was detected/handled by
> the drives, as I don't remember anything beyond the unit setting rotary
> switch on one of the boards of the RK05.

There was a pin on the drive interface connector which if high enabled 1-of-n
selects (it was normally pulled high by the terminator). If pulled low (done
by an RK11-D controller) the drive used binary selects.

-tony