8" winchester drive clock rates (was Re: DiscFerret: First working

I am assuming this is a real SA4000 from a PERQ 1 or 1a. That is, a 14" winchester drive. It doens't need a clock _input_ at all for reading. There is a differential pair 'PLO clock' which are outputs from the drive and consist of the sepeatated clock from the data separator on the drive. The Write Clock pair are inputs, but are only needed for writing (which I assume you are not goign to do). In fact the PERQ just links the PLO clock outputs back to the write clock inputs by traces on the controller board. I asusme you have the schematics of the PERQ CIO board, etc. One thing to watch for on a real SA4000 is the seek control logic. You can sent it step pulses for one track at a time and it works fine. You can sent it a burst of step pulses _faster than a given rate_ and it will buffer them and then move the heads the apporopraite number of cylinders/ But if yousend it a burst of pulses too slowly, it will miscount and the heads wil lnot be where youexpect them The Seek counter logic is horrible, put it this way, if anyone who'd been workign for me had come up with that design they would have been off ot the unemployment office... I have some information (including schematics) for the Micropolis 1200 series that were used on the PERQ 2 T1 machines. If you're working with onoe of those from a PERQ, note that there's an ICL-designed board on top of it that converts the interface to something sort-of like an SA4000, but with differnet control signals (the PERQ had a couple of ports to drive the SA4000 control lines, these were used to talk to this interface board, but the microcode used was different, so the signals are not the same). Again I have schematics, etc. -tony