Seek logic proven out in disk emulator version of the Alto disk tool

EMULATOR ROLE FOR ALTO DISK TOOL

I altered the VHDL for the seek simulation to avoid a race hazard in the way I set the timer value for the next countdown in the FSM, then tested again. I first forced the time for the arm movement to zero allowing me to verify the settling delay of 14.4 ms will work properly.

It was essential to first change the button signal to a one-shot otherwise the Strobe line is held on, the seek state machine hangs till that drops, which elongates the seek to the time it takes the button to mechanically switch on and off. With the one shot I removed that factor.

I now have a 15 ms seek time, which is basically the settle time, regardless of the span of the movement. Once the rest worked and I had validated the calculated difference in cylinders on the logic analyzer, I added in the arm movement delay.

It now appears to produce the correct time delays for various seek distances. I did uncover one defect in my simulation. If the Alto sets the requested cylinder address to the address the drive has already attained, the ReadyToSWR line should not go off at all. It is a no-op in that case. For any non-zero distance, the line should go off after it sees the strobe leading edge.

My logic to control the ReadyToSWR signal needs refinement, therefore I devised a change, compiled it and tested again. It now appears that I have exactly the behavior I wanted. All the times are right on spec and the related signals work well.

I moved on to working up the logic that produces the signals that would be seen under the read head. That means at each SectorMark, we begin with the 29 words of zero, see the sync word, shift out the two words of the header record while accumulating a checksum, shift out the checksum and then insert ten words of zero before the next record in the sector.

Writing from the Alto will be compared against the continual read process above, to sync up against, then we will aggregate and write the header words, etc into memory and validate the checksum sent to us. The write logic need do little except look for sync words at appropriate times, fetch the words from the Alto, build the checksum and store away the words. Much simpler than the read process.