Testing my new seek logic for the Virtual 2315 Cartridge Facility - virtual mode

RETEST UNDER VIRTUAL MODE TO BE CERTAIN IT WORKS PROPERLY

I ran the same loop seeking in and out by the same number of cylinders, initially 200 then other values - to be certain that the Virtual 2315 Cartridge Facility (V2315CF) is handling seeks properly in virtual mode where the physical disk drive is not spun up. 

I detected two problems. First, my logic is sending the seek requests to the drive even though it isn't spinning, and to my surprise the solenoids that lock the arm into various positions were buzzing away. Second, when single stepping through the program, the V2315CF tracked the intended disk arm position properly, but when I ran at full speed, it got out of sync. 

BLOCKING SIGNALS TO THE DISK DRIVE WHEN IN VIRTUAL MODE

It was simple enough to fix the first issue. I don't emit the -Access Go, -10/+20 Mil, and -Rev/+Fwd to the disk drive while in virtual mode. The only things I emit concerning seeks are +Access Ready feedback and the state of the +Home signal, sending those to the IBM 1130 disk controller logic. 

LOOKING INTO HOW THE SEEK SHADOWING WORKS DIFFERENTLY IN VIRTUAL MODE

I looked over my logic to see where it performs differently and how that might lead to miscounting. The obvious spot was that in real mode, after I update the cylinder number at the time when +Access Ready goes low, I wait the approximately 10 milliseconds for +Access Ready to return to logic high before deciding that a seek step is complete. In virtual mode, I immediately ended the seek when I updated the cylinder address as +Access Ready went low. 

That opens a window for glitches on the -Access Go line from the 1130 to trigger a spurious seek step. I changed the logic to wait until the seek timer gets to zero - that is when I return +Access Ready to logic high and that is the equivalent wait I do in real mode. The only difference is that +Access Ready comes from the disk drive in real mode and is derived from a 14.5 millisecond seek timer in virtual mode. 

RETESTING WITH CHANGED FPGA LOGIC

I introduced those changes and fired up the V2315CF in virtual mode to run my program that seeks back and forth by 200 cylinders. I was looking for the internal drive to remain quiet - no solenoid activations - and for the cylinder address shadowed by V2315CF to match what the program was doing. I tried this with 200 cylinder seeks and then with a few shorter ones so that I could move between two positions that did not include the home cylinder (0). 

I observed that the +Access Ready signal is not pulled up to 3V, it is sitting down around 1.4V when logically high. This is confusing the logic in the V2315CF trying to shadow the movements. There are no terminator resistors on this line, but if I add the standard pair back, virtual mode works properly. The consequence of that is disabling real mode, because real mode couldn't get the signal down close enough to ground to register with the IBM 1130 disk controller logi as a logic low. 

Rather than use a pair of resistors, pullup to 5V and pulldown to ground, establishing a termination impedance, I just installed a weak (10K) pullup on the board. This appears to have solved the issue for both modes - real and virtual.