Saturday, October 29, 2016

Continuing work on Write Sector, now questioning operation of disk drive circuitry


I completed the reinstrumentation to watch the behavior of the WriteSector and to ensure that the transition stream matches what I expect to see emitted. I will also watch at the terminator side of the cable to be sure I see the same data arriving at the drive. 

Watching the outgoing stream at the terminator gives me nice clean data matching my sector data exactly. I then switched the probe to watch the ReadData line coming it, during the write, to see if it appears to be the same data we sent out.

It does appear to align during the WriteSector operation, although I saw a few funny glitches near where I had written one bits. My WriteData&Clock signal looked very clean at the terminator, with no ringing. I zoomed in on the period with the '1' bit transition to see if I could see any signal issues that might cause problems. 

Sicne that looked clean, I next watched the ReadGate signal to trigger the scope and watched the ReadData input for the sector I had just written. I saw random bits showing up at times when I had not written anything - different from the ReadData line monitored during writing. 

I also looked at ReadClock and saw periods without a clock pulse. I set the scope to a point where I was seeing that dropout of the clock pulse (one time in the sequence of clock pulses), and then monitored ReadData. I was seeing quite a bit of spurious detected 1 bits surrounding the dropout. 

I am definitely going to have to move inside the Diablo during this investigation. I need to see what comes off the heads before the data and clock separator, since the separation may be malfunctioning. I also need to monitor the erase signal to be sure that the erase head is really turned on.

If the erase head is not active, I am just layering transitions atop existing transitions, which would give erratic clock pulses and bad data bit recovery. If it is active, then I have a different insidious problem. Time to dig into the schematics and finalize test points. 

I found a resistor that should show voltage drop as the erase current flows through one of the heads. I also found four test points that together will give me a view of what is coming off the heads and heading into the separator circuit.

My first test, to watch the erase current, showed that it indeed activates when WriteGate is asserted. I then switched the probe to the test point showing me the raw transitions coming in off the head while reading. I triggered on ReadGate, the start of reading sector 0 which I had rewritten, 

I saw low voltage, basically noise like signals coming from the head, In order to compare to a good sector, I moved the arm to Cylinder 1 and did the same capture. Quite different - now I had very wide swings consistent with the flux transitions of clock pulses. 

I looked through the schematics to see if I could spot where the problem occurred, I realized that if the erase current were off, or if the write selection current wasn't working, the drive would detect a WriteCheck condition and alert me. It hadn't. 

Heads and related circutis, top view
Select current to read/write on head, plus erase current drivers, middle of schematic
This moved me to look at the logic driving the signal on head bus A and B, which is what will actually produce the flux transition. As I looked along the left of the top schematic, I spotted a D flipflop and I could hear a huge "DOH" echoing in my brain. 

The flipflop takes pulses from the WriteData&Clock line, alternating the state of the A and B bus levels on each one. It uses the incoming pulse from the terminator to flip the state. Therefore, it is causing the transitions and I should not be doing that in my logic. Rather, I should be emitting simple pulses to cause the transitions to happen. Doh.

I had misread the spec and overly complicated my driver logic. All I had to do was emit a 100ns pulse whenever a transition should occur, not reverse directions of the output line. That would explain why my logic was working as I intended, toggling the driver line, but it wasn't writing intelligible data on the drive. 

I dove into the fpga logic and converted the output to what it should be, pulses at the proper times. With this changed and the bitstream generated by midafternoon, I tested again, first checking carefully on logic analyzer to see that the logic is accomplishing my newly corrected intentions. 

With the timing module working properly, I appear to be emitting the proper signals but still don't get a good read. Further, when I trigger on ReadGate and look at the raw signal from the heads on testpoint 1, it still looks like noise while other cylinders have large clear swings.

The manual asserts that a head which is not selected will have its center tap sit around -1V - which I confirm - and a selected head with WriteGate on will sit at +14V - also confirmed. However, a selected head with ReadGate (or absence of WriteGate) is claimed to sit at about +1V but I instead see the noise coming in from the head, or a similar noise pattern at much lower level, closer to ground. 

I still believe there is something wrong with the write circuitry in the disk drive, since I never leave any transitions on the surface that will be picked up by the head. I have the +14V power level and the erase function certainly seems to function, but I don't see magnetization of the domains on the platter.

I will have to hook the scope up to the two head bus lines, trigger on WriteGate and watch to see whether I am swinging the current inversely on the two lines. That would tell me that I ought to be able to write a flux reversal at that point. 

I did find a flaw in my serializer triggered by the change in my output pulse logic, requiring a quick fix but slow pass to create the new bitstream. Once it was ready, I tested again. Now I was producing exactly the pulse stream I wanted, although the outcome on the disk was still not perfect.

I am still not sure what is happening and need more investigation, probing of the drive circuitry and other testing. For example, the scope on the two head bus lines will tell me if I am truly writing or not. 

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