Sunday, December 11, 2016

Working on the drive and fpga to further improve reading accuracy


I bought some 9601 timer chips, the same as used in the Diablo board, in order to experiment with the timing and appropriate component values. The 'long' timer might be running a hair short, hard to say for sure based only on the logic analyzer traces. I tried to set up the scope to get a more precise value.

Using the scope and some calculations, I worked out some fine tuning to get the timers closer to the ideal timing for the Alto, reworked the board and validated the timings afterwards. Resistances increased to make the times a bit longer.

With that change in place, I did another ReadEntireCartridge transaction and uploaded the resulting image and checksum status files for processing. Immediately I saw that I had very long stretches where no errors at all occurred.

I did the postprocessing and analysis on my laptop, running the Python programs I derived from Ken's work, and looked at the success rate of reading this cartridge with the improved board. I am seeing differences in 4% of the sectors and 2.1% of the total words, statistically about the same as before, but actually a lot fewer sectors had errors.

The only checksum validation errors were on cylinders 5-15, 26 and 44, just 12 out of 203 cylinders on the cartridge. with the errors concentrated so that about half the sectors on each cylinder had errors.

I am going to tweak my ReadEntireCartridge logic to support additional retries and make it more robust. At this point, it is possible that there is some corruption on the cartridge on those low cylinders, because I have an exact match between the bitsaver archive image of the cartridge and the contents I read, other than on those 12 low cylinders.

The errors occur sporadically across the cylinders, not bunched together. If it were surface defects or deterioration of the media, I would expect to have errors on all records of the sector and several consecutive sectors, not the pattern I see; there are errors only on the data record and they are well distributed across each head and cylinder.

If I can get the errors down to essentially zero, I will be satisfied. Until then, I want to continue researching and honing the system.

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