Thursday, December 29, 2016

Validated new board and connectors, ready to be used on the Alto system to archive disk images tomorrow


I set up the testbed and began to check the functionality and signal integrity of the new board and connectors. Seeking was checked to be sure all the address bits were working - but in fact the 4 and 16 lines were not working. I found that the chip pins involved were not well soldered, fixed it and went back to test.

In addition, I have a Digilent Nexys2 FPGA board that was purchased by Al Kossow in order to give him a working disk driver and emulator. It had connectivity problems in the USB connector, with the connector itself bent and some other problems where it was attached. I used my heat gun to remove the connector, bought replacements and will surface mount the new part when it arrives.

The disk drive testing resumed and my seek addresses were all working correctly. Next up is getting the ReadGate and other signals tested. I am emitting '1' to ReadGate which should pull the output of chip U2 side 1 down to ground, thus activating pin 39 on the ribbon cable to signal pin E.

However, when I monitor pin E on the terminator, the line is not going down to ground. I have continuity from the fpga connector pin to chip U2, from the chip output to pin 39 on the connector, and all the way through the cable and drive to the terminator.

I might have a flaw with chip U2 such that it is not working, even though all the connections to and from it are good. I have to do a power-on test and verify the voltage levels at the chip. Watching the scope, I could see that the signal at the terminator changes as I probe the chip pins, indicating I probably have a poor solder contact there.

I decided to remove the chip entirely with my hot air rework gun, then resolder it properly. With that done, I verified good solid behavior, including the Seek, ReadSector and ReadEntireCartridge transactions. I uploaded the contents of the pack, along with the checksum validation array, so that I could check that the data was extracted properly.

The results were just as before, so I now have the production level board tested and ready to use tomorrow on the drive attached to the Alto and the other cartridges we have there.

As I packed up everything for transport, I decided to inspect the disk cartridge surface and the heads. I can see scraping and discoloration at the low cylinder numbers near where I am experiencing almost all of the checksum errors reading that cartridge.

Surface marking that is probably the cause of my errors reading some sectors

Surface marking on the other side of the platter
I will tentatively ascribe the errors to the surface condition, thus hope that my tool works well with undamaged cartridges on the other Diablo drive, the one that came with the Alto. We will know more tomorrow after we read and upload all the content. 

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