Saturday, September 2, 2017

Problems found in disk emulator and in Digibarn Alto; some corrected


I looked at the signal levels coming into my board from the Alto, which didn't look great. I took off the terminator from the Diablo drive that is used with the Alto disk controller board and found that it had almost no terminator resistors installed.

My interface board has sets of resistors for all the lines, which suggests that one of my problems is those terminators. I will modify my board to disable all the resistor pairs except the few lines that are terminated for the Diablo drive.

In addition I am going to do even more testing of wiring and component functionality on the board, here in my lab, before the next testing opportunity on the Alto.


We decided to try to boot the Alto system we are restoring for Bruce Damer and his Digibarn. We set up the ethernet network and LCM's server, to attempt to boot over ethernet. In addition, we brought our disk drive in our Alto over to connect its cable to the Digibarn Alto controller card, permitting a disk boot.

Neither worked. We didn't see any packets on Ethernet to suggest that the Alto was attempting a boot. When we tried a disk boot, the arm didn't seek at all. Putting a scope on the four signal lines that indicate the next task to perform highlighted one problem. One of the four lines wasn't being driven, instead floating at a low level.

Investigating the card that controls task switching we found a wire that was shorted to a random pin and broken off the backplane fingers. A quick repair gave us all four task lines (8, 4, 2 and 1) producing reasonable values. Still, no booting.

We put the Alto on the big table in preparation for wiring the logic analyzer probes to it. When we next resume sessions we can wire it up and take some traces to find what else is wrong inside.

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