Tuesday, October 31, 2017

Bottom platter of 7906 not damaged, working on drive emulator and diagnostics all run fine


7906 disc drive

I opened up the drive, removing the cover over the fixed lower platter to inspect it better. The really great news was that the lower platter is not damaged. It appears that the line I thought I saw was a reflection of the cover above.

Undamaged fixed platter surface
Thus, I have some crash debris on the top head for the removable cartridge and an unusable cartridge, but with luck, if the head can be cleaned properly, I can restore this drive to operation and use a fresh cartridge. The easy way is to get it clean in-situ, otherwise after removal I will need to do an alignment (without a CE cartridge). 

HPDrive emulated disc drive construction

In case I can't get the physical 7906 drive working, I am building a backup to still allow me to run the HP 1000 system with its RTE operating system. Ansgar K├╝ckes has created the HPDrive software that runs on a PC, in conjunction with an HP-IB adapter board, to act as an emulated disk drive (and more). 

This only works well with National Instrument HP-IB boards. They come in ISA and PCI flavors, but the ISA version forces the use of older machines and versions of Windows. Ansgar has written a driver for the PCI board, easily installed (albeit with a bit effort to override the Microsoft paternalistic ban on installing unsigned 64 bit drivers). 

I will need an HP-IB cable connected to the HP 12821A adapter board which supports HP-IB based disks on the HP 1000 mini. Initially I will borrow that board from Marc Verdiell, but eventually I will buy one once I see this working. I have the PCI board and a suitable windows desktop system on the way to me, courtesy of eBay and Amazon. 

Diagnostic failures

Each time I disturb the ribbon connector that runs from the CPU board and FAB below the IO cage up to the FEM board in slot 10, as well as disturb the backplane for peripheral cards, the machine has gone balky and won't work properly. I suspect the connectors and will clean them with Deoxit Gold - both the IO board to backplane and the FEM to ribbon cable ones. 

Everything cleaned, still some signs of the problem, so began board swapping yet again. It does appear that one of my 12966A serial cards will consistently cause the problem when plugged into the top IO cage slot. With that removed, I could run diagnostics.

As I tried to run the diagnostics for the floating point, scientific and fast fortran instructions (FPP/SIS/FFP) it would hang with the machine stopped but the console unresponsive. I couldn't switch to check other register contents until I did a PRESET on the machine. 

Something is hanging and not returning to the basic microcode - more reason to suspect connectors, either the ribbon cable between the FEM and main CPU board or the ribbon cable between the floating point box underneath and the console panel.

I ran the builtin microcode diagnostics for the FPP, SIS, FFP and VIS and quickly discovered that my likely candidate is the FEM board or its ribbon cable. I could run the FPP routine successfully but the system ignored the codes for the others, which are essentially firmware (microcode) routines. They didn't respond at all the codes. 

These tests are run by setting a code in register A (Accumulator), putting 0 in register P (instruction address) and hitting INST STEP. The codes and logic they test are:

  • 100000 - test memory and basic CPU
  • 105004 - test floating point hardware
  • 105337 - test the scientific instruction set
  • 105477 - test the vector instruction set
  • 105200 - test the fast fortran program instruction set
  • 105242 - test the RTE IV extend memory function
The first two routines run properly and report 102077 in the S reg at completion to indicate everything passed. The final four were nonresponsive. These worked previously thus the erratic cable/connectors is my suspicion, but we also could have a failure on the FEM board or in the ROMs. Since this spans multiple ROM chip sets on the board, the board or cable are the chief suspects. 

I moved the FEM board up to slot 11 so that I could use a different connector on the ribbon cable. Now, every test performed perfectly. I then booted the tape diagnostics in conversational mode and ran the full test for the FPP/SIS/FFP and got a fully successful run. Definitely flaky connectors/cables as I suspected. 

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