A 'back burner' project I have in mind is to repair an old HP-125 computer I picked up at a thrift shop quite a while ago. This computer has no onboard storage drives, instead coming paired with an HP-82901M floppy drive unit, which communicates over HPIB/GPIB/IEEE-488 (whatever you want to call it). These drives are either pure unobtanium, or are downright bankrupting, so I need to find a way to emulate it.
My requirements are as follows:
While I've learned how the GPIB protocol works, I have no idea what the command set is that either the HP-125 and/or HP-82901M speaks. I've found mention of the HP-82901M using the Amigo command set, as introduced with the HP-300 "Amigo", on this page about emulating an Amigo-compatible drive with a modern PC. That project requires dedicated GPIB cards that regularly run in the hundred+ dollar range, and requires you to have a desktop computer running alongside whatever computer you're trying to boot. This won't do for my requirements.
Unfortunately, they don't list the commands themselves, so my search had to lead elsewhere. One thing I did notice, is that the HP-7970E tape drive was listed as a compatible drive, and I found a listing of commands that it spoke. Unfortunately, most of them only apply to tapes, and not floppy or hard disks, and so I had an incomplete set of commands. The documentation on Bitsavers is sparse for the 82901, and nonexistent for the Amigo command set specifically.
One site I came across, that talks about a microcontroller-based approach, led me on a clue. Turns out this is a rare instance of HP not being the smartest in their documentation. They list the command set in the back of the service manual for the HP-9895A floppy drive unit, rather than its own manual as was done for the successors to the Amigo command set, CS/80 and SS/80.
The previously listed site does provide schematics and firmware, so it could end there. However, it's quite wasteful to use an entire SD card for one disk image. Another issue I have with it is it only provides storage for Amigo-compatible computers. At this point, and after all the effort I went to to find the command-set in the first place, it'd seem like a hollow victory to use someone else's single-use design. I decided to see just what I could do my own way.
So I now have a copy of the command set, and an idea. I've already been planning to make my own GPIB-connected storage modules, and I decided I could merge this into one of them.
My current plan, which is subject to change, is to include a switch on the front, to either start the module in 'Emulated' mode to work as an Amigo-compatible storage device, or 'Standard' mode to work with my own project computers. In 'Emulated' mode, each 'unit' of the drive will use one of eight disk images selected by a thumbwheel. In 'Standard' mode, all but the sectors containing disk images are present to computers running my own command set.
My requirements are as follows:
- Standalone operation, not requiring another computer to emulate
- Low cost parts, or using mostly parts I have on hand
- (Optional) Multiple uses, I don't want this to be a one-trick pony
While I've learned how the GPIB protocol works, I have no idea what the command set is that either the HP-125 and/or HP-82901M speaks. I've found mention of the HP-82901M using the Amigo command set, as introduced with the HP-300 "Amigo", on this page about emulating an Amigo-compatible drive with a modern PC. That project requires dedicated GPIB cards that regularly run in the hundred+ dollar range, and requires you to have a desktop computer running alongside whatever computer you're trying to boot. This won't do for my requirements.
Unfortunately, they don't list the commands themselves, so my search had to lead elsewhere. One thing I did notice, is that the HP-7970E tape drive was listed as a compatible drive, and I found a listing of commands that it spoke. Unfortunately, most of them only apply to tapes, and not floppy or hard disks, and so I had an incomplete set of commands. The documentation on Bitsavers is sparse for the 82901, and nonexistent for the Amigo command set specifically.
One site I came across, that talks about a microcontroller-based approach, led me on a clue. Turns out this is a rare instance of HP not being the smartest in their documentation. They list the command set in the back of the service manual for the HP-9895A floppy drive unit, rather than its own manual as was done for the successors to the Amigo command set, CS/80 and SS/80.
The previously listed site does provide schematics and firmware, so it could end there. However, it's quite wasteful to use an entire SD card for one disk image. Another issue I have with it is it only provides storage for Amigo-compatible computers. At this point, and after all the effort I went to to find the command-set in the first place, it'd seem like a hollow victory to use someone else's single-use design. I decided to see just what I could do my own way.
So I now have a copy of the command set, and an idea. I've already been planning to make my own GPIB-connected storage modules, and I decided I could merge this into one of them.
My current plan, which is subject to change, is to include a switch on the front, to either start the module in 'Emulated' mode to work as an Amigo-compatible storage device, or 'Standard' mode to work with my own project computers. In 'Emulated' mode, each 'unit' of the drive will use one of eight disk images selected by a thumbwheel. In 'Standard' mode, all but the sectors containing disk images are present to computers running my own command set.
Well. The way I have it with HPDisk is that I now have a 16-way switch that lets med change "disks". Amigo is a horrid protocol to emulate, SS/80 is so much cleaner. Currently using HPDisk with a HP9816 in SS/80 mode.
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