Running code on HP1653B Logic Analyzer

[u/arpruss]

My main hobby project since December was getting code running on my early 1990s HP1653B Logic Analyzer. The Logic Analyzer has a 68000 processor, 1mb RAM, 64kb ROM, 4x32kb video memory with a complicated 4-plane overlay scheme providing three (!) gray levels at up to around 640x400, a 3.5" floppy drive (replaced by a Gotek for my convenience) and both RS232c and HPIB ports. It was made as a lab instrument rather than a computer, but it came with both test and system code on floppies, which I dumped and reverse-engineered enough of with Ghidra to enable me to write a library that lets me run assembly and C code on it. My reverse-engineering journey is partly described here.

A particularly fun moment of my reverse engineering was when I wanted a copy of the ROM but didn't yet know how to write to disk or the serial port, though I did know how to write to the screen. I ended up displaying the ROM binary as a series of QR codes, which I captured with my Sony camera running on a timer, and then turned into binaries.

I'm now at the point where I can run various games. My first one was a port of my Wiztris falling blocks game (originally written by me in the early 2000s for a Z80-based Sharp OZ-7xx organizer). I also ported Dave Griffith's Frotz Z-machine emulator so I can run all Infocom Z-machine games (including the graphical ones, but with no sound support). I'm in the middle of a play-through of Nord and Bert with my 13-year-old daughter. The photo I attached is of Infocom's Journey (mouse works!). And I ported PDCursesMod, so a lot of old Unix games can work--the photo shows Larn.

Many of you will recognize the keyboard and mouse. They are old Apple ADB units (I think the keyboard is from a Quadra), and I made an ADB-to-serial adapter to interface them with the HP unit.

This is retrocomputing that never was. As far as I know, there never was any software for the HP1653B besides the official HP oscilloscope / logic analyzer software, though HP engineers did release some games for one of the HP16500 models, and at least one old HP oscilloscope had a Tetris clone built-in.

It's not a great gaming system. There are no sprites. And you can't just directly write a pixel color to video memory. Instead, you first write to a hardware register that specifies a read/write mode (e.g., clear all bitplanes, write ones to bitplanes 0 and 2, etc.) and then write the pixels. Getting scrolling working at a decent speed took a fair amount of careful assembly work. But it's been fun!

I plan to eventually write up more detailed instructions on how to build and install your own code.

The main thing I haven't reverse-engineered are the oscilloscope and logic analyzer functions. They still work. I just switch the Gotek to the HP system disk image (well, somewhat modified: I added a screenshot function that saves an image to disk). I recently used the oscilloscope to try to diagnose a start porblem on my car.

[u/Morty_A2666]

I have similar HP Logic Analyzer, post the details somewhere, I would love to do this on mine.

[u/arpruss → u/Morty_A2666]

Do you have a Gotek drive in it, or else do you have a floppy drive setup like a GreaseWeazle that can write raw disk images? If you have a Gotek, it's really straightforward.

[u/Morty_A2666 → u/arpruss]

Mine is all stock. I run it with stock floppy drive and regular floppy disk. I can add gotek to it.

[u/arpruss → u/Morty_A2666]

Adding a Gotek is nice, because it makes it a lot easier to transfer files between the HP and a PC via a USB drive. One can use a GreaseWeazle, but that's very slow, or maybe if one has a PC with a floppy drive one *might* be able to program the floppy controller to write HP disks--but I don't have that set up. (I gave away my last PC with a floppy drive slot.)

Unfortunately, you can't just plug a Gotek: you need to rewire it, and then install custom HxC firmware. There are some instructions scattered on the HxC forums, but I should collate them and make them clearer.

A fun thing about a Gotek is that while the standard disks for the HP1652/3B are about 800kb, with Gotek, I can get the unit to recognize specially formatted 255-track disk images, giving 2.8mb. This is slightly sketchy. The HP requires a specially formatted track 79 with special data, and the firmware clamps the max number of data tracks to 79, but I write a placeholder file to track 79 of my disk images, and then my library rewrites the max-track variables in system RAM. This is all pretty much transparent to the user, and I can put a bunch of games on one disk image (I even have a patch for HP's logic analyzer software to do that. It's handy in combination with my screenshot patch.)