In this post, I add bubble memory to my iPDS-100!
Background
I picked up the iPDS-100 from a fellow vintage computer enthusiast — I just couldn’t resist, a dual-CPU 8085 portable computer with built-in EPROM programming capability, and emulation pods for the 8051 and 8085. The one thing it didn’t come with though was bubble memory. The iPDS-100 had bubble memory as an option using the iPDS-120 multimodule adapter together with an iSBX-251 bubble memory board. My iPDS didn’t come with the iPDS-120 — it was an option and wasn’t part of the standard package.
There are several iSBX boards available, including the iSBX-251 bubble memory, as well as parallel IO, IEEE-488, etc. The multimodule adapter allowed you to plug in up to four such iSBX boards, either as four small ones, or two large ones. The bubble memory board is the large size, so up to two bubble memories could be accommodated.
Searching everywhere
I did manage to find a fellow user who had an iPDS-120 but he didn’t want to part with it (understandably so). I managed to spot an iPDS-100 on eBay that included the bubble memory board, but it sold for a high price, with high shipping. I knew that my quest to find an available board was unlikely to meet with success, so I set about building one. A couple of resources were helpful:
- A picture of the real iPDS-120 at bitsavers
- The full schematic for the iPDS-120 at Mark Ogden’s website
The picture, together with the motherboard in my iPDS-100 was sufficient to pull dimensions. Mark’s schematic gave me everything I needed to know to lay out the circuit. It’s fairly simple — a couple of 8259 for interrupt processing, plus a handful of 74LS245 and 74LS257 for buffering and multiplexing. The key complexity of the multimodule adapter is that it must be able to multiplex the modules between the iPDS-100’s two 8085 CPUs.
Cloning the board
I cloned it as exactly as I could:
I made a few substitutions — the type of headers the iSBX board used were not available, so I made due with standard 0.100″ headers that are 0.25″ square. I used headers and IDC connectors for the piggyback cables (Note — as shown this makes them a little bit too tall to fit the second module; more thoughts on that later). I used HCT logic instead of LS logic (more thoughts on that later, too…). Here’s what it looks like mounted to the iPDS-100 motherboard:
… and here’s what it looks like with an iSBX-251 added:
Booting
I did have to initialize the disk with IDISK (“IDISK :f4: S”) and then I had to copy several files over to it to make it useful — command files like DIR, COPY, ASSIGN, etc. I even had room to copy over the IPPS software to use for programming EPROMS. Then I booted it!
Resources
Below is a list of useful resources:
