The 8721 PLA, or programmable logic array, was one of the chips that had to be invented to make the Commodore 128, the last of the 8-bit computers that formed the leading edge of the early PC revolution, a reality. [Johan Grip] got a hold of one of these chips and decided to reverse engineer it, to see what the C-128 designers had in mind back in mid-1980s.
PLAs were the FPGAs of the day, with arrays of AND gates and OR gates that could be connected into complex logic circuits. [Johan]’s investigation started with liberating the 8721 die from its package, for which he used the quick and easy method favored by [CuriousMarc]. The next step was tooling up, as the microscope he was using proved insufficient to the task. Even with a better microscope in hand, [Johan] still found the need to tweak it, adding one of the new high-quality Raspberry Pi cameras and motorizing the stage with some stepper motors and a CNC controller board.
With optics sorted out, he was able to identify all the pads on the die and to find the main gate array areas. Zooming in a little further, he was able to see the connections between the matrices of the AND and OR gates, which makes decoding the logic a relative snap, although the presence of what appears to be an output block with latching functions confounds this somewhat.
The end result is a full Verilog HDL file that reflects the original 8721 logic, which we think is a pretty neat trick. And we’d love it if our own [Bil Herd] could chime in on this; after all, he literally designed the C-128.
All the retrocomputer love for Commodore machines seems to fall on the C64 and Amiga, with a little sprinkling left over for the VIC-20. Those machines were truly wonderful, but what about the Commodore machine that paved their way? What about the machine that was one of the first to be gobbled up in the late 1970s by school districts eager to convert a broom closet into the new “computer lab”?
The PET 2001 might be a little hard to fall in love with given its all-in-one monitor, cassette recorder, and horrible chiclet keyboard, but some still hold a torch for it. [Glen] obviously felt strongly enough about the machine to build a PET from current production parts, and the results are pretty neat. When trying to recreate a 40-year old machine from scratch, some concessions must be made, of course. The case doesn’t attempt to replicate the all-in-one design, and the original keyboard was mercifully replaced by a standard PS/2 keyboard. But other than that the architecture is faithfully replicated using new production 65xx chips and 74HCT family logic chips. [Glen] had to jump through some hoops to get there, but as the video below shows, the finished machine plays a decent game of Space Invaders.
Like parents standing on the porch waiting to see their children off to their first day of school we waited for what comes next in a release to production. Among our children: The C116 ($49 Sinclair killer), the C264 ($79 office computer), and the V364 – The computer with an interactive desktop that could speak (courtesy of [John Fegans] who gave us the lion’s share of what made the C64 software great).
Something happened then, and by something I mean nothing. Nothing happened. We waited to assist in production builds and stood ready to make engineering change notices, and yet nothing happened. It was around this time that [Mr. Jack Tramiel] had left the company, I know why he left but I can’t tell due to a promise I made. Sadly, without [Tramiel’s] vision and direction the new product releases pretty much stopped.
Meanwhile in Marketing, someone came up with the idea to make the C264 more expensive so that they could then sell it for a prohibitively high price in. They changed the name, they told us to add chips, and they added software that (at best) wasn’t of interest to the users at that price. They wanted another C64, after all it had previously been the source of some success. Meanwhile the C116 and the V364 prototypes slowly melded into the random storage of a busy R&D lab. We literally didn’t notice what had happened; we were too busy arguing against abominations such as the C16 — a “creation” brought about by a shoving a TED board into a C64 case (the term inbred came to mind at the time).
Ah, the humble Commodore 1530 Datasette drive. It never enjoyed much popularity in the USA, but it was the standard for quite some time in Europe. [DerSchatten13] still uses and loves his 1530. When a co-worker showed him some 7-segment bubble LEDs, he knew what he had to do. Thus the 1530 digital counter (translated) was born.
[DerSchatten13] started out by building his design on a breadboard. He used every I/O pin on an ATtiny2313 to implement his circuit. Tape motion is detected by a home-made rotary encoder connected to the original mechanical counter’s belt drive. To keep the pin count down, [DerSchatten13] multiplexed the LEDs on the display.
Now came the hard part, tearing into the 1530 and removing the mechanical counter. [DerSchatten13] glued in some standoffs to hold the new PCB. After rebuilding the circuit on a piece of perfboard, he installed the new parts. The final result looks great on the inside. From the outside, one would be hard pressed to tell the digital counter wasn’t original equipment.
Operation of the digital counter is identical to the analog unit – with one exception. The clear button now serves double duty. Pressing and holding it saves the current count. Save mode is indicated by turning on the decimal point. If the user rewinds the tape, the counter will stop the motor when the saved count is reached. Cueing up that saved program just got a heck of a lot easier!