In 2018 I started reading the patent for Hewlett Packard's HP-45 calculator, US 4,001,569, and thought it might be fun to try to implement it in discrete logic. I started with a few simple logic sketches, progressed to Visio diagrams, then to Verilog code. Ultimately, I built a working replica of the HP-35 in which I implemented the logic for each of the seven original ICs in its own 24-pin FPGA board, the LED display drivers in a mix of off-the-shelf ICs and discrete transistors, the power supply in modern switching voltage regulators, and a keypad of mechanical key switches with nearly that HP feel. To bring the three-year odyssey to closure, I presented it at Hackaday's Remoticon 2021 in time for the 50th anniversary of the HP-35's introduction.
Along the way, I developed a deep sense of awe for the giants of engineering who developed the original machine, some 50 years prior. The passage of time bestowed on me the advantages of knowing it could work, 50 years of semiconductor advances, and a treasure trove of detailed technical information contained in the '569 patent and many websites and forums dedicated to HP calculator history. The original designers had to rely on their imaginations, technical skill, and hard work.
Although the '569 patent is amazingly detailed, it's not a Heathkit manual in which each component is identified and each assembly step unambiguously described; I still needed to learn how every circuit worked, discover the exquisite timing relationships required, and fill in the pieces that were glossed over in '569. It was this 'filling in' that exposed the gaps in my knowledge about bygone techniques and now-ubiquitous design principles that I had up-to-then taken for granted.
In this blog, I'm going to describe some of what I saw while looking over the giants' shoulders.
No comments:
Post a Comment