Testing chips for the HP-35 in '73

Fred K. Beckhusen has written a document The HP-35 “Bluebird” tester with many interesting details of how the HP-35 chips were tested - secretly, in the sense that Mostek applied a need-to-know basis for the operators.

The HP-35 calculator was a huge sales success in 1972, the year I graduated from high school. There is a lot of information online today about this marvelous machine, but very little about how it was manufactured, or how it was tested. I wish I had know back then what I know today, because I worked on the HP-35 for a long time, without even knowing about it. So here it is, 40 years later, and I am writing about the HP-35 chip set and architecture, and the Bluebird tester we tested them with.

This photo from the 1973 Mostek data book:

The HP-35 famously shipped with a bug in the ROM, and HP did a product recall - see for example this page on the late Jacques Laporte’s site all about the HP-35.

Images above from the very interesting Remembering The HP-35A by Richard J. Nelson.

You can run both versions of the microcode in this in-browser emulator by Ashley Feniello, who demonstrates both versions in this video:

via hpmuseum forums


Again via hpmuseum forum:

HP had two competing sources of PMOS chips for the calculators they developed prior to the HP-27 and HP-30 series, Mostek and AMI. Both vendors were given the engineering requirements and did their own circuit design and layout. Generally the corresponding chips from the two vendors were interchangeable, but there were occasional exceptions.

Eric links to a new write up of this adventure, published last year:

The previous write up, a pdf, has moved or vanished so I’ve updated the head post to link to the Internet Archive instead.