« New Insights in the Search for Planet Nine | Main | What Shall We Call the Hill? »

Friday, April 16, 2021

CONTINUITY: HP 9825 Repair Part 3: Logic Analyser and a 43 Year Old Patent to the Rescue

After going about as far as possible with an oscilloscope (although back in the day we went way deeper into the woods with just a ’scope), it's time to hook up a logic analyser and see what the CPU and memory are doing. Aiding in the process is U.S. Patent 4,075,679 [PDF], granted in February, 1978 and assigned to Hewlett-Packard, whose 606 pages contain, inter alia, a complete commented source code listing of the ROM and extensive logic, circuit, and timing diagrams. How deep was the damage to this vintage machine when its power supply went all berserker?

Posted at April 16, 2021 13:42