Hewlett Packard HP9845b

Hewlet Packard HP9845b


The HP9845 series computers were introduced in 1977 as a continuation of the successful line of scientific and business calculators and computers from Hewlett Packard.  The HP9845 had many unique features and options.   The processors (actually it has two) are custom built 16bit chips from HP on a ceramic substrate, and operate in parallel.  One handles the language and the other handles the I/O functions. Options for the video display include monochrome ("B / T" model) and color graphics ("C" model).   For storage the B and C models have dual tape drives and there is a built-in thermal printer (which also has a HP custom made microprocessor controller called a NANO processor).  A complete history and specifications can be found on the hp9845 website: https://www.hp9845.net/9845/history/story/ .

I first worked on the HP9845 in 1980 on a project with a company that was trying to develop a PC board CAD program on this system.   I wasn't the programmer at that time, I worked on setting up the hardware which had the 9845C color graphics model and an HP plotter attached on a GPIB (HPIB) interface.

I was able to recover a 9845b model and various parts over the years and have been working on restoration.     It was possibly stored in a barn or outdoors as it had a mouse nest inside. I disassembled it completely and gave it a thorough cleaning. I found problems with several of the boards so it may have been a "parts donor" with boards swapped to repair other systems. The CRT had the "cataract" problem so I rebuilt the CRT face (there are good examples for this repair on YouTube).


 I replaced all the EMI filter caps a few dead transistors, voltage regulator and several TTL IC's. Some of the ribbon cable connectors were damaged from liquid (mouse urine?) I had to replace. Even though the boards are gold plated I still had some spots with green corrosion to clean up. Several connectors on one of the ROM trays are a total loss. It's missing the air filter holders, the printer lid and the paper advance keycap. I had several bad ROM's on the PPU RAM/ROM board so I built a temporary piggy-back board and burned a ROM image set with the diagnostic ROM code (from the HP9845.net website), and was able to boot up to the diagnostics. The processors and RAM tested okay. I have also repaired the tape capstans but tape drives will still need more work. I am now looking into the printer, which seems to be completely nonfunctional, which is why I was looking for information on the NANO processor.