HP 85 Scientific Desktop Computer

Hewlett-Packard HP 85

The HP 85 is one of my favorite scientific computers of the pre-PC area. Designed for easy programming of scientific calculations and test automation, this all-in-one machine is a delight to use in the lab, still today. They make excellent test equipment automation and data acquisition controllers, and I use them extensively in my HP test equipment setups.

Internally, the HP 85 uses a proprietary 8-bit CMOS processor, which is a specialized scientific calculator processor rather than a generic purpose processor. It natively operates on extended precision floating point numbers. HP also developed an entire custom chipset to go with the processor, including an external IO controller, a tape controller, a CRT controller, and a keyboard controller. There is none of the typical glue logic on the motherboards, just a few large custom chips all designed to work harmoniously together.

There are two 85 models, the more common original HP 85 with a max of 32K RAM, also retro-actively named the HP 85A. The later and less common HP 85B came with 64k RAM and built-in I/O, Mass Storage and EDISK ROMs. You could also add the EMS ROM that support later 3.5" floppy disk drives which use the SS/80 protocol (such as the HP 9122C) instead of the early 5.25" Amigo drives (such as the HP 82901). It is possible to add the mass storage and EMS "B" ROM to an "A" model using the PRM 85 plug in which I highly recommend, and your A will be like a B, except with less max memory. HP later produced other variants to try to fend off the mounting personal computer threat: the HP 86 and HP 87. They both lack the built in tape and the printer. In exchange the 86 had a composite video output for a larger external TV monitor, and the 87 had an integrated monitor like the 85, but twice the width. In my opinion, neither of these later PC-imitating machines have the charm and usability of original HP 85 all-in-one design.

HP 85 Restoration

Vintage HP 85's have known issues. Neither the tape drive nor the printer will probably work. The tape drive capstan rubber invariably turns into goo, and the belts for the printer disintegrate, crippling both of these essential peripherals. Additionally, the 3M DC100 tapes used by HP 85 are hard to find and almost universally non-functional, even when new in the box. It is convenient to modify to the tape drive so it can use the almost identical, but far more available and reliable DC2000 tapes. I posted my videos below on the topic of HP mini cartridge tape drive mechanisms, showing different techniques to repair the capstan, modify the tape drive, and refurbish dead DC100 and DC2000 tapes.

The printer is a more straightforward repair, and I have videos that show how to change the bands and re-adjust the printer properly.

The display monitor failed on one of my units, so there is also a video about repairing that.

HP 85 Schematics

HP 85 Demo Videos

Demo videos of the HP 85 in action

  • video 11: playing Slot and Maze on the just repaired 85
  • GPIB equipment control demo. It takes just two lines of code!
  • HP 85 system demo (with HP 82901 diskette and HP 7225 plotter)
  • HP 85 draws anime demo (using a HP 7225 plotter)
  • HP 85 ASCII art demo (using a HP 2631G matrix printer and a HP 7970E 9-track tape drive)

Recreating LIF floppy disks for the HP 85

The HP 85 can be connected to an HP 82901 5.25" floppy drive. But the floppy format is different from the (then not yet existing) standard PC format. This video shows one method to recreate a LIF formatted 5.25" floppy readable by the HP 85, using a regular PC and imgdsk

HP 85 Restoration Videos

These introductory videos shows how to open the case and give a tour of the innards.

HP Mini Tape Drives and Tape Cartridges Reference Videos

These reference videos explains how to restore several variations of the HP miniature tape cartridges and drives

  • Video 1: Refurbishing drive capstans (preferred method) and upgrading for DC2000 usage
  • video 2: How to refurbish DC100 and DC2000 tapes

HP 85 Specific Tape Repair Videos

These HP 85 specific how to repair the tape drive capstan and modify the tape for DC 2000 tapes.

This shown an early method I used to repair the capstans. I do not recommend it anymore. See the later videos for better and simpler methods.

  • video 6: taking the tape drive apart
  • video 7: explanation of the DC100 vs DC2000 tape cartridge differences
  • video 8: tape capstan repair (not the recommended way anymore)
  • video 9: PCB mod for the DC2000 write current
  • video 10: fixing a tape stall error
  • Better capstan repair with an O-ring. Still, my preferred (and simpler) method now is to recoat with plasti-dip, as explained in the above reference videos.

HP 85 Printer Repair Videos

These 3 videos show how to repair the printer.

  • video 2: removing the printer belts
  • video 3: changing the printer head
  • video 4: new belt installation and printer adjustment

HP 85 Monitor Repair Video

My HP 85 display monitor failed in a weird way. This video shows how I repaired it.

Burning the service ROM for the HP 82929 PROM Drawer

One of the most useful but very rare ROM is the service ROM, which enables you to diagnose every part of the HP 85. Here I show how to burn it into a PROM, and put it in the HP 82929 drawer to boot the HP 85 in the service mode. The ROM files and a project to make a reproductions HP 82929 drawer by Tony Cox are in the documentation below. Another good ROM to have if you are a hard core programmer is the Assembler ROM, also included below.

HP 82929 PROM Drawer Reproduction

In the video, I use a genuine HP 82929 PROM drawer. But these are hard to find. Fortunately, Tony Cox has made a reproduction of it. The documentation for his PCB are in the file below: