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 supports later 3.5" floppy disk drives which use the SS/80 protocol (such as the HP 9122C) instead of the early Amigo drives (such as the HP 82901 and the HP9121). 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 overview articles

The articles below are great architectural overviews of the HP-85 and HP-86.

HP 85 Restoration Hints

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 Expansion / Connection

On its own, the HP-85 is a lovely programmable scientific calculator with graphics. But it becomes way more interesting when you add the expansion ROMs and interfaces. Then it truly transforms into the great lab machine it is known to be, able to control a myriad of HP-IB test instruments, plotters, printers, save onto discs, etc... In order to do that, you'll need to plug in a few drawers in the 4 expansions slots at the back of the machine. See the full list in the accessories and configuration doc below. But I summarize it here for convenience.

My "must haves" that I like to put in all my machines are:

  • a 16k RAM expansion drawer (upgrades the total RAM to the max of 32k supported by the HP-85A)

  • a ROM drawer, preferably with the advanced programming, mass storage, IO, and the printer/plotter ROM.

  • An HP-IB interface module (lets you connect HP-IB printer, plotters, mass storage disks and test instruments, provided you have the ROMs above)

Optionally you can add all sorts of modules, which I use rarely:

  • the serial interface

  • the general purpose IO interface (GPIO)

  • the printer interface (Centronics/parallel port)

  • the PROM modules (I programmed 2: one with the Service ROM, one with the Assembler ROM, and one with the System Monitor ROM). See how to build a modern reproduction of this rather rare drawer below.

  • the MODEM module

  • the speech generation module

  • HP-IL interface

  • CP/M system (very rare, adds a Z80 running CP/M to your HP-85!)

Or you could get yourself a PRM 85 plug in, a modern day module that has all the ROMs and transforms your HP 85A into and HP 85B, and also has the Service ROM which could be quite a help in troubleshooting a faulty HP-85.

HP 85 Accessories and Configuration

The documentation below summarizes all the add-on accessories and configuration options available for the HP-85. Further detailed documentation for the specific modules or accessories is provided in the HP 85 documentation below.

HP 85 Documentation

HP 85 Main Machine Documentation

This folder includes the user and service manuals.

HP 85 Schematics

HP 85 Interfaces Documentation

These are the various interfaces and plug-in modules that fit in one of the 5 slots at the back of the HP-85. These manuals usually cover the installation of the hardware. The user's manual describing the commands to use them are in the expansion ROMs manuals further below. You generally need to add the corresponding ROM to the ROM drawer at the same time as you add your interface module in order to use it.

HP 85 Expansion ROM manuals

These are the manuals for the expansion ROMs that go in the plug-in ROM drawer. Some are pure basic language extensions (Advanced Programming and Matrix ROM for example). Others are the drivers and language extensions for the interface modules, and the true "user manual" for your interface is really in the documents below. The actual ROM binary file dumps are further below as documentation to the video episode where I burn a PROM for the EPROM drawer.

ROM Binary Files Dumps

These are the binary ROM dumps. I show how to use them in the episode where I burned the service ROM for the HP EPROM drawer (see video further below "Burning the service ROM for the HP 82929 PROM Drawer").

Assembly Language and Capricorn CPU Docs

With the Assembly ROM, you can program the HP-85 in assembly and make binary. This was extensively used for games, or if you want to add extensions to the BASIC language or write new ROMs.

Software PAC Docs

Several software PACs were available on tape cartridges. Most of them can be recreated on tape using the .hpi archives that are part of the HP-85 emulation package from Everett Kaiser.

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: