A new computer joins the flock

August 17, 2021

Heathkit Christmas Catalog 1984

Recently I was investigating S100 floppy disk controllers as a project to build a fully working S100 CP/M system from one of my IMSAI chassis. I have Commodore 128 and Apple ][ systems that do or can support CP/M. However, my first CP/M systems were Northstar Horizon and IMSAI 8080 with a Disk Jocky 2D/B controller. Most of my experience was with 8″ floppies, except for Northstar which uses 5.25″ 10 hard sector diskettes.

The challenges are

  1. Getting or building a boot disk in the correct disk format with BIOS for the floppy controller card, and system size and configuration (memory and I/O) to load a CP/M system that will work with the particular hardware configuration in the system.
  2. Copying from one format to another – i.e. 8″ to 5.25 inch HS Northstar. Of course files can be moved via serial transfer and load once you have a working system with basic software available.

I came across references to the Z-207 Floppy controller from the H100/Z100 computer system by Heathkit/Zenith. It fully supports 8″ and 5.25″ disks out of the box. So I found a working controller to play with.  I also got a set of OS disks, and discovered the H100/Z100 series supports 3 operating systems – CP/M 85, CP/M 86, and MS-DOS 3 (ZDOS).

This seemed very interesting, so I started learning about the H100 series, a unique 8/16 bit computer which has both an 8085 and an 8086 CPU on the motherboard. The other interesting features are that includes a bit mapped video controller (composite output) and an S100 bus. CPU, I/O, Memory (192kb or 768kb), and video are on the Motherboard and attached video board. Keyboard is built in. Floppy or Winchester HD controller are S100, as are optional add on memory and modem cards.

At Christmas 1984, the Zenith  ZF-111-22 with 192k memory and two 320 kb floppy drives (assembled) and no monitor sold for $3499, with MSDOS. That is $9,193.82 in today’s dollars.

I found an H-111 model in the H100 series from 1984-5 locally in the Los Angeles area with the 768kb motherboard and low profile floppy disks. Including the monochrome amber Heath monitor – Just like the catalog picture above. And it actually works with all three operating systems!!! This version appears to be one of the last models of the H100 made before Heath/Zenith  switched to 16 bit only “IBM compatible” PC models.

The only other widely sold computer I know of that supports both 8 and 16 bit operating systems is the Compupro 8085/8086 CPU S100 card. And it does not perform well on 8085 mode. It supports CP/M 80 and CP/M 86 – but I don’t know if it actually supports MS-DOS.

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