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The H-89 was sold under the two names: Heathkit H-89 and Zenith Data Systems Z-89. The H-89 was sold in kit form, the Z-89 came assembled.

It originally came with 16 KB of memory, later versions provided up to 48KB on the main CPU board (in groups of 1 KB chips). Zenith and Heathkit offered a 16 KB expansion card ($120) for a total of 64 KB when using CP/M.

The system was identical to the H-19 video terminal but had an additional CPU board between the CRT and the terminal board. (Really identical because Heath offered upgrade kits to convert an H-19 to an H-88/H-89 computer).

It used hard sectored disks with a built-in card controller. Under either H-DOS or CP/M, disk capacity was of 90 KB. Another model the H-88 was identical to the H-89, but did not include the floppy drive or controller. It had a cassette port.

A couple of years later, the H/Z-37 soft sectored controller and ROMS came out, then was replaced with a double 5.25" floppy disk drive called H/Z-87 (102 KB, 250 ms). A double 8" floppy disk drive called H/Z-47 (1 Mb each) and a hard disk called H/Z-67, it contained one 10MB 8" winchester drive and one 8" floppy drive (like the one in the H/Z-47).

It ran under HDOS or CP/M (the operating system used 16 KB of RAM).
HDOS was originally written for the H-8, it ran without modification on the '89. This was a single-user OS written by J. Gordon Letwin for Heath. It included a Basic interpreter and assembler.
For CP/M, H/Z wrote a custom BIOS in assembler that the new user could further customize for his specific hardware and assemble right on the machine.
A version of MP/M was also available for the system.

A lot of extension boards were available for this computer including 64 KB memory boards, hard-disk controller cards, 3-port serial I/O board, H19 terminal board, etc.
A third party small upgrade card was also offered which doubled the processor speed to 4 Mhz.

An assembler/debugger was given with the DOS. A paper tape reader was available as well. Microsoft has adapted its various programming languages (Basic, Fortran, Cobol) for this computer. Borland also offered a version of Turbo Pascal that worked great with the CP/M.

Dennis reports:
The base H-89 had no graphic modes, just 33 graphic characters. At least two different add-on boards were created that gave the H-89 bitmapped graphics capabilities, but I'm pretty sure they both required hardware modifications to tie them in to the terminal board - this was not a simple plug-in expansion card