Zenith Z90

From: Christian Fandt <cfandt_at_servtech.com>
Date: Wed May 6 11:11:22 1998

At 19:23 05-05-98 +0100, ard_at_p850ug1.demon.co.uk (Tony Duell) wrote:
>I am now the proud (?) owner of a Zenith Z90 computer, and I wonder
>if anyone else knows anything about it. It's an all-in-one desktop
>computer, about the size of a VT52 terminal.

I've had experience with most of the Heath/Zenith 8-bit machines. I have an
H-8 and an H-89 (which actually is the good ole Heathkit version of your
Z90). The Z90 was a Zenith Data Systems-branded machine which is Heath
factory-built. It had a bit different setup than the factory-built or kit
versions bought from Heath (the H-89). Don't quite know what those
differences were though, but the H-89 and Z-90 were still very similar.

Still have the equipment, most all docs, H-8 and H/Z-89-related magazines,
etc. but memory fading from lack of use of it the past five or seven years
:( Same goes for my H/Z-100 and H-8 machines.

The H/Z-100, incidentally, could be considered Heath/Zenith's answer to the
transition from a CP/M world to the then-new Messy-DOS world. Had an 8-bit
8085 running CP/M-85 and a 5 MHz 8088 running M$DOS version 1.25. Expansion
bus containing floppy/hard disk and video cards is S100. Was well-respected
and somewhat revolutionary for its time. Kinda pricey when new. US military
bought several thousand. Mine has the later third-party IBM PC compatible
conversion boards and 30 meg HDD and 768K RAM modification.

H/Z-89/90 enclosure actually is the H/Z-19 terminal Heath designed in '79
or so to replace the H-9. Does actually fully emulate the VT52 when put in
that mode.

You can disconnect the serial cable to the Z90 Mainboard and use it as an
H/Z-19 or VT52 if you need a terminal in an emergency.

>It's _very_ tightly packed inside. I've not pulled it to bits yet (not
>enough time), but it seems to contain
>A Terminal Logic Board (probably a Z-19 or similar) which contains a Z80,
>6845, RAM, ROMs, TTL, etc. This talks to the keyboard (just a matrix of
>switches) and the video driver board (essentially a monitor) in the base
>of the machine
>The Terminal board talks to the main CPU board via a serial link. The CPU

>board contains another Z80, 48K RAM (but see below...), ROMs, etc.
>Plugged into the CPU board is a 16K RAM expansion card (obvious), a
>triple RS232 port card (3 8250's) and a pair of disk controllers. One is
>a hard-sectored controller using a sync serial chip and the other is a
>double-density controller using a 1797. I've not figured out how they
>connect to the single internal drive and the 2 external drives.

Are the floppy drive cables disconnected Tony?

>That's pretty tight already, but there's more. There's a 3rd party
>graphics board fitted on top of the CRT (seriously). It links into the

Vaguely recall a 3rd party graphics board. Can you see a name, etc. on it??
I may be able to lookup an old advertisement or something for you on it.

>PSU/video cableform from the Terminal logic board to the monitor, and
>also into 8 of the RAM chip sockets on the CPU board. The RAMs you pull
>to plug in the cables go into the graphics board, which contains
>_another_ 16K of RAM as a bitmapped display buffer.
>I got some manuals with it. The most useful is a hardware reference
>manual for the basic computer - schematics/parts lists/circuit
>descriptions. Alas sheet 2 of the schematic is missing (it would show the

I think I could help you on this if I can find my library of H/Z-89 docs.
We just moved into a new house you know, so I have to determine just which
box it is!

>terminal logic board and keyboard, I guess), but the circuit description
>is so complete that I could almost draw out the schematic from that
>alone. No schematics of the disk controllers either. There's also a CP/M

Love the VERY thorough manuals Heath Corp wrote. Generally agreed amongst
the electronic collecting community that there is nothing like them from
anybody else. So, up until the early 90's or so, when Groupe Bull bought
Zenith Data Systems, Zenith's manuals and some hardware features on their
computers had been top-notch as a result of Heath's engineering. Not so
after Bull took over.

>manual/ disks and an HDOS manual set with disks. HDOS looks to be a
>little like RT11 :-). Oh, volume 2 of the HDOS set contains ROM sources
>for the H8, H88, H90, etc computers :-).

I think it was sort of based on RT11. Somebody either here or on the Heath
list mentioned that. You see, around the time the 8080-based H-8 was
introduced, Heath had already (or just about to) released the H11 which is
an LSI-11 based machine. It used a DEC CPU board and a modified RT11 OS
called HT11 which was distributed on paper tape. Later just RT11 itself
IIRC was distributed. RX01 drives came along too.

I haven't used HDOS (the Heath-written OS for the H-8) much at all but I
*think* some of the functions look like RT11 too. Gotta check that more.
I've got a couple of H11 systems which I need to restore. Need a decent DEC
OS on RX01's first as they came with no disks :( (Suggestions anybody?)

>Does anyone know this machine? Any hints/tips on getting it working again
>(assuming it doesn't...)

I can help some Tony, and I think there's others here who have some
Heath/Zenith 8-bitters. Send a private email if you want to discuss any
details (unless if of course you feel the group would be interested, then
post here).

Have fun with it, Chris
-- --
Christian R. Fandt, Electronic/Electrical Historian
31 Houston Avenue, WE Phone: +716-488-1722 -Home
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Received on Wed May 06 1998 - 11:11:22 BST

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