Few things on the Heathkit/Zenith Z-100/110/120 by Nick
My first real job was at Heathkit Electronics (store #30, East Detroit, Michigan) in 1982, when I was 16 years old and still in high school. I started working at Heathkit very shortly after the Z-100 came out, and it was still new to everyone at the store. I fell in love with the thing instantly, my mentor and I learning everything we could about the amazing system. I got my first Z-100 in 1983 (yay! employee discount!), and now had one at home and at work. I used the Z-100 through
college (EE degree, Michigan Technological University), and continued to use it for some time even after I got a 286 laptop (though mostly as a terminal -- something it did well with that keyboard) after graduation. I was certainly not the top Z-100 expert at Heathkit, but I was #1 at our stores, and supposedly one of the top network engineers/techs/troubleshooters by the time they kicked me out (by shutting down the division!) and made me get a real job in 1992. (I showed them, I'm still enjoying what I do for a living!
• The all-in-one (H/Z-120) version actually had three screen color options -- Amber, Green and
white. However, in ten years of working at Heathkit, I have NEVER seen the white screen in operation, however I have two white tubes from the kit, new in the box (from our first shipment of Z-120! We got two green, two amber, two whites. The whites never sold...and are probably the best screen!). Going to have to install 'em one of these days in one of my H-120s. :)
• The Z-100 didn't run at 4.77MHz. because they were to freaking cheap to put more crystal oscilators in the PC, so they picked one frequency that could be divided down into all the frequencies needed on the board. The H/Z-100 had something like ten or so crystals and crystal oscilators in the box. The original machine ran at a flat 5.000MHz, the later one ran at 8.000MHz, and more than a few of the old ones were upgraded to 8MHz simply by replacing the processor and the processor crystal. Zenith later had an official upgrade to the later rev of the main board (which took 256K memory chips) which replaced a number of parts with "high-speed" versions, but my experience was that it wasn't needed.
• The keyboard was INCREDIBLE on these things. Several friends of mine consider it the best keyboard on a mass-market machine ever.
• Interesting bit of Zenith lore: Company legend was that the machine's release was held up pending final ratification of the
• Yes, the machine was darned fast. There was only one design compromise that I am aware of having been made for "IBM compatability", and that was the
320k/360k floppy disks. Zenith had been using 640k 80 track floppy drives for some time on their earlier systems. As the machine was in development at the same time the PC was, it is kinda curious that it "backed up" to the 40 track 5.25" floppy disks, corporate lore has it that MS hinted that things might go better if they stuck to 40 track floppies on the "new machine". HOWEVER, Zenith in no way gave up on the 80 track floppies, they could be installed on the Z-100 with just the flip of a switch on the controller card, or with a simple jumper modification, one 40 track and one 80 track could be used. All the Zenith-provided OSs supported the 80 track floppies.
• Some of the very earliest Zenith Z-100 systems actually came with only
32K RAM chips on the video board -- actually, "half-defective" 64k chips (either the top or bottom half had a bad spot in it). Very few of these machines were shipped before it was decided this was a "bad economy" and 64k video chips went in all the systems.
• The video system was incredible for its day -- purely a graphics machine, no "text mode", text was just dots drawn on the screen to look like a character. And yet, in spite of this, it actually outperfomed the IBM PC's video display for many things. This machine did not suck. Standard graphics mode was 640x225, but by tweeking the registers on the video card, you could run the thing at up to 640x512 on an interlaced monitor. There was no "semi-graphics" mode or "text mode" -- but rather, a "block graphics", emulating the old H/Z-19 terminal chars for apps which didn't wish to deal with the graphics directly (the ROM would handle it for those apps).
• Some more OSs provided by Zenith: UCSD p-System, MP/M-86, MS-DOS v3.1. A Zentih engineer, Barry Watzman, also made his own MP/M-86 (Before Zenith's was released) and a CP/Mv3.0 .
• The machine had privisons in it from day one to support 16M RAM, for both the 8085 and 8088 processors, by way of bank switching. Remember, this was well before EMS.
• I believe this machine was not anticipated as the "ultimate" in a series, but rather
a "bridge" frrom the old 8-bit CP/M world to the new "16 bit" world. It is very clear from the design that the 8088 was not chosen for price (as there are few other cost-cutting things in the machine), but for compatability with the 8085
processor. The processor swapping system on this machine is very simple and elagant -- something that wouldn't have been possible if they had used the 8086 and a Z-80. (ok, MAYBE one could interpret "simple and elegant" as "cheaper".) I was ONCE told by a Heath/Zenith exec that they had seen the "successor" to the Z-100 in the labs, I unfortunately do not recall the specs (and as I recall, I did not believe them at the time -- meaning the whole story is in doubt), but it was impressive. Keep in mind, the Z-100 had a 16-bit ready expansion bus that could handle 16M directly -- it could do everything an 80286 would have demanded of it, and realisticly, it could handle everything the market demanded out of consumer PCs until the early 1990s, when the PCI bus finally took over.
• CP/M on the Z-100 actually used the 8088 for I/O -- disk and video I/O was all handled by the 16 bit processor. This was more than just a cute trick -- the 8085 processor could not reach the video RAM itself, however it had the bonus of giving the system the one of the largest CP/M 2.2 program spaces around, as all the hardware I/O was off in the 8088's RAM space, not in the first 64K.
• The technical manuals (sometimes included with early versions of the machine, extra cost option later) included a copy of the IEEE-696 spec -- along with spec sheets for EVERY SINGLE non-trivial chip in the machine, and complete schematics, and a complete ROM source code listing. These manuals were a gold mine of info. The chips were virtually all "off-the-shelf" components, the few that were custom logic arrays had full logic equations published in the tech manual.
There were NO secrets in this system.
• The machine was completely socketed. Every chip was in a socket and could be removed and tapped into for "other uses". And many of us did incredible things with that flexibility.
• ok, one lie. There was one undocumented board -- the data separator card for the hard disk controller (my defense: it was an OPTION!). The hard disk system consisted of three parts: the controller card which plugged into the S-100 bus, the data separator card (which basicly extracted the data from the bit stream flying out of the hard disk) and, of course, the hard disk itself. For reasons that I do not fully understand, Zenith released full schematics (included with the HD kit!) for the disk controller, but there was apparently something they felt was intelectually valuable about that data separator card, and they didn't release any info on it, other than which cables went where. In spite of this, I was able to reverse engineer a small part of the data separator card and modify it to support four MFM hard disks...and once again, found Zenith had planned ahead, and the disk controller and several OSs were sitting there, waiting for me to provide a Z-100 with more than two hard disks. :)
• Zenith made an network board available for the thing, a bizzare tree-structured ArcNet. Each computer would have three jacks on its back, you could plug the computer into any other computer using any of the jacks. And any other computers into any other available jacks. And so on. Yes, you had to have all the computers on to have a complete network, however (ouch!). Network OS was a hacked version of MSDOS v2 on the server (before MSDOS v2 was ever supported on the Z100!), with a client package on the workstations running MS-DOS v1 (a.k.a., Z-DOS).
• Can you tell I love this system? I still have my first, and several others I
picked up. I can't get rid of the things... A friend of mine still has one in kit form, new in the box.
Jim Mallabar reports:
The Z-100 made history in being one of the first
PCs given to Students by a college. Clarkson University
"gave" these machines to all incoming freshmen from 1983-1985
(there was a $300 damage deposit you could get back at graduation).
Upgrading to 768K memory was very popular, as then you could compile
FORTRAN programs without 2 floppy disks, ('83 and '84 models came with
just 1 drive).
The machines were famous for being solidly built (did the Navy use these
machines as well?). They could handle being dropped off of desks
and lofts, being stepped on, and having kool-aid (and other
liquids) spilled in them. Mine smelled like cherries every time I
turned it on after an unfortunate incident with some Kool-Aid Powder.
I even used the system for 3 or 4 years after graduation to run some
programs for grad school and to keep my wife's resume up to date.
As the first computer I owned, I found it more powerful and fun to use
than the TRS-80, Commodore64, Apple2C, and IBM PC that I had used in
high school. I have very fond memories of this "old
Steven Vagts H/Z-100 newsletter:
The H/Z-100 is still in use and I still publish a newsletter, the Z-100 LifeLine, which has been published since 1989, with 6 issues per year. We are still making changes to this fantastic machine and are presently researching/constructing an IDE controller for the Z-100 to replace the MFM hard drives which are becoming very difficult to find.<br>
I also have a web site devoted solely to this machine! Check it out for additional information. [See
the links section]
Peter Rowe's memories:
Nick Holland (who I might well have met at that eastside store, or one of
the HUG meetings sometimes held there) is right about the keyboard on this
machine. Still the best I've ever had. I fact, my H-100 served
me well until 1992, longer than any other computer I've owned. (The
previous H-89 was pretty good too, until it was stolen in a house break
in. This was, after all, Detroit...)
The number of add ons that various
people came up with for this puppy was amazing. I had not only the
built in 6 inch floppies running, but a pair of old 8 inchers that some
guy in Detroit had hooked up and modified the bios for. And then
that S-100 bus made finding odd controllers and "engineering"
boards easeir then, even than it is today.
Using easily available wirewrap boards, you could even come up with your
own simply, since support code for add on boards was simple to find or
modify. I had a commercial temp controller board from an old CPM
machine that worked fine, even on the 16 bit side, as well as a kiln
controller running my lost wax casting burnout kilns (I was a jeweler at
the time). Never got around to having it control the house temp, but
The other things I recall that were
interesting are that because it was in development at about the same time
as the first IBM computer (but it was released earlier), when they'd
gone to Microsoft to see about an operating system, I was told they ended
up rather without intending to, with almost the same operating system as
IBM. Z-Dos was MSDos, and the main difference between it and the IBM
dos was in the way the bios ran the video. You could read and write
IBM format discs and files, but if you tried to load and run a program
compiled for the IBM, you got a screen display that showed an endlessly
repeating message of "wild interrupt", and that's all.
But it turns out that the source code was very often compatible with
either machine, so for some programs, all the programmers had to do was
recompile the program on a compiler designed for the H/Z-100, and the
result usually would be just fine. not always, if things needed
specific ports or the like, but most of the time.
Zenith had a contract as well, with both the navy and the Air force, to
supply these computers to them. Not sure in what capacity.
probably general "office use" sorts of things, but that contract
went for a number of years, lending strong support to the machine.
Also, Clarkson college may have been the first to give these things
cheaply to incoming freshmen, but as I recall, there were a couple other
schools too that also tried that idea. Not sure to what extent.
And, again if memory serves me correctly, we saw initial experiments with
a "windows" type operating system ( as a shell running under
dos, of course) not from microsoft, but another company, called X-windows.
Didn't last long, but it was the first system I recall that could run
multiple programs simultaneously for the user, switching between them.
Also, as i recall, the early microsoft windows products, though somewhat
unimpressive to me, excited others in part because they felt it ran better
on the H/Z machines than on the IBM machines. There was, at the
time, a strongly felt rivalry between Heath/Zenith users and the IBM camp.
We all very strongly felt we had a much better computer. I think we
were right, too. Just didn't have as strong a company behind it
In fact the downfall of the machine
for me was that some outside company introduced an add in board that
allowed the H/Z-100 to run IBM programs, and I'd gotton dependent on some
of that software. When that board failed, and I couldn't get it
fixed, even though the basic H/Z machine was fine once that board was
removed, I then sorta had to upgrade to a newer system (a 386 from Zeos,
if I recall...) The new machine was, of course, a lot faster, as
that converter board slowed the old machine down a LOT, but I still
missed that old H/Z. Still do, in fact, every time I'm faced with
some technical or software challenge on my current HP box, and know there
is no way for me to currently even begine to decipher either the hardware
or software I now have. Too complex, no documentation, little
Sigh. I'd have stayed with Heath/Zenith if they'd stayed around...