Evolution of the Species

my H-8

This page was last updated on 06/16/2001

The H8 captured the imagination of the hobbyist computer geeks.  And Heathkit just kept adding more stuff.

The basic H-8: A Backplane, a CPU Card and a Front Panel

The H-8 backplane had ten identical 50-pin connectors made from two tandem 25-pin single-inline rows of pins on 1/10-inch centers.  The first and last connectors were spaced differently from all the rest, so at first appearance, you had 8 slots for cards and two connectors for cables.  The front panel had 25-pin cables attached which plugged into the first connector; the CPU card plugged into the second slot and you had seven more slots for controller and RAM cards.  The cabinet was a metal base, front panel and backplate and plastic sides and contained a husky power supply.  Power was un-regulated; each board had its own three-terminal regulators.

The original H8 CPU card used an 8080A with the standard Intel "glue" logic – a pretty straightforward design.  The CPU card included a 1Kbyte ROM containing the front panel and tape I/O software. This ROM monitor program was called PAM-8.  It occupied locations 000.000 through 003.377.

The front panel consisted of four LED indicators and nine MAN-1 seven-segment displays and displayed data and addresses in bytewise octal notation as a six-digit address and a three-digit data (or a two-letter register code):

Drawing of DIsplay

There was also a 16-button keypad which was labeled like a decimal calculator. The keys also had funny labels like “REG” or “OUT”. It was a bit intimidating until you got used to the context-dependent decoding of keystrokes.  Then you discovered that you had complete control of the CPU’s registers and of RAM contents.  You could enter programs from the front panel, single-step them, and read or write the cassette tape.

The keypad (Z80 version)

The price for the H8 (as of Spring, 1978) was $375.  Such a deal!  Of course without memory or I/O it was just a very high-tech paperweight...

Memory and I/O Options: the Early Days

When the H8 was first introduced, the RAM card was the H8-1.  This was a static RAM card with sockets for two banks of 4Kx1 static RAM chips.  The card came standard with one bank of RAM and you could purchase the second bank as an optional chip-set (The H8-3.) The H8-1 was $125 and the H8-3 was $85.

For I/O at the time, you had your choice of a combination serial port and tape cassette controller card, the H8-5 (at $110) or a parallel port card, the H8-2 (at $150).

The H8-5 card provided a single serial port based on an 8251 UART chip and an audio-tape storage interface that used a PLL to encode data at 1200 baud.  The tape interface was a variety of the "Kansas City Standard" audio tape technology of the era: the KCS system recorded 8 or 4 cycles of 1200 Hz or 2400 Hz at a rate of 300 baud; Heathkit's implementation was to record 1 or 2 cycles of the same frequencies and get four times the data rate.  In my experience, the tape was pretty reliable for the era but still not good enough for serious computing.  And this was with a high fidelity grade cassette recorder (ECP3801, which cost $60).

The H8-2 was a most unusual beast indeed! It provided three parallel I/O ports by having three 8251 USARTs feeding three UARTS. For each channel, the CPU did I/O to a USART which then was connected to the serial in and serial out pins of a UART; the parallel port was the data and control pins of the UART. Heathkit did this so that the parallel port would use the same I/O routines as the serial port (which was a clever, if strange, idea).


Initially, the devices available from Heathkit were pretty primitive: there was the H9 video terminal (all CAPS!) and the H10 paper tape unit (Does anybody remember paper tape? It was on the way out even in 1978!)  At the time, I had a KSR33 teletype for a console and was working on a design for a video terminal which used a surplus 12" TV set for the display. So I gave those clunkers a pass.

Along comes the H-17

The three-drive version of the H-17

When the H-17 floppy disk system arrived, it was a separate box nearly the same size as the H8 which connected to a controller card via a ribbon cable.  The floppy disks were 5-1/4 inch ten-sector, hard-sectored (meaning there were eleven holes punched in the diskette instead of just one) single-sided; the capacity of a floppy was 100 Kb (40 tracks, 256 bytes per sector). The original H17 cabinet held two drives, mounted horizontally.  [A side note: I later learned to wrap aluminum foil around the ribbon cable – the RFI with the unshielded ribbon was terrible!]

The controller card was basically just a parallel port, a small buffer memory and a USART, but it also included another 2Kbyte ROM: the H17’s BIOS, if you will. The RAM was allocated 024.000 through 027.377 and the ROM code was in 030.000 through 037.377 – hence, to boot from the H17, you typed “REG, PC, ALTER, 0, 3, 0, 0, 0, 0, ALTER, GO”. The code in the ROM took it from there, launching the Heath Disk Operating System (HDOS). Writing to RAM required first doing an I/O operation to un-protect the RAM.

People eventually got tired of so many keystrokes to boot the disk.  Heathkit sold them a replacement PAM-8 ROM called “PAMGO” that provided a one-button boot capability.

A year or two later, the H-17 cabinet was redesigned to support three disk drives, mounted vertically. This was the H17-3 modification kit.

I bought the H-17 when it first came out.  At that time, it was only available pre-assembled. The two-drive WH17, with controller card and the first version of HDOS (Heath Disk Operating System) came to $1000 – just about what I paid for the basic 16K tape-based H8 in the first place.

Memories are made of this...

Before long, Heath introduced a bigger RAM card: the WH8-16 held a whopping 16,384 bytes of static RAM memory; three of these babies and one of your old H8-1/H8-3 cards and you filled the memory space. The WH8-16 was $395 in 1979.

Of course somebody had to realize that there was an opportunity in the making... That somebody was a company called Tryonix Electronics. They were one of the earliest third-party hardware vendors for the Heathkit computers. Their MH-8 Dynamic RAM card hit the scene in early 1980. This one card could hold up to four banks of 16K bytes each and (for the 16 K configuration) cost $225. Put all four banks on the card and you had a full memory map -- plus some! -- in one bus slot.

(Which I did. At that point, I traded a 40K system using four bus slots for a 64K system using only one, though until the Org-Zero modification came out, I could only use 56K of the RAM.)

Sound and Pictures: Multimedia in an eight-bit universe

When I first got my hands on a computer (in 1969, at college), I wrote a program that played music. So when the HA-8-2 came out, I snapped it up.

Made by New Orleans General Data Services, the card was a pair of digital-to-analog converters wired as a two-byte write-only memory. (The on-card DIPswitch defined which two bytes: the default addresses were 000.000 and 000.001.) But bundled with the hardware was a four-voice wave-table synthesizer program -- making the card a stereophonic sound synthesizer. At $151 in April, 1981, it wasn't on the market for long before I had one in my system. I spent many hours transcribing sheet music for the card. Lots of fun! The sound was fair -- the sampling rate was only a few kHz, so you didn't want to use complex waveforms for voices in the upper octaves. I coded perhaps four hours worth of music for the system, using an ascii-coded musical notation similar to what was eventually used in GWBASIC.

NOGDS also made a graphics card, the HA-8-3. Basically, the card allowed video-game-like graphics on a TV set. I bought one, but never did much with it. (There wasn't much software written for the card either, as far as I know.) $265 in October, 1982.

"It looks to be terminal."

The green-screen Zenith version of the H-19

The H-19 terminal was a major win for Heathkit. I think they sold more H-19s than they ever sold H-8s. It seemed like virtually every 8-bit box of that era had one attached. (The H-19 is still in the UNIX termcap database!) This was one of the first terminals available to the hobbyist which would do gasp! lower case letters -- with descenders, even. And the price was pretty reasonable: $675 in July of 1980. (It had been out for awhile when I finally broke down and bought one. I'd been using a 24-line by 72 character home-brewed video terminal, so I wasn't desperately in need of something better, I thought -- that was, until I had a chance to see an H-19 in action!)

Features? How about a nice easy-to-read screen -- viewing area around 10-1/2" diagonal -- with enough bandwidth in the video circuits so the dots were good and crisp, a keyboard that was really a joy to type on, inverse video, line-art graphics capability, a 25th line that was essentially a separate one-line display... Let's just say it was a professional-grade terminal.

(I guess it must have been -- when Heathkit created the H-89, they basically packaged a one-board Z-80-based computer inside an H-19. When they built the 8088-based H100 system, the text mode display was a software emulation of the H-19, with minor adjustments. They knew enough not to mess with success.)

Hard-copy Made Easy

To be continued…

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