Heathkit's H8 is an Intel 8080A-based microcomputer sold in kit form starting in 1977. The H8 was similar to the S-100 bus computers of the era, and like those machines was often used with the CP/M operating system on floppy disk.
first eight-bit Heathkit H8 computer (collection www.yesterpc.org)
The main difference between the H8 and S-100 machines was the bus; the H8 used a 50-pin bus design that was smaller, more robust and better engineered electrically. The machine also included a bootstrap ROM that made it easier to start up, including code for running basic input/output and allowing input through a front-mounted octal keypad and front panel display instead of the binary switches and lights used on machines like the Altair 8800.
The H8 required a separate terminal to be truly useful; Heathkit introduced several terminals as well. A successor model, the "All-in-One" Heathkit H89, combined a Z-80 processor board and a floppy disk drive into the cabinet of an Heathkit H19 terminal. This model also was sold in fully assembled form as the WH89. These were later sold by Zenith Electronics with their name on the front as the Zenith Z-89.
MITS announced the Altair 8800 in January 1975 and started selling kits soon after. Marketed to electronics hobbyists through trade magazines like Popular Electronics, the company founders felt there would be limited appeal and expected to sell only a few hundred systems. Instead, they received orders for thousands in the first month. Sales were so much greater than expected that MITS was unable to clear the order backlog for the better part of the year.
The Altair sparked off such intense interest in the microcomputer world that a number of other companies jumped in to fill the sales backlog, building machines that were clones of the Altair. The primary component of this design was the S-100 bus, so named because it used a 100-pin edge connector that MITS found at bargain prices when they were designing the machine. Unfortunately the pins were connected from the backplane with no real thought put into their layout, and it had a number of problems that made it unreliable.
In spite of any design flaws, standardization led to a flourishing of companies selling into the S-100 market. The introduction of floppy disk controllers and the disk-based CP/M operating system dramatically improved the system's capabilities and started the process of turning them into practical small-business tools. By the late 1970s they were starting to displace minicomputers and other systems in a number of roles.
Heathkit was a long-established player in the electronics market, making kits for products that had proven themselves in the market. Some of these were quite complex, including a color television. In 1977 they decided to enter the microcomputer market, and designed the H8. The machine was announced in July 1977 and started selling that fall at a price of $379.
To be useful, the user also need to purchase a 4 kB SRAM card ($139) and some form of storage controller; at a minimum this would be the H10 paper tape punch/reader or the H8-5 Serial I/O card ($110) which controlled a cassette tape, using a 1200-baud variant of the Kansas City standard format. Another common accessory was the H9 video terminal, which was also driven by the H8-5 card; although any serial terminal would work. Unfortunately, the H9 was inexpensive but ugly in appearance, was limited to upper case characters and 12 display lines, and used a cheap array of switches for its keyboard. It was eventually superseded by the H19 terminal, a nicer design ergonomically and capable of lower-case and graphic-like characters. The H19 became a major product line of its own. The H17 floppy disk system became available in 1978, normally sold with one drive but expandable with a second (and later to three). Use of the H17 required at least 16 kB of RAM. The H8 could use CP/M, and often did, but early machines required either a special version of CP/M that was "org'd" at 8K instead of zero, or a small hardware modification and an updated ROM to do so. Heath also had its own DOS, unsurprisingly named HDOS, which was written by J. Gordon Letwin. Letwin later went to Microsoft to become the chief architect of OS/2.
At the time the H8 was introduced, the computer market was in the midst of a shift from the hobby market that had spawned it to a "user" market that purchased pre-assembled machines. Heath followed this trend and introduced the WH8 in fully assembled form for $475. Like the H8, the WH8 would need to include several other cards to be useful. The disk drive system was also available fully assembled as the WH17. For the CP/M operating system, Heathkit provided the WH67, a 10 MB eight-inch hard drive and the H47 eight-inch floppy disk system.
In 1978 Heath introduced the Heathkit H88 which combined the H19 terminal and a new Zilog Z80-based single-board processor into the case of the H19. A version with a disk drive incorporated to the right of the terminal screen became the H89. The machines bore a strong resemblance to the TRS-80 Model III and similar all-in-one computers. The H89 was available both in kit form for $1595, and fully assembled form as the WH89 for $2295.
Soon after the introduction of the H89, Heathkit was purchased by Zenith to enter the microcomputer market. They continued sales of the H89 with their own labeling on the front as the Zenith Z89. Eventually, Zenith Data Systems (Heathkit plus the computer division of Zenith) was purchased by Bull HN (CII Bull, Honeywell and Nippon Electric) because they needed a US maker of microcomputers to comply with government purchase requirements. Kit sales were ended soon after that purchase.
Heath chose not to implement the S-100 bus and instead created their own, known as "Benton Harbor Bus" after their home town. The bus was based on a 50-pin connector and laid out to avoid the electrical problems of the S-100 system (like +5V and ground being placed beside each other). The H8 was packaged in a box-like chassis with pressboard sides and metal sheeting for the rest of the case. The top sheet was heavily perforated to form cooling vents.
The machine was built up from the backplane mounted on the right-hand side panel of the case, with ten 50-pin card slots. The first and last slots were spaced differently from the rest, and the power supplies occupied some of the space needed for the last card. This meant that the last card not only had to accommodate the narrow spacing but also could not be full-length, leaving eight "standard" slots available for full-length cards. The front panel plugged into the first slot and the CPU plugged into the second, leaving seven for further expansion. The card slots were arranged on an angle, which allowed the case to be reduced in height. Each card contained its own voltage regulators, using the Z-shaped mounting bracket as a heat-sink. (Power distribution on the backplane was unregulated +8V and +/-18V; the cards regulated these to their requirements, typically +5V and +/- 12V.)
Another notable change was the replacement of the front-panel toggle switches and lights of a standard early-model S-100 system with a keypad and seven-segment LED display (early S-100 machines like the Altair or IMSAI 8080 contained no ROM and when they were started the user had to "key in" a program via the toggle switches to read a paper tape. Once this "loader" program was ready, a paper tape containing a more complete loader would be read in, allowing the user to load programs from cassette or floppy disk).
On the H8 all of this code was already pre-installed in a 1 kB ROM in a monitor program known as "PAM8", occupying locations 0 through 3FF16 and the H17 disk I/O drivers used for booting, occupying a 2 kB ROM occupying locations 180016 through 1FFF16 The ROM contained code to control the keypad and display, booting it directly into an operable state. Several versions of the PAM-8 ROM were sold as upgrades; at one point Heathkit switched to using 2 kB ROMs, occupying through 7FF16 and subsequently to a 4 kB ROM occupying through FFF16. The ROMs interfered with the operation of standard CP/M, which assumed it could write the memory near location 0, in particular the interrupt handler pointers.
PAM8 and portions of HDOS used an unusual address notation called "split octal" where 16-bit numbers were split into two 8-bit numbers printed in octal: the first location was "000.000" and the location after "000.377" was "001.000". Most mini and micro computers used either straight octal (377 was followed by 400) or hexadecimal. With the introduction of the optional HA8-6 Z-80 processor replacement for the 8080 board, the front-panel keyboard got a new set of labels and hexadecimal notation was used instead of octal.
Benton Harbor BusEdit
The 50-pin "Benton Harbor Bus" was considered an improvement on the S-100 bus. The 50-pin bus of the H8 contains sixteen address lines, eight data lines, 5 interrupt lines, and the system control lines. The bus does *not* supply +5V -- each card is expected to have its own local +5V regulator powered from "unregulated" +8V on the bus. 
- MITS (June 1975). "MITS advertisement". Digital Design. CMP Information. 4 (6). Retrieved 1 January 2008. "There was a subsequent article in February's Popular Electronics and the MITS people knew the Altair was here to stay. During that month alone, over 1,000 mainframes were sold. Datamation, March 1975." "By the end of May, MITS had shipped over 2,500 Altair 8800's"
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- Dave Wallace,  Archived 23 July 2011 at the Wayback Machine, 29 September 2001
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|Wikimedia Commons has media related to Heathkit H8.|
- "Heathkit Manual for the Digital Computer Model H8", Heath Company, 1977
- "In your Free Heathkit Catalog", early Heathkit computer advertisement
- "Introduction of the Personal Computer", Heathkit brochure for the H8 and H11