Heathkit H8
Encyclopedia
Heathkit
's H8 was an Intel 8080
-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
. 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 was a successful design but 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 Z89
.
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 minicomputer
s and other systems in a number of roles.
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 only 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 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 (who 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.
-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 in order 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 in order to comply with government purchase requirements. Kit sales were ended soon after that purchase.
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. Also, the power supplies occupied some of the space needed for the last (which 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 "type in" a program via the toggle switches in order 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. Over time, 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.
Heathkit
Heathkits were products of the Heath Company, Benton Harbor, Michigan. Their products included electronic test equipment, high fidelity home audio equipment, television receivers, amateur radio equipment, electronic ignition conversion modules for early model cars with point style ignitions, and...
's H8 was an Intel 8080
Intel 8080
The Intel 8080 was the second 8-bit microprocessor designed and manufactured by Intel and was released in April 1974. It was an extended and enhanced variant of the earlier 8008 design, although without binary compatibility...
-based microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...
sold in kit form starting in 1977. The H8 was similar to the S-100 bus
S-100 bus
The S-100 bus or Altair bus, IEEE696-1983 , was an early computer bus designed in 1974 as a part of the Altair 8800, generally considered today to be the first personal computer...
computers of the era, and like those machines was often used with the CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
on floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
. 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
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
that made it easier to start up, including code for running basic input/output
Input/output
In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
and allowing input through a front-mounted octal
Octal
The octal numeral system, or oct for short, is the base-8 number system, and uses the digits 0 to 7. Numerals can be made from binary numerals by grouping consecutive binary digits into groups of three...
keypad and front panel
Front panel
A front panel was used on early electronic computers to display and allow the alteration of the state of the machine's internal registers and memory. The front panel usually consisted of arrays of indicator lamps, toggle switches, and push buttons mounted on a sheet metal face plate...
display instead of the binary switches and lights used on machines like the Altair 8800
Altair 8800
The MITS Altair 8800 was a microcomputer design from 1975 based on the Intel 8080 CPU and sold by mail order through advertisements in Popular Electronics, Radio-Electronics and other hobbyist magazines. The designers hoped to sell only a few hundred build-it-yourself kits to hobbyists, and were...
. The H8 was a successful design but 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
Computer terminal
A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system...
. This model also was sold in fully assembled form as the WH89. These were later sold by Zenith Electronics
Zenith Electronics
Zenith Electronics Corporation is a brand of the South Korean company LG Electronics. The company was previously an American manufacturer of televisions and other consumer electronics, and was headquartered in Lincolnshire, Illinois. LG Electronics acquired a controlling share of Zenith in 1995...
with their name on the front as the Zenith Z89
Zenith Z89
The Z-89 was a personal computer produced by Zenith Data Systems in the early 1980s. It was based on the Zilog Z80 microprocessor and ran the HDOS and CP/M operating systems...
.
Background
MITSMicro Instrumentation and Telemetry Systems
Micro Instrumentation and Telemetry Systems was an American electronics company founded in Albuquerque, New Mexico that began manufacturing electronic calculators in 1971 and personal computers in 1975. Ed Roberts and Forrest Mims founded MITS in December 1969 to produce miniaturized telemetry...
announced the Altair 8800
Altair 8800
The MITS Altair 8800 was a microcomputer design from 1975 based on the Intel 8080 CPU and sold by mail order through advertisements in Popular Electronics, Radio-Electronics and other hobbyist magazines. The designers hoped to sell only a few hundred build-it-yourself kits to hobbyists, and were...
in January 1975 and started selling kits soon after. Marketed to electronics hobbyists through trade magazines like Popular Electronics
Popular Electronics
Popular Electronics was an American magazine started by Ziff-Davis Publishing in October 1954 for electronics hobbyists and experimenters. It soon became the "World's Largest-Selling Electronics Magazine". The circulation was 240,151 in April 1957 and 400,000 by 1963. Ziff-Davis published Popular...
, 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
Edge connector
An edge connector is the portion of a printed circuit board consisting of traces leading to the edge of the board that are intended to plug into a matching socket. The edge connector is a money-saving device because it only requires a single discrete female connector , and they also tend to be...
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
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
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 minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
s and other systems in a number of roles.
H8
HeathkitHeathkit
Heathkits were products of the Heath Company, Benton Harbor, Michigan. Their products included electronic test equipment, high fidelity home audio equipment, television receivers, amateur radio equipment, electronic ignition conversion modules for early model cars with point style ignitions, and...
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
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
. 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
Kansas City standard
The Kansas City Standard , or Byte standard, is a digital data format for audio cassette drives. Byte magazine sponsored a symposium in November 1975 in Kansas City, Missouri to develop a standard for storage of digital computer data on inexpensive consumer quality cassettes, at a time when...
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 only 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 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
HDOS
HDOS is an early microcomputer operating system, originally written for the Heathkit H8 computer system and later also available for the Heath H89 and Zenith Z-89 computers...
, which was written by J. Gordon Letwin (who 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.
H89
In 1978 Heath introduced the Heathkit H88 which combined the H19 terminal and a new Zilog Z80Zilog Z80
The Zilog Z80 is an 8-bit microprocessor designed by Zilog and sold from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes...
-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 in order to enter the microcomputer market. They continued sales of the H89 with their own labeling on the front as the Zenith Z89
Zenith Z89
The Z-89 was a personal computer produced by Zenith Data Systems in the early 1980s. It was based on the Zilog Z80 microprocessor and ran the HDOS and CP/M operating systems...
. 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 in order to comply with government purchase requirements. Kit sales were ended soon after that purchase.
Description
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. Also, the power supplies occupied some of the space needed for the last (which 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
LEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....
display (early S-100 machines like the Altair or IMSAI 8080
IMSAI 8080
The IMSAI 8080 was an early microcomputer released in late 1975, based on the Intel 8080 and later 8085 and S-100 bus. It was a clone of its main competitor, the earlier MITS Altair 8800. The IMSAI is largely regarded as the first "clone" computer. The IMSAI machine ran a highly modified version of...
contained no ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
and when they were started the user had to "type in" a program via the toggle switches in order 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. Over time, 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.
External links
- "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
- "www.yesterpc.org", The computer museum with large Heathkit computer collection
