Heathkit H8

Background 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 is the S-100 bus, so named because it uses a 100-pin edge connector that MITS found at bargain prices when they were designing the machine. Unfortunately, the pins are connected from the backplane with a disorganized layout, and it has a number of problems that make it unreliable. 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 beginning to displace minicomputers and other systems in a number of roles. H8 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. The company had considered designing a kit computer as early as 1974, but concluded that it was not a good fit for their traditional market. The successful launch of the Altair changed things, and in 1977 Heathkit decided to design a kit similar to the Altair but addressing its more obvious shortcomings. The H8 was announced in July 1977 and started selling that fall at a price of $379. For full functionality, the system also requires a 4 KiB 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 controls a cassette tape, using a 1200-baud variant of the Kansas City standard format. 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. ==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 first and last slots are spaced differently from the rest, and the power supplies occupy some of the space needed for the last card. This means that the last card not only has to accommodate the narrow spacing but also cannot be full-length, leaving eight "standard" slots available for full-length cards. The front panel plugs into the first slot and the CPU plugs into the second, leaving seven for further expansion. The card slots are arranged on an angle, which allows the case to be reduced in height. Each card contains its own voltage regulators, using the Z-shaped mounting bracket as a heat-sink. (Power distribution on the backplane is unregulated +8V and +/-18V; the cards regulate these to their requirements, typically +5V and +/-12V.) Another notable change is 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 contain no ROM and when they are started, the user "keys in" a program via the toggle switches to read a paper tape. Once this "loader" program is ready, a paper tape containing a more complete loader can be read in, allowing the user to load programs from cassette or floppy disk). On the H8, all of this code is already pre-installed in a 1 KiB 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 KiB ROM occupying locations 180016 through 1FFF16 The ROM contains 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 KiB ROMs, occupying through 7FF16 and subsequently to a 4 KiB ROM occupying through FFF16. The ROMs interfere with the operation of standard CP/M, which assumes it can write the memory near location 0, in particular the interrupt handler pointers. PAM8 and portions of HDOS use an unusual address notation called "split octal" Heath/Zenith ended H8 manufacturing in 1981 because its design did not comply with FCC Part 15 regulations. == Benton Harbor Bus ==

The 50-pin Benton Harbor Bus was considered technically superior to the S-100 bus. The 50-pin bus of the H8 contains sixteen address lines, eight data lines, five interrupt lines, and the system control lines. Like the S-100 bus, it does not supply +5 V; each card is expected to have its own local +5 V regulator powered from "unregulated" +8 V on the bus. The bus is laid out to avoid the electrical problems of the S-100 system (like +8V and -16V being placed beside each other). H8 engineers cited several reasons for not usisng the S-100 bus: They had not heard of it when H8 design began, the bus would not have fit the cabinet, the Benton Harbor bus was both electrically superior and less expensive, and Heath did not want to encourage customers dissatisfied with "weird hardware accessories" to blame the company. Regardless of the reason, H8 customers became dissatisfied with not being able to use the many S-100 products; Heath attempted to compensate by releasing Benton Harbor Bus-compatible accessories. A Heath owner complained in 1983, however, that Heath/Zenith's prices were much higher than equivalents for other computers. ==Benton Harbor BASIC==

Heathkit also introduced their own dialect of the BASIC programming language. Two versions were available, Benton Harbor BASIC that supported the most basic commands and lacked string variables, and Extended Benton Harbor BASIC which required at least 24 kB of memory and added string variables, integer types, and commands for working directly with the floppy disk without having to exit to CP/M or the monitor. Modelled on Dartmouth BASIC, as opposed to popular later variations like HP Time-Shared BASIC or Microsoft BASIC, the language had a number of idiosyncrasies. ==See also==