S-100 bus

S-100 bus
Cromemco · CC BY-SA 3.0 · source
Name	S-100 bus
Invent-date	1974
Invent-name	MITS
Width	8-bit, 16-bit

S-100 bus. The S-100 bus, also known as the Altair bus, was a pioneering early computer bus architecture that became a de facto standard for the first generation of microcomputers. Introduced with the MITS Altair 8800 in 1974, it provided a flexible expansion system that enabled a burgeoning industry of third-party hardware manufacturers. Its open design was instrumental in the growth of the homebrew computer club and the early microcomputer industry, influencing numerous subsequent systems.

History and development

The bus was created by MITS engineers, including Ed Roberts, for the Altair 8800, which was based on the Intel 8080 microprocessor. Its specification was largely published in hobbyist magazines like Popular Electronics, encouraging widespread adoption and modification by the community. The lack of formal standardization initially led to compatibility issues, prompting the IEEE to later codify it as standard IEEE-696 in 1983. This period saw intense activity from companies like Processor Technology, IMSAI, and Cromemco, all producing competing S-100 systems and cards.

Technical specifications

The architecture originally defined 100-pin edge connectors, with signals largely following the pinout of the Intel 8080 CPU, including a 16-bit address bus and an 8-bit data bus. The IEEE-696 standard later refined this to support 16-bit data transfers and 24-bit addressing, accommodating more advanced processors like the Zilog Z8000 and Intel 8086. Key signals managed memory-mapped I/O, direct memory access (DMA), and vectored interrupts. Electrical specifications outlined +8V, ±16V power rails and defined timing for synchronous and asynchronous operations.

Hardware implementation

A typical system consisted of a passive backplane with multiple card slots into which active processor, memory, and interface cards were inserted. Prominent manufacturers like Cromemco and North Star Computers produced sophisticated cards for dynamic RAM, EPROM, and floppy disk controllers. Specialized cards for graphics, such as the Cromemco Dazzler, and for IEEE-488 instrument control were also common. The physical construction often involved wire-wrapped prototypes and large card cages, reflecting the era's hands-on engineering ethos.

Software and programming

Early software development was performed via front-panel switches, but the bus soon supported bootloaders and monitor programs stored in EPROM. The CP/M operating system, developed by Digital Research, became the dominant software platform for S-100 business systems, requiring a specific disk controller and BIOS implementation. Programming languages available included Microsoft BASIC, FORTRAN, and Pascal, with development tools often distributed on 8-inch floppy disks. The architecture influenced early work at Microsoft and the development of the SCP 8086 system.

Applications and systems

Beyond the iconic MITS Altair 8800, the bus was used in a wide array of systems, including the IMSAI 8080, the Sol-20, and the Cromemco System Three. These machines found roles in small business accounting, scientific laboratories, and early computer-aided design. Specialized applications included typesetting with machines from Mergenthaler Linotype Company, and real-time control in industrial settings. Systems from Vector Graphic and Morrow Designs were also notable in the small office market.

Legacy and influence

The S-100 bus ecosystem demonstrated the viability of an open hardware standard, directly paving the way for later expansion buses like ISA in the IBM Personal Computer. It fostered the growth of countless hardware and software startups during the late 1970s and early 1980s. Many pioneers of Silicon Valley, including members of the Homebrew Computer Club, cut their teeth on S-100 systems. While obsolete by the mid-1980s, it remains a historically significant artifact in the History of computing hardware and the birth of personal computing.

Category:Computer buses Category:Microcomputers Category:IEEE standards