Book created: 03-12-2017
This project is about designing, building, programming and utilizing a mini-computer based on Zilog Z80 microprocessor.
LC-81 is a hobbist retro project, a home-brew minicomputer designed and built with technologies of the 1970s. Based on Zilog Z80 eight-bits microprocessor, if features a modular, extensible architecture and a legacy, nostalgic taste.
The motivation for this project is both technical and cultural. In the technical side, I tried to produce a computer machine so simple that I can understand entirely from application software to chips and wires. In the cultural side I wanted to have a taste of an epoch (1970s) when minicomputers of this kind were popular and lovely.
LC-81 was conceived to be both simple and "historical correct" with respect to the assumed retro context. Unfortunately some legacy components such as punch tapes and magnetic tapes are out of my reach, so I will need to replace them with others such as audio cassettes and floppy disks, but the aim is still to keep the machine as old fashioned as possible even with these limitations.
The Minicomputer concept
Minicomputers was not a trivial concept in the 1960s. On the contrary, making computers with less power seamed to be the opposite of what computer manufacturers had been doing steadily from the very beginning: producing machines with increasing speed and capabilities.
Not even Science Fiction was able to anticipate a turn in this pattern. Movies such as “The Forbin Project” and “2001 a Space Oddysee” depicted centralized giant machines capable of producing human-like though just because of their inmense computer power. And power were directly related to size, hence the image of machines the size of a building.
The path that eventually lead to minicomputer has its roots, perhaps, in the SAGE project. In those days, the Air Defense asked for a sophisticated ground control system capable of making sense of radar data to anticipate the approaching of enemy aircrafts. With this end, an experimental computer named “Whirlwind” was designed and built at MIT using government funds. This machine was big and powerfull, actually the fastest machine ever built by that time, but the key to its success was due in part by the use of 16 bits word length, in contrast to 48 to 64 used in conventional main frame computers. More important, perhaps, is the intereactive fashion in which Whirlwind was operated. This inspired Ken Olsen, one of the participant engineers, to rethink computing all together and found Digital Equipment Corporation in 1957 to start manufacturing, two years layer, a new kind of computer soon known as “minicomputer”.
Minicomputers had small word length: from 12 to 18 bits and little memory: between 4 and 64 Kilo words. This contrasted to big mainframes with one or two megawords, 64 bits each. Minicomputer also lacked the generouse troughput of large mainframes and they came with little software. The name “mini” was more than justified, but this also applied to the price tag, about two or three orders of magnitud less expensive, and that were a definitory advantange.
These more afordable machines made their ways into lands where conventional main frames were forbiden, such as clinic laboratories, machinary control and schools. They got closer to the public than never before so common people started to get involved. That was long before personal computers (micro-computers) entered the scene.
The LC-81 mini-computer consists of various rack mount units interconnected together by the mean of a proprietary 37-lines parallel bus called EXT-BUS.
A minimal configuration comprises the Master Controller (MC) and some other unit. The MC provides the intelligence to the system containing the microprocessor (Zilog Z80 CPU) and the first 24 KB of memory. The other units provide I/O resources, memory extension and, perhaps, specific functionality for a specific application.
As many units as needed can be connected throughout the EXT-BUS to conform a large equipment. Examples of possible extra units are: Tape Drive Controller, Disk Controller, Serial Ports, Printer Controller, Real-Time Timer, GPIO and many others.
All circuits are built on prototyping boards conveniently mount on industrial 19 inches rack-mount trays. Blank panels were used to make the Console front.