Information Technology Limited

Computer Technology Limited or CTL, was a British computer company founded slightly later than Digital Equipment Corporation (DEC) in the United States.

Founder Iann Barron had worked for Elliott Computing but left to form CTL when he couldn't persuade Elliott to incorporate his ideas in their next generation of computers. He left in 1973, going on to form Inmos and develop the transputer.

The first CTL computer (the Modular One) appeared for sale in 1968.

The Modular One was a 16-bit computer built with Emitter Coupled Logic (ECL) and was competitive with other first generation minicomputers.

A key feature, from which it derived its name, was that it was composed of separate processor, memory and peripheral modules sharing a common interface and physical form factor, so allowing them to be put together in any combination, housed one or two high in modular racking. Standard modules were roughly 50cm wide and deep, 70cm tall, and complete with power supply, typically weighed in excess of 25kg. Modules were interconnected using a single type of interface, comprising two identical cards to be plugged into two modules to be connected, these cards themselves linked by a flat ribbon cable either one or two metres long. Thus, memory was just another peripheral (such as a printer) but was both input and output.

Every interaction over these interfaces comprised a 3-way handshake, which in the case of a processor accessing a memory module, consisted of send address, receive data, and send new data, a scheme well suited to the destructive read followed by rewrite required by magnetic-core memory of the time. These three phases were mediated by voltage edges rather than pulses, as this was thought to be faster. Furthermore, the input and output impedances of ECL were comparable to the characteristic impedance of ribbon cable. This, together with the small voltage swings between the "0" and "1" states made for low noise, reflection-free communication.

Processors naturally had a number of interface slots bussed together, allowing connection to memory and peripheral modules as required. Memory modules had several, allowing them to be accessed by more than one processor as well as by disc controllers for DMA. Disc controllers could be connected to two processors as well as memory modules. All modules had a "1.x" type designation, for example, the original processor module was 1.11, memory modules were 1.2x, character peripherals were 1.3x, discs were 1.4x and magnetic tape devices were 1.5x. The standard interface was designated 1.01.

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