Ferranti Sirius

During the 1950s there was widespread interest in the use of magnetic amplifiers as a solid state switching device.[2] The amplifiers used the saturation points and hysteresis curves of a magnetic core to sum a number of inputs and settle to a single output state. The various logical functions were achieved by linearly adding the input signals on control lines and generating an output signal if the sum exceeded a fixed threshold, defined by the saturation property of the magnetic core. This process came to be known as "Ballot Box Logic" due to the way the inputs created a majority-rule on the output. One difference between magnetic logic and conventional tube or transistor systems is that it is the current that defines the logic levels, not voltage.[3]

Since the magnetic cores were open in the middle, any number of control lines could be threaded through them. This was particularly useful when implementing a best two-out-of-three, a common logic circuit used in binary adders.[3] Another possibility is to use the same core as the switching element in several different portions of the machines logic. For instance, a single core could be used as part of the system that reads instructions from memory, and then again as part of the ALU, as long as both functions don't operate at the same time (as they would in an instruction pipeline).

Interest in magnetic amplifiers lasted only a short time through the 1950s. When they were first being studied, transistors were expensive and unreliable devices, but the introduction of new manufacturing techniques in the late 1950s started to address both of these problems.[3] In spite of their other advantages, magnetic amplifiers quickly disappeared as transistor based logic became increasingly common, and only a few computers based on these systems were produced.[4]

One group working on the magnetic amplifier design was Gordon Scarrott's team at Ferranti R&D labs in West Gorton, Manchester.[3] This team had a longstanding partnership with Manchester University, after commercializing the Manchester Mark 1 and several follow-on designs.

As the prices of transistors dropped, Ken Johnson, an engineer at the lab, proposed a new type of transistor-based logic that followed the same conventions as the magnetic amplifiers, namely that binary logic was based on well known currents instead of voltages. Like the magnetic amplifiers, Johnson's design could be used to control several different inputs. Better yet, the system often required only one transistor per logic element, whereas conventional voltage-based logic often required two or more.[3] Although transistors were falling in price they were still expensive, so a machine based on Johnson's design might offer similar performance at a much lower price. He dubbed the concept "Neuron" due to its resemblance to the neurons in the brain.[5]

Interest in Neuron was high, and the team decided to build a small machine to test it, known as "Newt", [6] for "Neuron test". This machine was successful, and the lab was so impressed that they decided to expand the testbed into a complete computer. The result was the Sirius, which was much less expensive than similar machines using traditional transistor logic.[3] Sirius was announced on 19 May 1959 with claims that it would be the smallest and most economically priced computer in the European market.[7] The Sirius was marketed in England for £20,000, a deal compared to its competitors, the Elliott 803 at £35,000 and the ICT 1301 at £120,000.[8] About 20 were produced in total.[9]

Convinced that Neuron was a major advance, Ferranti R&D proposed a much larger machine based on the same logic, one that would have even greater price advantages over traditional designs. The new machine was aimed at the business market, not their traditional high-performance niche, and Prudential plc signed up as a launch customer while several other large insurance firms followed. Emerging as the Ferranti Orion in 1961,[10] the system proved to be a disaster. As the machine was much larger than the Sirius physically, it had longer wire runs and thus required larger currents to operate the Neurons.[3][6] Electrical noise and settling times were major issues, and Orion was much slower than promised.

Engineers at other Ferranti offices were concerned about the Neuron-based design from the start, but were never able to convince management to stop the effort.[6] When Orion failed, these teams were able to convince Prudential that they could deliver a machine five times as fast at the same price point within three years. A ground-up redesign using traditional transistor logic followed and emerged in 1964 as the Orion 2.[11] However, the losses caused by the Orion project were too great for management's taste, and the computer divisions had already been sold off to International Computers and Tabulators in October 1963.