Ferranti Sirius

Ferranti's Sirius was a small business computer released in 1961. Designed to be used in smaller offices without a dedicated programming staff, the Sirius used decimal arithmetic instead of binary, supported to ease programming, was designed to fit behind a standard office desk, and ran on UK standard mains electricity (then 240 V) with no need for cooling. It was also fairly slow, with instruction speeds around 4,000 operations per second, and had limited main memory based on delay lines, but as Ferranti pointed out, its price/performance ratio was difficult to beat.

During the 1950s there was widespread interest in the use of magnetic amplifiers as a solid state switching device. 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.

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. 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. 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.

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