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Integrated Magnetic Circuits for Synchronous Sequential Logic Machines

01 March 1960

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The ferrite memory core has many attractive features as a switching element for digital computer circuits.1,2'3'4 5 6 It combines a memory function with the circuit equivalent of a switch, and it is small and inexpensive. Furthermore, as a magnetic amplifier it can provide gain. It is also believed to be reliable, the only ambients to which it is susceptible being temperature, external magnetic fields and mechanical stress. In spite of this, its use in logical circuits has been mostly limited to access circuitry for magnetic memories. 295 296 T H E HELL SYSTEM TECHNICAL JOURNAL, MARCH 1 1 ) 0 0 Why is this? One reason is that the maximum rate at which presentday cores can be cycled is around 1 mc, the limitation being the rise in temperature produced by hysteresis losses. However, this speed is sufficient for many systems, and materials are being continually improved in this respect. A more pertinent reason is to be found in the circuit approaches that have been used. Additional circuit components, and diodes and transistors in particular, are usually included. The result has been that the total count of semiconductor components in some cases -- for example, shift registers -- is not materially reduced compared to the equivalent all-semiconductor circuits. Thus, the reliability and cost are set mostly by the semiconductor components. In addition, in order to provide a correct impedance match between the magnetic and semiconductor components, it is necessary to use multiturn windings on the cores, and this considerably increases the cost of assembly.