Marconi Transistorised Automatic Computer (T.A.C.)

The Marconi Transistorized Automatic Computer was Marconi Company's first computer, designed and manufactured from around 1959.^[1] They employed Germanium transistors which by this time were sufficiently reliable with room temperatures kept below about 23 degrees C.^[2] Apart from transistor failure other common faults included Power Supply capacitors 'drying out' resulting in excess ripple and poor connections on the computer input output highway. Paper tape peripherals had their own poor reliability influenced by degree of operator usage. The type S3301 was a 500 kHz clocked 20 bit word machine with two Mullard core memory stores providing 4k of 20 bit data, see Ref. 4. The iinternal CPU logic was synchronised to even and odd clock signals and special signals generated via the microinstruction diode boards. The memory logic had slow and fast loops to speed the transfer of sequential data bursts. Its microinstructions were programmed on diode bosrds. A facility was provided to microstep through instructions to help with fault finding. Processor status bits were provided with machine instructions being decoded from 6 bits in the current address memory word. Double word data had the MSB designated a sign bit coded as binary fractions ( -1 to +1) for the square root, multiply and divide instructions. The instruction set had the usual functions based on three registers named A, B and D (C was the current address in memory register, M). An additional instruction assisted with checksum calculation for data transferred to and from main data stores (viz. Sperry Rand magnetic drums.^[3]).^[4]

Applications included marking up radar screens with aircraft info ^[5] and providing data processing for operators in a Nuclear Power Station.^[6] Surviving computers (ex Power Station) are on display at Bletchley Park National Museum of Computing (operational)^[7] and Jim Austin's collection near the University of York.^[8] Addition manuals and documentation exist at the School of Computer Science, University of Manchester and the Manchester John Rylands Library, the original performance summary specification at.^[9] Marconi went on to develop the Myriad series of computers^[10]

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