Elliott 803 - Hardware Description

Hardware Description

The 803 is a transistorised, bit-serial machine; the 803B has more parallel paths internally. It uses ferrite core memory in 4096 or 8192 words of 40 bits, comprising 39 bits of data with parity. The CPU is housed in a single cabinet about 66 inches long, 16 inches deep and 56 inches high. Circuitry is based on printed circuit boards with the printed circuits being rather simple and most of the signalling carried on wires. There is a second cabinet about half the size used for the power supply, which is unusually based on a large nickel-cadmium battery with charger, an early form of uninterruptible power supply. A third cabinet (the same size as the power cabinet) holds the extra working store on machines with 8192 word stores. There is an operator's control console, Creed teleprinter and high-speed paper tape reader and punch for input/output, using 5-track Elliott telecode code, not Baudot. Tape is read at 500 characters per second and punched at 100 cps. The operator's console, about 60 inches long, allows low-level instructions to be entered manually to manipulate addresses and data and can start, stop and step the machine: there is a loudspeaker (pulsed by the top bit of the instruction register) which allows the operator to judge the status of a computation. The system requires air conditioning, drawing about 3.5 kW of power in a minimal configuration.

Optional mass storage is available on an unusual magnetic tape system based on standard 35 mm film stock coated with iron oxide (manufactured by Kodak). At the time this was in use by the film industry to record sound tracks. Elliott's factory at Borehamwood was close to the Elstree film studios which explains the use of the 35mm sprocketed media. The 1000 foot reels held 4096 blocks of 64 words per block (4096 x 64 x 39 = 10,223,616 bits, or the equivalent of about 1.27Mbytes).

Another unusual feature is the use of magnetic cores not only for memory but also as logic gates. These logic cores have 1, 2 or 3 input windings, a trigger (read) and a output winding. Depending on their polarity, current pulses in the input windings either magnetise the core or cancel each other out. The magnetised state of the core indicates the result of a boolean logic function. Two clock phases designated alpha and beta are used to trigger (reset to zero) alternate cores. A change from a one to a zero produces a pulse on the output winding. Cores which receive alpha trigger pulses (alpha cores) have inputs fed from gates which are triggered on the beta phase (beta cores). Transistors were expensive at the time and each logic gate requires only one to amplify the output winding pulse; however a single transistor drives the inputs of a small number of (typically 3) other cores. If more than 3 inputs are to be driven, up to two additional transistors can be driven by each core.