An Electronic Digital Computor Using Cold Cathode Counting Tubes for Storage

An Electronic Digital Computor Using Cold Cathode Counting Tubes for Storage By R. C. M. Barnes, B.Sc., E. H. Cooke-Yarborough, M.A., A.M.I.E.E., and D. G. A. Thomas, M.A. Electronic Engineering - August 1951. A small sequence-controlled computor is described; it has storage capacity for up to 90 numbers each of eight decimal digits. Cold cathode counting tubes (Dekatrons) are used as storage elements. Parallel operation is employed, and the speed of operation is comparable with that of a desk calculating machine. Apart from speed, however, the computor provides most of the facilities found in larger, faster digital computors. Information is fed into the machine from perforated paper tapes and a teleprinter or tape perforator records the results. SEVERAL large-scale computors have been described in recent years, including the ENIAC with hard valve circuits, the EDSAC with mercury delay storage, the University of Manchester computor with cathode-ray storage and the Birkbeck College computor with magnetic storage. These computors are intended primarily to carry out enormous computations, too lengthy to perform by other means. In addition smaller scale relay computors have been built for general work. The machine described here is not intended to deal with the long calculations for which the large computors have been designed. It is intended rather to do the work of a few operators with desk calculating machines where the work to be done is of a routine or repetitive nature, As a result, simplicity and reliability have been regarded as of more importance than speed of operation and the machine performs the operations of addition, subtraction, multiplication and division little faster than a desk calculator. Fig 1 is a schematic diagram of this machine. The design can, for convenience, be regarded as consisting of two parts, the first includes the means by which the machine receives orders, controls the sequence of operations and the routing of numbers to and from the stores, and prints the results, the second part consists of the means of storing numbers and, closely allied with this, the arithmetic unit in which the arithmetical processes take place. In the present machine these two parts are almost entirely separate. Because a relatively low operating speed is accepted, it becomes practical to read orders and other data from the perforated tape as they are needed in the calculations (this is done in the B.T.L. machine). Design is thus simplified and much less storage capacity is needed. The low operating speed also makes possible the use of relays for routing and sequence control. Since each relay may carry many contacts this leads to simplicity in setting up connexions. The fact that the techniques are well established and of proved reliability is of great importance. Page Number 1 of 9 Numerical data obtained in the course of the calculation must be stored in the machine. The use of relays for number storage is not attractive, as many relays would be needed. Other methods were considered, largely for their suitability for use in a computor with relay routing, and it was eventually decided to make use of cold cathode counting tubes ("Dekatrons"). These were recently described and are coming into increasing use for scaling and counting purposes. In these tubes, a gas discharge may be stepped round in to any of ten stable positions, and each tube is therefore capable of storing a decimal digit. The tubes have the advantage that numbers can be fed into them, or obtained from them, by use of relatively simple circuits. Moreover the numerical information is in the form of large amplitude pulses (tens of volts), so the possibility of spurious pulses being introduced from external sources is much reduced by comparison with other storage methods. Use of these tubes permits the computor to work on a decimal, rather than a binary, basis. This is convenient from several points of view; it is particularly advantageous in a parallel computor, since the number of digits and therefore the number of parallel arithmetical units, is considerably reduced. Provided that the number of stores required is not too large, this method of storage is thought to compare favourably with other methods on the basis of cost, size, and power consumption per digit. When the computor is fitted with its full storage capacity (90 eight-digit numbers, equivalent to 2,700 binary digits) just over 800 tubes will be used. The power consumption is about 150µA per tube at 370V. From the fact that the tubes operate at this low current and from life tests to date, tube life is expected to be many thousands of hours. Each storage address consists of nine tubes, corresponding to eight digits with one additional tube to indicate the sign. The decimal point is regarded as being after the first figure of the number stored whether this is a significant figure or zero, and the capacity n of each store is +10 > n > -10. Numbers are fed into stores and handled in the arithmetic unit as trains of pulses. Negative numbers are handled as complements except on the punched tape and when finally printed, when they appear as modulus and sign. The basic process of addition (or subtraction by addition of the complements) is carried out electronically in order to save the wear that would occur if high-speed relays were used. Multiplication and division is by a sequence of multiple additions (or subtractions), with the operation of a relay circuit to shift the decimal point as each multiple addition takes place. No provision is made for reading of tables of functions and the low operating speeds will often make it impracticable to calculate the required functions ab initio. This must be regarded as a limitation of the present machine; it may eventually be overcome by providing a special table reading attachment. The computor has been operating with 20 eight-digit stores for some months. This has made possible the establishment of most of the electronic and relay circuits. Although the machine has not yet been put into service it seems appropriate at this time to publish details of the design which has been established. The computor contains about 380 relays, 18 Dekatrons, 80 thermionic valves and 40 cold cathode triodes, plus 28 relays and 90 Dekatrons per ten stores. It occupies three 7 ft. Post Office racks, together with an additional rack per four groups of ten stores, one smaller rack for the power supplies and a table for tape readers and printers. Total power consumption is about I kW. FACILITIES The computor operates solely under the control of orders and data fed in from perforated paper tapes, and no manual switching or plugging up is necessary. Provision is made for a maximum of eight input tapes read by separate tape readers. Each tape may contain orders, input data, and constants. A typical allocation of tapes is expected to be a master programme tape, a tape giving the data relevant to the particular problem and a number of closed loops containing the longer sub-routines. The output of the computor is fed into either a perforator or a modified teleprinter operating as a page printer. A maximum of eight printers and perforators can be accommodated although only a few of the positions are used at present. It is possible to feed the perforated, tape from an output perforator into an input tape reader to give a long-term storage when the nature of the work permits. The first order is always read from tape reader No. 01 and subsequent orders come from the same source until an order specifies a change. If the source of orders is changed to a Dekatron number store, orders are drawn from successive stores until the sequence is interrupted, Orders are normally only stored in the Dekatron stores when they take the form of short sub-routines or if they are to be subjected to arithmetical operations; the majority of orders will be read directly from the tape readers. Page Number 2 of 9 Orders appear on the tape as blocks of five (decimal) digits and each digit is represented by one row of perforations. Numerical data are perforated as blocks of eight digits preceded by a sign character. Blocks of digits are separated by a character which can be either a space or one of ten block-marking characters. A search facility is provided by which any tape reader can be made to search along its tape until it finds the required block-marking character. ARITHMETICAL ORDERS Arithmetical orders consist of one digit defining the arithmetical operation to be performed, a sending address (two digits) and a receiving address (two digits), The sending address may be a Dekatron store, the accumulator, a tape reader, or the rounding-off circuit The receiving address may be a store, the accumulator, a printer or perforator, or a drain for unwanted numbers. Seven operation digits have been allocated as follows: (1) Add the contents of the sending address into the receiving address, and leave the contents of the sending address unchanged. (2) Add the contents of the sending address into the receiving address and clear the sending address. (3) Subtract and do not clear the sending address. (4) Subtract and clear the sending address. (5) Multiply the contents of the sending address by the contents of the “receiving address" clearing the latter, and add the product into the accumulator. (6) Divide the contents of the accumulator by the contents of the sending address, form the quotient in the receiving address and leave the remainder in the accumulator.

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