The design philosophy of Pegasus, a quantity-production computer1
W. S. Elliott / C. E. Owen / C. H. Devonald B. G. Maudsley
Summary The paper gives an historical account of the development of the packaged method of construction of computers, and the advantages of this method are discussed. The packages used in the computer Pegasus are described from both an electronic and a mechanical point of view. The specification of the machine is given and the arguments which led to this specification are discussed. The detailed logical design procedure leading from the specification to the wiring lists is described. The method of maintenance and some reliability figures are given.
The development of standard plug-in unit circuits ('packages') for digital computers began in this country [England] in 1947, and some of the advantages of the method have been discussed in earlier papers [Elliott, 1951; Johnston, 1952; Elliott et al., 1952; Elliott et al., 1953]. The advantages start in the design stage of a new computer project and follow through production and commissioning to maintenance.
In the design stage, what is known as 'logical' design is separated from engineering design. Once the packages have been designed by electronic engineers and the rules for their interconnection have been laid down, the 'logical designers' (usually, but not necessarily, mathematicians) can begin organizing the packages into various computers to carry out different functional requirements. The electronic and mechanical design work invested in the packages is thus drawn on for more than one computer design, and each computer can be assembled from stock parts without further engineering effort. Design time and cost are therefore much reduced.
In production, whether we consider one design of computer or several designs using the same packages, costs and time are also much reduced. Quantity production lines for the relatively few types of standard package are set up, and are common to different computer designs, thus reducing inspection and planning costs. Standard cabinet work has been designed for Pegasus, and this too can be taken from stock or established production lines to make other computers.
In commissioning a computer, because all the packages have been pretested, when power is first applied to the complete machine it is known that a large part is already fault-free. It remains to detect a few errors which may have been made in the interconnections.
Perhaps an even more important consideration is ease and speed of maintenance. Test programmes will usually indicate the part of the machine in which a fault is occurring. Several monitor sockets are located on the front of each package, and by inspection the faulty package is speedily found and replaced.
The package method has been criticized on the grounds of the cost and questionable reliability of plugs and sockets, and some redundancy of components.
The authors believe that the many advantages far outweigh the cost of plugs and sockets. The present trend is to use copper-etched printed circuits, and these fall naturally into the plug-in unit idea, the plug contacts being part of the printed wiring; there has been no trouble in Pegasus from plugs and sockets. Component redundancy in Pegasus is about 10% of the diodes and a few resistors, the cost of redundant components being about L150.
Electrical design of the packages
Circuits used for arithmetic and switching operations
Historical. A previous data-processing machine [Elliott et al., 1952; Elliott et al., 1956b] used 330 kc/s serial-digital circuits; they had originally been designed for 1 Mc/s operation, but 330 kc/s was chosen to suit an anticipation-pulse cathode-ray-tube store. This frequency has been retained to the present time because it suits the magnetostriction delay-line store [Fairclough, 1956] and the magnetic-drum store [Merry and Maudsley, 1956]. Experience with the data processor led to work (commenced in 1951) on a new set of circuits [Elliott et al., 1952], particular emphasis being
1Proc. LEE, pt. B, vol. 103, supp. 2, pp. 188-196, 1956.