Chapter 9 The design philosophy of Pegasus, a quantity-production computer 181

panels. Warm air from the main source of heat, the valves, is prevented by the valve panels from reaching the more temperature-sensitive components, such as diodes, secured to the component panel.

The back panel wiring. For locating long signal wires between sockets a system of plastic strips is used, which hold the wires at definite positions given by the instructions on the wiring lists. The exact route of every wire is predetermined, thus making wiring and inspection more reliable and fault finding and maintenance easier.

Final assembly. The completely wired frame is assembled in its cabinet, which has already been fitted with the control and auxiliary supply circuit unit, heater transformers, fuses, cooling assembly and cableforms. The work of connecting the cableforms, heaters and earths can be done by relatively unskilled labour working to clearly written instructions and diagrams.

The cooling system. Each cabinet has its own cooling system as an integral part of the construction; there is therefore no difficulty in cooling cabinets added to existing computers. Two axial-flow turbo blowers are mounted in the base beneath an airtight pressure chamber, each providing 300 ft³/min of air at a total pressure head of 1 in (water gauge). The maximum temperature rise is 10^o C.

The power supply. A separate cubicle houses metal rectifiers, shunt stabilizing valves and control circuits. The power is obtained from the mains through a motor-alternator set, the output of which is stabilized to 2%, the main purpose of this set being to act as a buffer against switching surges and other mains voltage variations. The valve heaters in the computer are energized from the stabilized alternator output, which is expected to extend the valve life.

All digital computers so far have a fault rate which cannot be ignored. When the best has been done in the choice of components, circuits and mechanical construction, attention must be paid to the following points to get the best out of a machine:

There are parity-checking circuits on both the main and the high-speed stores. Errors of a single digit in the stores stop the machine. The fault can then be quickly located by examination of the monitors.

For other faults the general method is to run a test programme (assuming the fault is not in the main control) which will indicate the area of the fault. Detailed examination can then be carried out with the monitors.

All outputs of circuit units are readily accessible at monitoring sockets on the front of each package, and in addition about 80 points can be directly selected by switches from the monitoring position: these include all store lines and a number of key waveforms. Fault-finding is normally a matter of tracing 0's and 1's through the machine with reference to logical diagrams rather than electronic circuit diagrams.

A variety of triggers can be selected for the monitor time-bases, these including

These triggers and some other monitoring facilities are produced by 19 standard packages and are found to be well worth the extra equipment.

Once a faulty package has been located, the machine can be got working again immediately by replacement of the package with a spare; repair of the faulty package can be done at leisure with the aid of a package tester. With this equipment a package can quickly be given a series of standard tests; each is selected by switches, and the performance is measured either by observation of meters or a built-in oscillograph.

During commissioning not one case was found of the first machine doing other than what one would expect from the logical diagram (except for a very few cases of incorrect wiring).

The machine h.t. supplies are reduced while the test programmes are being run. This marginal testing shows up incipient faults such as deterioration in valves, crystal diodes or resistors. The machine is at present kept in good running order down to 10% margins