The BTM/ICT 542 & 550 Calculators
These early commercial computers which were introduced in 1954 & 1956 respectively, were controlled by a replaceable plug-board program and read their input data from previously punched 80 column slotted hole cards, the results being punched into a different part of the same card. The computer used plugged-in two double triode valved "turrets" as logic elements, partial products being retained in capacitor storage and extant for that card passage only. The separate Reader/Punch unit was cable connected to the Arithmetic Unit and operated at about 100 cards per minute, the several calculations being performed while the card moved from the brush sensing stage to the punching stage, a matter of a few inches.

These pages presently show the following images :-

1) Pages from an ICT salesman's 542/550 brochure dated- late fifties

2) Photographs of a "turret" i.e. one of the machine's pluggable valve technology logic elements. These turrets are little known and represent a "stepping stone" technology between totally hard-wired machines and those using removable printed circuit boards.

3) This photograph shows the "gates" into which the turrets are plugged, the control panel and the plug board.

The author is also in possession of some ICT Letchworth Training School "hand-outs" for this machine, but they are far too large for reproduction here.

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All text © 2007 D.C.Adams

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