A bedstead is a system of pulleys round which the tape is driven by a
sprocket wheel at about 2000 sprocket-holes per second, so as to be scanned by
There are four bedsteads A,B,C,D: to ensure simultaneous scanning of
corresponding places on different tapes, their four sprocket wheels are on a
(b) Bedstead Drive
To reduce the tearing of sprocket-holes, two of the pulleys are driven at
the correct speed. For the same reason the drive is applied gradually when
starting, and removed gradually when stopping ( by means of relays).
The tapes are draped loosely on the pulleys, centrifugal action tends to
tighten them, and they may need to be slackened. After a long run tapes may
Between the "Gate" and the sprocket wheel the tape moves past two engraved
marks: to ensure that the tape is correctly placed these are aligned with an
appropriate pencil mark on the tape [ Fig.58(iv) ]
The tapes, which are of course continuous loops, are jointed flexibly with
One spring switch is used both for starting and stopping: it is thrown down
( and released) for start, up for stop.
(c) The Gate
Each bedstead has 13 photo-electric cells which scan the tape as it passes
(downwards) through the "gate". The gate is placed as near as possible to the
driving sprocket to reduce the effect of stretched tapes.
One of the photo-cells scans the sprocket-holes, permitting the counters to
add 1 or 0 at each sprocket hole.
In each position of the tape 10 of these cells scan the 10 dots and crosses
in two consecutive places on the tape, the 10 outputs appearing in 10 jacks on the
plug panel ( the output from each bedstead in the 10 jacks immediately below the
corresponding letter), and nowhere else.
(d) Start and Stop Signs
The remaining two photo-cells look for the start and stop signs which are
punched in the 4 1/2th and 3 1/2th impulses of the tape, exactly as on Colossus.
A start sign causes the machine to start counting.
A stop sign causes the machine to stop counting, transfer the count to
relays, and prepare for the next start sign.
Only one start sign and one stop sign are used as such at any time; these
are not necessarily taken from the same bedstead: they are selected by the
switches above the plugboard. Start ( on A,B,C, or D) by the first four: Stop (on
A,B,C, or D) by the second four.
(e) Position Counter
The start signs are used also for finding the relative positions