The start sign is a hole between the 3rd and 4th impulses, which
(i) causes the machine to start counting (the start sign has to be punched 2 1/2
sprocket lengths before the first place to be counted),
(ii) sets X, Mu, PSI, in motion.
The stop sign is a hole between the 4th and 5th impulses, 1 1/2 places
beyond the end of the text, which
(i) causes the machine to stop counting, and generally prepares for the next start
(ii) transfers to relays the score which has been counted.
150 blanks between atop and start give Colossus time to prepare for the next
(b) The Bedstead
The bedstead is a system of pulleys round which the tape is driven by
friction at about 40 feet or 5000 sprocket holes per second, so as to pass through
a gate where it is scanned by eight photo-electric cells, one for each impulse and
one each for sprocket hole, start and stop.
Each Colossus has two bedsteads; while one is in use a tape can be put on
the other. An on-of switch by each bedstead controls both its driving motor and
its lamp. There is a switch on the selection panel, whereby either bedstead can
be selected (near or far), not both at once.
The maximum length of tape which a bedstead can carry is either 11,000
(short bedstead) or, on Colossi 5,6,7,8,10, 30,000 (long bedstead). On a long
bedstead the voltage applied to the motor can be adjusted to maintain the correct
speed whatever the length of tape and number of pulleys in use.
Although shorter tapes can be put on the pulleys, Colossus does not work
well with a tape less than 2,000 long.
53C THE X, Mu, PSI, STREAMS
(a) The triggers
The twelve wheels constituting X, Mu, PSI, are set up in 'triggers' of
length 41,31 etc. These triggers, except the wheel-breaking panel, are
inconveniently situated at the back of Colossus.
For each character of a wheel there are two small sockets: a cross is
represented by short-circuiting these with a U-shaped pin ("putting in a pin"); a
dot by leaving them vacant. (Fig 58(xviii)).
There are several alternative triggers for each wheel. The X, PSI patterns
have five triggers (a, b, c, d, e) each selected by a switch on the selection
panel. X a must be used with PSI a.
The Mu pattern has seven triggers a,b,c,d,e,f,g, any one of which can be
used with any X, PSI, trigger. This discriminatory treatment of Mu was arranged
when only the Mu's changed daily.