The photographs in chapter 58 show the layout both of the whole machine
and of individual panels, far more clearly than verbal description, which is
Colossus makes counts concerning certain streams of teleprinter letters.
One, denoted by Z and represented on a punched tape, is wholly arbitrary; the
others, denoted by X, Mu, PSI and represented electrically, are specialized and
composed from certain fixed periods. These patterns X, Mu, PSI, do in fact
represent the 12 wheels of the German Tunny machine, and move in the same way.
Their 12 components will here be called wheels. For given "settings" there will
be, corresponding to each place on the tape Z, definite positions on all the
Colossus counts the number of places of Z where a condition involving some
or all of these streams is satisfied. An essential feature is that the counts can
be made in rapid succession with the various wheels in different relative
Colossus cannot count a condition involving two different places in the
stream except in a limited way by memory circuits, used mostly for delta-ing.
The sum Q of any number of the three 5-impulse streams Z, X, PSI, either
differenced or undifferenced, can be switched into the Q panel: the switches of
this panel suffice to impose the majority of the 2^32 app. = 5,000,000,000
combinations of conditions which are theoretically possible. Less specialized
conditions can be imposed by the plugboard.
Note 1 Although the streams are named Z, X , Mu , PSI, these are not necessarily
used as the real Z, X , Mu , PSI of Tunny. In a short wheelbreaking run (25) the
pattern set up in X is really delta X except in the wheel for which the run is
made, where it has only one cross and is used merely to select in turn the
characters of that wheel.
Note 2 A tape is required in every case, because it controls counting, but Z need
not occur in the conditions imposed (e.g. X test runs).
53B THE Z STREAM
(a) The tape
The tape is a continuous loop of five-impulse tape carrying the Z stream,
the usual sprocket holes, a start sign, a stop sign, and 150 blanks.
The sprocket holes are utilized
(i) to cause the machine to count if the conditions imposed are satisfied: the
machine counts once, at most, for each sprocket hole.
(ii) to maintain the correct motion of X , Mu , PSI.