MAS is the master switch (upper row, second switch from right; labelling
obscured in the photograph). Unless this switch is thrown Colossus can neither
count nor step. It is however possible to set wheels and to reset counters.
The switches labelled X, Mu, PSI, are the stepping switches 53D(c). The
switches labelled mult are multiple test switches 53L(g). X5, PSI5 are oddly
The other switches are
PMH Print main heading [53G(g)] PCO Printer cut-out [53G(i)]
SET Set wheels [53D(a)] Lc/o Lamp cut-out [53G(d)]
RESET Reset counters [53G(j)] LC Letter Count [53G(h)]
REC Rectangle [53M(f)] KL Cancel lights [53G(d)]
53P. COLOSSUS TESTING.
Any account of the methods used by the engineers to test Colossi would be
entirely out of place in this report, but it is appropriate to refer to the
methods used by Wrens, chosen to carry out routine tests.
Owing to the complexity of its operations Colossus can produce results so
erroneous as to be useless without arousing suspicion till valuable time has been
Runs have therefore been selected such that a machine faulty in any respect
is unlikely to give correct scores, and these have been done on Colossi known to
be in good order, using selected standard wheel patterns and a selected standard
tape. One set of triggers on each Colossus is now assigned to these
standard patterns, and standard tapes are kept in stock : the runs are repeated
on all Colossi at frequent intervals.
A single fantastic run could doubtless be devised to check everything, but
it is preferable to use a number of runs, which in themselves will aid in locating
faults. Z and X are first tested without PSI.
Of course when there is a fault the ordinary chi and psi tests [ 23K(d) ]
will fail, thus providing a crude test of Colossus very frequently.