Z one forward is rubbish, it is in use, for each stretch of 9's, from one place
before the first 9 to the last 9. Colossus however, because it deltas backwards,
treats this as being from the first 9 to one place after the last 9; and
accordingly if not 99 is used with Z not deltaed by Colossus one place will be
lost unnecessarily at the end of each stretch of 9's.
(h) Start Units
These carry a permanent dot or cross as labelled.
(i) Addition Field
These are of course used to add impulses: there are three rows, 26 columns.
All but one of the impulses to be added are plugged into consecutive jacks
in the top row.
The odd one is plugged in the middle row below the first of the other plugs.
The output (sum) is taken from the bottom row below the last of the other
The columns used thus are isolated so that several additions can be carried
To plug any impulse to equal a cross, add a cross and plug normally.
(j) Common Jacks
There are on each Colossus six or more commons of five jacks each. An
impulse put in can be taken out four times: if this is insufficient common jacks
can be linked together.
(k) Counter Jacks
There are 8 jacks for carrying conditions to each counter.
There are also 6 jacks carrying conditions to all counters. One of these is
marked TM because it will not work if the TM switch on the Q panel is thrown.
There is a special counter jack (Multiple Test Doubting) used only for
multiple testing on special patterns.
(l) Examples of the use of the Plug panel
(1) Before not 99 was available it was usual, on corrupt texts, to switch Q = Z,
Q not = 9, and plug all wheel-breaking runs.
(ii) The wheelbreaking run, Delta-X1 + Delta-Z1 + Delta X6 = . is normally done
by plugging (delta X6 is usually set up in the X2 trigger).