S.T.C. 30 & 100 Line
P.A.B.X.

The uniselector S will respond to the first digit dialled and operate a relay combination according to the digit dialled.

If the tens digit dialled is 2, 3 or 4, S will drive to the beginning of the required tens group.

S then responds to the second (units) digit dialled and will come to rest on the terminals of the required extension line.

Direct access to the public exchange is obtained by dialling "9." This causes a free junction JC to hunt for the calling line which will be found by the line finder JF. The connecting circuit C is released when the calling extension is found by JF and the extension line is extended via JC to the public exchange.

Numbering scheme
The numbering scheme is as follows:-
Extensions, 20-49.
Tie lines, 8.
Direct access, 9.
Manual board, 0.
Other levels, N.U. tone.

The 100 line equipment functions in a similar manner.  The line finders in this equipment have access to 50 subscribers and consequently are divided into two groups - those serving extensions 20-69 such as TI in Fig. 23 and those serving extensions 720-769 (such as T2). The selectors have access to all the 100 subscribers.

An 11 point marker switch responds to the first (tens) digit dialled and pilots the selector S to the beginning of the required group.

Selector S responds to the units digit and comes to rest on the terminals of the required line.

If, however, the first digit dialled is 7 then S will stop at the beginning of the 720 group and the marker returns home.

In this case the second digit dialled will be routed to the marker switch which pilots S to the beginning of the required tens group. The third digit dialled will actuate S and step it to the terminals of the required extension line.

This equipment is arranged in two 50 line units, the second being added when the capacity of the first is exceeded.

Direct access, tie lines and manual board calls are obtained by dialling 9, 8 or 0 respectively.