ABC path setup

Call control also instructs all free outgoing supervisory relay sets to put out a signal to the C-switch selector, indicating that they are available for selection.

A C-switch having a free outgoing supervisory relay set associated with it is chosen. The access relays of the C-switch operate and extend the Test and Mark wires of the chosen C-switch to the B-switch selector.

The test wires enable the switch selector to determine which of the B-C trunks are free by examining the potential on the H-wires of the B-C trunk multiples.

The mark wires enable the selector to prepare the chosen switching path by marking the appropriate cross-point relay in the chosen C-switch.

The marker relay in the line circuit operates, and causes the A-switch access relays to operate, extending the test and mark wires of the A-B trunks to the B-switch selector. The B-switch selector now chooses a B-switch which has a free path to the chosen C-switch and to the A-switch

The supervisory relay set then

initiates the switching of the path between it and the

subscriber's line circuit. The K relay in the sub-

scriber's line circuit is operated by call control and is

held by an earth via the P-wire from the supervisory

relay set.

The private (P) wire

is used as in a step-by-step exchange for busying the connection and metering the call.

The hold (H) wire

is used to switch and hold the cross-point relays which establish the path through the switch.

The pairs of resistors marked 19K and 33K form potential dividers between the H-wires and a 50-volt positive battery supply.

Free Trunk (vertical in this diagram)

The junction of each pair of resistors is connected via an access relay contact to a test (T) lead which goes to the switch selector. A free trunk has no potential on its H-wire; therefore the switch selector detects positive potential on the T lead.

An "in-use" or busy trunk (vertical in this diagram)

A trunk which is in use has negative battery potential applied to its H-wire (by the subscriber's line unit), and this will make the resultant potential on the T lead negative also, thus indicating to the switch selector that the trunk is not available.

When a trunk has been chosen, it is ' marked '

This marking is done by the switch selector, which applies a resistive earth to the mark (M) wire of the chosen trunk. When a complete path through an A, B and C-switch has been marked,

switching is initiated by a positive potential which the supervisory relay set applies to the H-wire. The positive potential causes a current to flow through the cross-point relay of the C-switch and via the associated diode to the marking earth. Operation of the cross-point relay joins the horizontal and vertical multiples and extends the positive potential, developed across the marking resistor, along the H-wire to the B-switch. The marked cross-point relay in the B-switch operates, followed almost immediately by the cross-point relay in the A-switch. The subscriber is thus linked to the supervisory relay set.