The connect circuit is siezed by the caller's loop being extended from its associated linefinder on the - and + wires. This operates relays A and AA from the battery and earth via the barretter. A1 operates relay B. B5 returns a holding earth on the incoming P wire and earths the power hold lead. B1 operates relay CD. B4 connects dial tone to the 570 ohm winding of the AA relay which by transformer action returns the tone to the caller. B3 earths the ring start lead. B7 and CD3 operate relay E.
The caller now dials a single digit. The release of the A relay energises the uniselector magnet and the uniselector steps to the dialled outlet. The original operate circuit for relay C is disconnected as soon as the uniselector steps but each time A2 releases it provides a current in the second winding which holds CD. The 100 microfarad capacitor across the second winding makes relay C slow to release so that it holds during the dial train but releases when relay A stays operated at the end of the digit. When relay CD releases the following four actions may occur.
1) The outlet dialled is spare. In this case the release of CD2 operates the NU relay via the strapping on the bank 4 outlet. NU2 connects NU tone to the AA relay and the caller. Any further digits dialled are ineffective as relay CD has released and disconnected the magnet circuit. Relay E releases slowly but has no function if NU has operated.
2) The outlet dialled is allocated to a BT line call. When CD2 releases, relay EL is operated via the strapping on the bank 4 outlet. Relay EL switches the caller's loop to the exchange line via relay L which operates. L1 holds relay B. Relays A and AA release but are inoperative. Relay D is operated by EL4 and in turn contact D! disconnects the ring start lead. The caller is now connected to the BT line and can continue to dial out or answer an incoming call as appropriate.
3) The outlet dialled is a busy line. When CD2 releases it is unable to operate the PT relay as the P wire will be earthed. Relay E at E4 then disconnects the testing path for relay PT. E2 released connects busy tone to the caller.
4) The outlet dialled is a free extension. When CD2 releases, relay PT operates via the strapping on the bank 4 outlet to the K relay battery on the P wire. PT1 operates relay H, PT2 holds relay PT. H4 earths the P wire and operates the line circuit K relay, so removing the bridging apparatus. H2 returns ringing tone to the caller. When relay E releases, ringing and ring return battery are connected to the called extension's line.
When the called extension answers, Relay F operates to the direct current now flowing round the loop. F5 ensures that relay F stays operated. F1 and F3 connect the called extension loop to the D relay which operates. F6 removes the ring start condition.
The two extensions can now speak to each other.
Release : When the caller clears, relays A and AA release. In turn relay B releases. B7 causes the release of relays H, PT and F. The release of H causes the release of relay D and allows the release of relay K in the called extension's line circuit.
With all relays released, the magnet drive circuit is completed and the uniselector drives home. During the drive home, the incoming P wire is earthed to hold the linefinder until the connect circuit is available for another call. Similarly relays A and AA are not reconnected until the uniselector has returned home.