The UAX13 - Dialling the First Digit
When the caller hangs up, the next selector removes the earth from the P wire which allows the group selector and its associated linfinder to restore to normal.
Should a caller listen to dial tone without dialling (often this will be a fault condition), the selector will force release the call after around one minute and leave the caller "parked" onto the P relay in his line circuit. This will light the PG alarm lamp when the alarm button is pressed on the C unit.
When the group selector is normal it returns a testing battery back to the linefinder, allotter and control relay set via the P wire. If the selector is in use (B relay operated), off normal (N2 springs operated), busied (red link removed from test springs 13 and 14), or the selector fuse has blown, the selector and its linefinder is busied to the allotter and control relays set.
Setting up a Call from a Regular Line Circuit
If free, the selector is initially taken into use by an earth received over the - wire from the control relay set. This operates relay A which in turn operates relay B.
B3 replaces the battery on the P wire with an earth.
B5 operates relay CD
B2 and CD1 operate relay CC
CC3 and CD3 connect dial tone to the 570 ohm winding of the A relay, which acts as a transformer to return the tone to the caller.
Dial pulses release and reoperate the A relay. When A is released, A1short circuits relay B which does not release as this action makes it slow to release.
Assuming this is a call from a regular caller there will be no holding effects from conditions on the vertical marking bank for that level and this will allow CD1 to release relay CC.
When a 150 ohm battery condition on the P wire is found marking the first free outlet, relay HX operates to this condition.
H7, H3 and H2 extend the P+- wires through to the next selector or relay set. Relay A is disconnected and releases.
When B has finally released, the B3 earth is removed from the P wire which is now through to the next selector and is receiving a holding earth from there.
CC2 connects the M wire through from the line circuit to the following selector. This wire will later be used to feed pulses back to operate the caller's meter if required.
The selector is left with the - + P and M wires connected through from the line circuit to the following selector. The earth returned on the P wire from the following selector holds the H relays in both the group selector and the linefinder and by this means holds the selectors in place and switched through.
The caller now dials further digits into the next selector or relay set
When the caller hangs up, the selector or relay set further along the connection removes the holding earth from the P wire. This releases any H relays holding to this earth.
H4 connects up the rotary magnet self drive circuit to the release alarm earth connected to U15 on the diagram. This will cause the selector to drive on out of the bank and eventually to restore to normal. In turn this will place the N springs back to their normal position and at N2 reconnect the P wire battery to make the selector available again.
Note that to release, the selector needs a low resistance earth on the release alarm wire. This is provided by the release relay in the alarm circuit so long as there has been an earth on the release relay wire immediately prior to clear down. Contacts B1 or H5 provide this earth.
Failure to Dial First Digit
The line circuit P relay holds to the loop and prevents the L relay from reoperating. The caller is said to be "parked" with P1 providing a PG alarm to the C unit.
All Outlets Busy
If all outlets on a level are busy, the group selector will drive around that level looking in vain for a P wire battery. Eventually the selector steps onto the 11th outlet of the level. At this point the S springs are operated within the selector.
Should the caller continue to dial, the pulses to the A relay are ineffective as the magnet path has been disconnected at CD4.
Callers can be connected to the exchange as "ordinary" or "coin box" lines. These two classes of lines can then be given access to different levels of the group selector. Generally this means that coin box lines are barred from some levels.
The linefinder levels 3-7 are used by ordinary lines and level 8 is used by coin boxes.
There is a complication. Level 7 and level 8 are teed together and between them can only be connected to 10 line circuits.
The CB wire 150 ohm battery condition is used within the group selector to determine whether or not to allow the connection to continue.
The calling coin box hears the NU tone and generally clears down. If he does not the selector will eventually time out using relays TM and H resulting in the coin box line being parked back in his line circuit.
Note that at Norchard the coinbox feature is used to discriminate on rack A1 between on site phones and off site phones, and on rack A2 between on site phones and museum demo phones.