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Functions

(1) To return a holding and guarding earth on the incoming P wire when siezed by a calling loop on the incoming - and + wires. Note that an earth on the - wire from a 50 point linefinder and selector hunter will also sieze the selector.

(2) To return dial tone to the caller should the group selector be used as a first selector.

(3) To step vertically in response to the received dial pulse train.

(4) To drive horizontally into the bank on the selected level at the end of the dial train.

(5) To search for a free outlet on the selected level. This will be denoted by a "dis" P wire condition. Busy and spare outlets will have earths on their P wires.

(6) To sieze the first free outlet and to gaurd that outlet with an earth on the P wire until the following selector can return a holding earth.

(7) To extend the calling loop to the next selector or relay set via the outgoing - and + wires.

(8) To hold to the earth on the P wire from the following selector or relay set.

(9) To restore to normal when the earth on the P wire is removed. The selector will also replace the earth onto its incoming P wire during the restoration of the mechanism so that it cannot be siezed until it is ready to respond to dial pulses.

(10) To drive to the eleventh step, operate its S springs and the level overflow meter, and return busy tone to the caller if all outlets on a level are busy.

Note : 20 outlets are provided on each level giving this selector 200 outlets in all. Each of the ten steps around the level must therefore have two outlets available.
Six wipers are needed, three for the odd -+P and three for the even -+P wires.
The diagram below shows the way the outlets are arranged.

The selection of the required outlet is performed by two switching relays, HA and HB.

If only the odd outlet of a pair is free, HA switches the call.
If only the even outlet of a pair is free, HB switches the call.
If both odd and even outlets of a pair are free, HA switches the call.
If both the odd and even outlets are busy, the selector steps forward on the level to test the next pair.

Note : While stepping horizontally from one contact to the next, relays HA and HB hold via the operate windings.
When the rotary magnet interrupters RM1 operate at the end of each stroke, relay HA is left connected to the P2 wire and relay HB to the P1 wire for testing.
When the appropriate relay releases due to the "dis" on the P wire, the "other" relay holds over its 1500 ohm winding to the P wire earth.
If both the odd and even outlets are free, then both HA and HB release.

Page provided by John Bathgate This page was last updated on 9th May 2008