Selector Stepping

1

• Selector Stepping
• All calls made through a Strowger exchange
  require that pulses from the customer's dial step
  the magnets in the selector mechanisms. The main
  requirements are
• 1) that the vertical magnet in group selectors
  be stepped under the control of the dial
• 2) that in a final selector the vertical magnet
  be stepped by the penultimate dial train and the
  rotary magnet be stepped by the final dial pulse
  train.

2

• This circuit shows a typical group selector
  stepping circuit.
• Relay A operates when the selector is seized
  from the previous selector.
• When relay A operates to the customer's loop
  being extended from the previous selector stage,
  A1 operates and in turn operates relay B to the
  150 ohm battery supply.
• Relay B in turn earths the P wire to hold the
  previous selector stages and pre-operates the CD
  relay.

3

• When the dial pulse train arrives, relay A
  releases and re-operates a number of times.
• On the first release of relay A, the A contact
  short circuits the B relay and extends the B
  earth via the CD contact to the vertical magnet.
  The vertical magnet operates and the selector
  takes a step up. This operates the N springs
  which immediately short circuit the 700 ohm CD
  relay winding.
• Note that both the B and CD relays now have
  short circuited windings. This makes both of the
  relays slow to release so that they will remain
  operated during the rest of the pulse train.

At this point the CD relay has about an amp of
current flowing through its 5 ohm winding to
maintain the relay's flux. When the A relay
re-operates, the magnet current virtually ceases
and relay CD holds due to the slugging effect of
the short circuited 700 ohm winding. The magnet
and the 150 ohm resistor still supply sufficient
current to re-energise the B relay and allow it
to rebuild its flux.
4

• During pulsing, relay B is energised during the
  "make" period and held by a short circuit during
  the "break" period. Relay CD is energised by the
  magnet current during the "break" period and held
  by the short circuit across the 700 ohm winding
  during the "make" period.
• When the dial pulse train ceases, relay A
  remains operated, and so does relay B as it is
  receiving current from the magnet and the 150 ohm
  resistor. Relay CD however releases (slowly) as
  its 700 ohm winding is still short circuited and
  the 5 ohm winding is no longer receiving magnet
  current.

The release of relay CD then instigates the
driving of the rotary magnet so that the wipers
enter the dialled bank level in the search for a
free selector from the next rank. This aspect
will be dealt with in a later paper. The
rectifier in the 150 ohm operate circuit for the
B relay prevents the resistor from slugging the
vertical magnet during stepping.
5

• This is a variation for a group selector
  stepping circuit as used in PABX equipment.
• In this circuit, the A relay operating causes
  only the operation of relay B from the magnet
  battery supply. Relay B does its usual job of
  holding an earth on the P (or H) wire.

6

• When the pulse train is received, the initial
  release of relay A short circuits relay B as
  before and also extends an earth via the 4 ohm
  winding of the CD relay to energise both relay
  and vertical magnet. Relay CD therefore only
  operates when the dialling begins.
• Relay B holds during the "break" period due to
  the short circuit placed across it by the A
  contact. The N springs operate as the selector
  moves off normal and prepares a circuit for the
  drive of the rotary magnet, but this is delayed
  as the C contact is now operated.

At the end of the first pulse the A relay
reoperates. This removes the operating current
from the vertical magnet and reinstates the
current to relay B. It also places a short
circuit across the CD relay to make it slow to
release so that it will hold during the "make"
period. During pulsing, the B relay is short
circuited when the A relay is released and
receives current when the A relay is operated.
The CD relay is short circuited when the A relay
is operated and receives magnet current when the
A relay is released. At the end of the pulse
train, relay CD will slowly release when the
magnet current ceases. This allows its contact in
the rotary magnet circuit to start causing the
switch to drive into the dialled bank
level. This simpler circuit is only suitable for
PABXs etc where line conditions can ensure that
good pulsing limits from telephones are
available. This CD relay circuit would be more
likely to fail than the previous circuit used in
public exchanges.
7

• Final Selector Stepping
• Final selectors have a more complex stepping
  circuit as two pulse trains are used to step
  first the vertical magnet and then the rotary
  magnet.
• An additional relay E is required to change over
  the stepping circuit between the two pulse trains
  from the vertical magnet to the rotary magnet.
  Rotary off-normal (NR) springs are also required.
  
• This circuit is a little simpler than most
  actual circuits as the E relay can also be
  involved in the metering circuit element.

8

• When contact A1 operates on siezure, relay B
  operates to the 200 ohm battery supply. B4 then
  operates the CD relay.
• During the first pulse train, the vertical
  magnet is operated from the B1 earth when the
  "break" of A1 occurs, CD1, the 5 ohm winding of
  relay CD, E4, and NR1. When the selector moves
  off-normal N2 short circuits the 700 ohm winding
  of the CD relay, making CD slow to release and
  dependent on the flow of magnet current though
  the 5 ohm winding. Relay B holds as it either
  short circuited by A1 released or is fluxed by
  the current from the magnet and the 200 ohm
  resistor during the time that A1 is operated.

During the initial pulse train therefore, relays
B and CD hold and at the end of the pulse train,
relay CD releases with the cessation of vertical
magnet current. C3 in releasing connects relay E
to the magnet battery and relay E operates.
Contact E6 changes over and removes the short
circuit from the 700 ohm winding of the CD relay
which therefore re-operates. Relay E holds via E1
and CD3 both operated. Relays B, CD and E are
now all operated and the rotary magnet is now
selected for the reception of the final pulse
train.
9

• Relay E has an armature end slug which makes it
  both slow to operate and slow to release. The
  slow to operate feature ensures that the relay is
  well fluxed before it operates, so making the
  slow to release feature reliable as soon as the
  relay is operated. This means that the momentary
  "dis" that appears in its holding path when the
  CD3 contact change over will not cause the
  release of relay E. The slow to release feature
  is also used later in the metering sequence.
• When the final pulse train is received, the
  rotary magnet responds and as the selector steps
  into the bank, the NR springs operate. With NR2
  and E6 both operated, the 700 ohm CD winding is
  again short circuited making relay CD dependent
  on the rotary magnet current pulses.
• At the end of dialling therefore, relay CD
  releases again. C3 releases relay E, slowly.
  (During this release lag the called customer's
  line is checked to see if it is busy, free or,
  possibly, spare). When relay E finally releases,
  relay CD again re-operates with the short circuit
  removed from the 700 ohm winding.

Should the caller dial any further digits, they
will be ineffective as contact NR1 will have
disconnected the vertical magnet and E4 will have
disconnected the rotary magnet. When all dialling
has ceased, relays B and CD remain operated, but
relay E is released. (Relay E is now available to
time the metering pulse to the caller when the
called customer answers).