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.
1
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.
2
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.
3
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.
4
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.
5
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.
6
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.
7
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 offnormal 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.
8
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 reoperates 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).
9