Pulsing

1

• Pulsing
• Dial Limits The nominal dial characteristics
  are speed 10 pps, ratio 2/3 break, 1/3 make.
  
• In practice dials are set to maintenance limits
  of speed 9-11 pps, ratio break 63-70 break.
  
• Equipment is designed to accept pulsing at 7-12
  pps, ratio break 63-70.
• Factors Affecting Pulsing The dial pulses, in
  the end, operate and release an A (pulsing) relay
  in selectors and relay sets. The speed is
  generally set by the dial but the ratio is
  affected by many other conditions.

2

• Line resistance This reduces current and flux
  values and their rate of growth. Generally the
  operate lag of the A relay is increased and the
  release lag is reduced. the break pulse tends to
  increase.
• Line inductance This also slows growth of
  current and flux and again tends to increase the
  break pulse.
• Line leakance This holds the flux in the A
  relay to a certain minimum value and keeps the
  relay partially fluxed. This tends to slow the
  release of the relay and therefore tends to
  reduce the length of the break pulse.
• Line capacitance This provides a path for the
  back EMF of the relay when the break pulse
  disconnects it. This tends to hold the relay
  operated thus reducing the break pulse. However
  it also restricts the rate of growth of current
  and increases the operating lag of the relay. The
  break pulse therefore tends to be displaced in
  time.

3

• Dial springs spark quench This is usually a
  100 ohm resistor in series with a capacitor of
  about 2 microfarads across the dial springs. The
  capacitor has no real affect on the make as the
  capacitor will be short circuited. On the break
  though the capacitor charges from the line
  voltage and provides a circuit for the A relay
  back EMF. This slugs the release of the relay and
  tends to hold it operated and so reduces the
  break period.
• Exchange battery voltage Low voltage acts in a
  similar way to line resistance in reducing relay
  flux and the rate of growth of flux, again the
  operate lag of the A relay is increased and the
  release lag reduced, leading to an increase in
  the break pulse length.
• Transmission bridge capacitors Final selector
  capacitors are disconnected during pulsing and
  therefore have no effect. Auto to auto relay sets
  have two capacitors connected in series across
  the A relay during pulsing, which act rather like
  line capacitance and therefore cause the pulsing
  to be displaced

4

• Typical Selector Stepping Circuit In a
  stepping circuit, relays B and CD have to remain
  held during the reception of a train of
  pulses.In particular relay B has to remain held
  during the break period of the pulse. During this
  time the B relay is short circuited by the A1
  contact and the slugging effect provides a
  release lag of around 225 ms.
• This is achieved by using the whole winding
  space for the 1300 ohm coil and allows the relay
  to hold to a 46 volt supply, 12 pps and a break
  that can reach or exceed 95. The relay can also
  have a high number of springsets with its high
  mechanical load on the armature.

Relay CD has to remain held during the make
period of the pulse when the magnet current
ceases. This is achieved by having a short
circuit 700 ohm winding which provides a release
lag of around 150 ms. Generally the magnet will
fail to step before the CD relay will
fail. Modern selectors overall have a great
amount of tolerance to pulse distortion.
5

• Junction calls
• When calls are made involving more than one
  exchange, the circuits interconnecting the
  exchanges are called "junction" circuits
• (Connections between the customer and the
  exchange use "local"lines).

A transmission bridge is required in each
exchange that the call passes through. The A
and B relays in each exchange hold the call
through that exchange via the B contact earth on
the P wire holding any group selectors that will
have been used to set up the call. Local lines
and junction circuits are simply two wire
circuits, but each exchange connection requires
the third P wire circuit in order to test and
hold the call through the exchange. Please
recall from the "Transmission Bridge" paper that
barretters can serve both caller and called
customers when a junction is involved in the call.
6

• On a "junction" call the originating exchange
  uses an outgoing auto - auto relay set to convert
  the three wire exchange connection to the two
  wire condition of the junction.
• Any intermediate exchanges that the call passes
  through will also require an auto - auto relay
  set. The terminating exchange uses a final
  selector to provide the necessary transmission
  bridge.

Any auto - auto relay set therefore has to
provide facilities to convert the three wire
exchange condition to the two wire junction
condition and this requires that the relay set
has a transmission bridge. Any pulsing must
therefore be repeated across the transmission
bridge to the junction and the following
exchange.
7

• The Auto - Auto Relay Set This diagram shows
  the essential features of an auto - auto relay
  set that permit P wire holding and pulse
  repetition to a junction.
• The P wire normally has a low resistance battery
  condition on it to denote that the junction is
  free.

The relay set is seized by the customer's loop
being extended from a group selector level on the
- and wires and relay A operates. A1 in turn
operates relay B which earths the incoming P wire
to hold the connection from the group selector.
8

• Junction Limits
• A2 also loops the junction with the 400 ohm high
  impedance I relay. This "initial pickup" is the
  condition that usually limits the resistance
  permitted in the junction pair. This limit can go
  up to 2000 ohms with the total loop resistance
  being presented to the distant A relay being 2400
  ohms. Above this value the A relay will not
  operate reliably.
• During pulsing the I relay is short circuited
  and the distant A relay will receive somewhat
  higher line current.
• A junction of 2000 ohms however implies a
  transmission loss of perhaps 20 db and this will
  not be acceptable in nearly all cases. To
  overcome this the junction will need amplifiers
  to reduce the loss to between 3 and 6 db
  depending upon where in the network the junction
  is.
• In turn, amplifiers generally mean a four wire
  junction being used which in turn reduces the
  line resistance by half.
• To offset this gain though, many transformers
  would be introduced into the signalling path
  along with capacitors across the signalling path.
  These factors generally reduce the junction
  signalling limits to around 1200 ohms.

9

• Pulse Repetition
• When the caller dials, the A relay releases
  during the pulse break periods. Contact A2
  repeats the break to the junction, contact A1
  operates relay C and C2 operates relay CA.
• These two relays hold during the pulse train.
  Contact C1 short circuits relay I and, on the
  next operation of relay A, presents a zero ohm
  loop to the junction. Pulsing continues with
  either a break or a zero ohm loop being extended
  to the junction.

At the end of the pulse train, relays C and then
CA release slowly. C1 removes the short circuit
from the junction and leaves the I relay and a
400 ohm resistor in parallel across the line.
Flux builds in the I relay whilst the distant A
relay is held by the current flowing via the 400
ohm resistor. When CA1 releases the line current
will all flow via the I relay, but as the relay
has been partially fluxed, the line current will
not drop below the "hold" value for the distant A
relay. This arrangement is known as "Two Stage
Dropback" and is an essential feature in all loop
disconnect relay sets.
10

• If the 400 ohm resistance circuit had not been
  provided, then the drop back from the short
  circuit to an unfluxed I relay would have caused
  the line current to have dropped momentarily to
  zero.
• This would have caused a momentary release of
  the distant A relay and so would have produced an
  extra pulse. eg if six had been dialled, seven
  pulses would have been received.

The A relay in the auto - auto relay set could
also be adversely affected if a "one stage drop
back" had been employed. During pulsing the
transmission bridge capacitors build a charge
from the A relay battery and earth via the short
circuit of C1. When C1 releases the capacitors
would discharge to the junction line potentials
and this would produce a current in the A relay
in opposition to the line current. The A relay
may therefore itself produce an extra break which
it would repeat to the junction. This again would
be an extra pulse.
11

• Trunking
• The diagram and write up assumes that the auto -
  auto relay set is provided on a one per outgoing
  junction basis. This is generally true,
  particularly in the smaller exchanges.

However when large exchanges with many junction
groups are involved, it is usually more economic
to provide the relay sets further back in the
selector chain, between the group selector
ranks. There are usually more junctions
connected to the levels of second selectors than
there are second selectors. The outgoing side of
the relay set is then connected to a second group
selector which in turn will select which junction
is used. Contact B3 in the auto auto relay set
provides the forward holding earth to busy and
hold the next group selector. The forward B3
earth is also generally taken to the bush of the
test jack of the selected junction to mark it as
busy to any testing engineer.