AU483003B2 - Improvements in or relating to electronic switching circuits - Google Patents

Description

.69,144/74 This invention relates to outpulsing cfrcuits for use with

telephone subscribers' instruments.

It has become fashicnable to replace the dial of a telephone

instrument by a push-button unit, especially where V.F. signalling is used. However, it is desirable to be able to offer the

facility of push-button "dialling" to subscribers served by

exchanges which can only accept loop-interruption irpulsing.

Hence the object of this invention is to provide an outpulser

which can derive impulse trains from the outputs of a push-button

unit. According to the invention there is provided an outpulsing

unit for a telephone subscriber's instrument of the type having a

push-button unit, which includes storage means into which the

digits corresponding to the push-button operated by the subscriber

are entered, and an electric impulse generation circuit associated with said store and to which digits extracted from the store one

at a time are sent, wherein the impulse generation circuit includes!

a first transistor whose emitter-collector path is in series with

the line loop to the exchange, which transistor is in its

conductive state between impulses of a digit and during the inter

digital pause, and a second transistor controlled in accordance

with the digits to be sent such that said first transistor is

rendered non-conductive tor the duration of each impulse to be sent.;

Embodiments of the invention will now be described with

reference to the drawings, in which Fig. I is a block diagram of

an outpulser according to the invention, and Figs. 2 and 3 are two

forms which one of the blocks in Fig. I can take.

The outpulser shown in Fig. i is generally similar to that

shown in Fig. 3 of Application No. 55095/73, with the difference

that where the above-mentioned circuit uses an impulsinc, relay to

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69,144 174 send the impulses, inthe present case an electronic circuit is

used. The outoulser OP is connected in series with the telephone portion, which latter includes the usual items, i.e. the microphone MIC, the receiver REC and the transmission circuitry TRANS. The outpulser can also, as in the case of the other circuits shown in

our above-mentioned application, be in parallel with the telephone

portion. When the instrument is on-hood, line terminals [i and L2 are

connected to the ringer BR via a capacitor. As soon as the subscriber lifts his handset, the hook switch S disconnects the

ringer and the ioop is completed through the outpulse terminals

X and Y. The capacitor C is thus connected to the loop via the impulser IMP which at this time is rendered conductive so that

the capacitor charges via the protection circuit PC and the

impulser IMP. The capacitor C is charged to a level determined by the Zener voltage of the diode ZDI and at a rate determined by

the exchange battery, the feed bridge resistance, and the line

resistance. This charge-is limited by the size of C, but is such that when the loop is interrupted during impulsing there is enough

energy to keep the outpulser's electronics operating during the

break. W/hen the loop interruption ends, C regains the charge

which it lost during an impulse. The unit MOS contains electronic circuitry which receives and

stores digits, and is driven by the clock driver and powered from

the line or from C. When the charge on the latter reaches the threshold level of the gates used in MOS, the unit MOS is reset to

a rest condition, provided that the supply to the clock driver has

also reached the required voltage. This unit MOS is a single-chip circuit assembly using 14OS technology, though it will be

appreciated that other forms of integrated circuitry can be used.

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69.144/74 Push-button keyblock KB is connected to MOS via inputs refresented by IL. This is shown as one line, but would be a number of lines, dependent on the coding used. In the present there are four input lines on which case digits keyed in are entered in a binary-coded decimal form, and a fifth line which is a strobe input, used by MOS to scan for the presence of a digit input. Each digit keyed in from the block KS is entered into a store in MOS, under control of the strobe input, and MOS commences to cutpulse as soon as it detects that its store contains a digit, subject to a delay known as a predigital pause. This latter allows time to complete the loop and for any transients to expire. While it is outpulsing the unit MOS can accept further digits, as long as its store never contains more than 18 digits. When a digit has been outpulsed it is cleared from the store, an interdigital pause is controlled, and the next digit dealt with. It is possible, if desired, for the entire number to be placed in the store in MOS, and then outpulsing to commence in response to a start signal applied to MOS. When outpulsing is about to start, MOS operates a relay driver via one output: this operates a latching relay RLB, which closes its contacts RLBI, which short-circuits the speech circuitry for the duration of outpulsing so that annoying impulsing noise does not reach the subscriber. This corresponds to the off-normal contacts of a standard dial. When outpulsing of the wanted number ends, MOS causes the driver to release the relay RLB. The other output of 1.IOS controls the block IMP, which has already been mentioned. As will be seen later, this includes a transistor .;ith its emitter-collector path in s-ries with the line and which the loop is completed. When an impulse is to be sent a sicnal on the output from MOS to IMP renders the above-mentioned

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69,i 44F transistor non-conductive for the duration of the impulse. Thus

the loop is broken as "seen" by the exchange, or to be more accurate is rendered hioh resistance.

The protection circuit PC includes a diode bridge connected

to the line in such a way as to protect the circuitry against polarity reversals of the exchange battery, and also surge-

suppression-elements to protect against induced high voltage

transients. This latter also protects the electronics against

damage due to the ringing voltage if the ring-trip at the exchange

fails.

As the outpulser is in series with the speech elements, the

loop extends from L2 to LI via the hook switch S, the speech

circuitry and the outpulser. The outpulser could be short-

circuited out during the speech part of a call if this is consideredi

desirable. In addition to contact RLBI, there is another contact

RLB2 shown connected across the receiver REC so that the latter is

also shorted during impulsing. This contact may not always be

necessary.

Fig. 2 shows one form which the circuit block IMP can take.

Here the block B corresponds to the elements to the left of the

block IMiP in Fig. I, which control the block IMP. The circuit

includes an NPN transistor TRI whose emitter-collector path is to

be controlled, an'N-channel depletion-mode field effect device TR2

and another NPN transistor TR3. Initially when the circuit

receives no input from the block B to the base of TR3, the latter

is in its off or non-conductive state. Hence the gate electrode

of TR2 assumes a voltage equal to the voltage on its source

electrcde due to resistor RI and diode DI. Therefore base current

is supplied to TRI, which is therefore in its on or conductive

state. Hence the circuit is completed (Fig. I) from L2 via PC and 48 3.003

.69,i44P IMP to the rest of the circuitry. When an impulse is to be sent, the outout from B to the base of TR3 is energized, so TR3 is switched to its conducting state. This brings the voltage on the gate of TR2 to a potential negative to that on its source, which turns TH2 off, and this in turn turns TRI off, thus effectively breaking the loop to the exchange. When the impulse is to end, the energization of the base of TR3 is removed, which cuts TR3 off. This in turn renders TR2 conductive, whereafter TRI is switched on to complete the loop. The diode Dl is included to ensure that the gate of TR2 is unable to become excessively more positive than the source of TR2. In the circuit of Fig. 3, TR4 in series with the loop is an NPN transistor device, TR5 connected to its base is and TR6 is another NPN device. a PNP transistor With no power supplied from the box B, TR6 conducts due to current supplied to it via resistors R2 and R3. The collector of TR6 is also drawing current from the line to the exchange via R2 and R4. The collector also draws current from the base of TR5, which is therefore turned on, and this in turn renders TR4 conductive since the current through TR5 enters the base of TR4. Note again that TR5 is the reverse conductivity type to TR4 and TR6. When an impulse is to be sent, the base of TR6 is suitably energized from the box B, with the result that TR6 is cut off. When this happens the voltage drop across R2 due to current drawn via R3 is insufficient for TR5 base to supply current; so TR5 and TR4 are turned off. This process is reversed when the impulse ends, in which case TR6 is switched on, followed by TR5 and TR4 to re-complete the loop. In Fig. 2, it should be noted that transistor TRI could be replaced by two transistors connected tcgether as a Darlington pair:

69'14 4174 such an arrangement is, of course, a three-terminal device. In this case the function of diode Di is provided by two diodes connected in series. A Darlington pair can also be used in Fig. 3 in place of TR4.

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Claims (1)

1. 691144/74 The claims defining the invention are as follows:-

   1. An outnulsing urft for a telephone subscriber's instrument

   of the type having a push-button unit, which includes storage means

   into w-hich the digits corresponding to the push-button operated by

   the subscriber are entered, and an electric impulse generation

   circuit associated w,.ith said store and to which digits extracted from the store one at a time are sent, wherein the impulse

   generation circuit includes a first transistor whose emitter-

   collector path is in series with the line loop to the exchange,

   which transistor is in its conductive state between impulses of a

   digit and during the interdigital pause, and a3 second transistor

   controlled in accordance with the digits to be sent and so

   connected to said first transistor that said first transistor is rendered non-conductive for the duration of each impulse to be sent.;

   2. An outpulsing unit as claimed in claim 1, wherein the base

   of said first transistor is connected to the source of a normally-

   conductive FPT which forms said second transistor, and wherein said FET is controlled by a third transistor.

   3. An outpulsing unit as claimed in claim 1, wherein the base

   of said first transistor is connected to the collector of a third

   transistor of the opposite conductivity type to said first

   transistor, and wherein the third transistor is controlled via a said second transistor from the stored digits.

   4. An outpulsing unit as claimed in claim 1, 2 or 3, modified

   in that the first transistor is replaced by two transistors

   interconnected as a Darlington pair.

   An outpulsing unit substantially as described with

   reference to Fig. I and 2, or Figs. I and 3 of the accompanying

   drawings. l~~

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