No. 52 PAGE No. 28

Circuit Laboratory, Engineering Department
January 1966

This press button telephone will work to any loop/disconnect system capable of operation by the conventional dial, and can also be adapted to systems permitting much higher digit signalling rates than are mechanically possible using the dial. No exchange modification is necessary - Pressetel is the simpler of two possible design approaches fulfilling this requirement, and is thus most economic for general subscriber use.

Press button signalling is being introduced into telephone systems to enable the subscriber to save time in the setting up of calls. In practice this saving is more than that conferred by increased signalling speed alone; with the ability to deal with the digit sequence more quickly there is less strain on the short-term memory performance of the subscriber. Wrong numbers due to subscriber error - a major source of lost time when they occur - are therefore less likely. A parallel improvement is the lessened preoccupation with the mechanics of signalling, as compared with use of a dial. Simple layout of the press buttons, larger digit areas and the absence of distracting movement are factors contributing to this.

In the provision of press button facilities there are two main approaches. The first is in the type of instrument which permits the user to operate the press buttons at speeds as fast as his physical capabilities allow. A multiple store is necessary to enable the digital information to be re-transmitted to line at the rate required by the exchange equipment. Such a store with its associated sequencing elements is necessarily complex and, at the present state of manufacturing technique and component costs, an instrument of this type will be too expensive for some subscribers.

The second approach is represented by a simple and more economic type of instrument, using a single digit store. Successive digits can be keyed into this store at a rate governed by the requirements of the exchange. The information corresponding to each digit is converted into line pulses and, when these have been transmitted, the next digit may be keyed in. The function is analogous to that of the conventional dial, but electronic techniques permit a keying rate to be achieved which is the actual maximum the exchange will accept; certain features of dial operation preclude this possibility.

The principle of single-digit storage is used in the Pressetel, which is primarily designed to replace the dial instrument; its output is of loop/disconnect form and it will work to any exchange accepting this form of signalling via the normal 2-wire line connection. Although such exchanges will usually be of Strowger type, the output pulsing speed is widely adjustable to cater for other systems. The design is in fact adaptable to pulsing speeds and corresponding digit signalling rates which exceed the subscriber’s fastest keying capabilities, and make it comparable, as regards facilities, with the more complex instrument discussed above.

A principal operating feature of Pressetel is that keying of a digit into the store can take place during the interdigital pause period. To permit the user to take full advantage of this facility and so achieve maximum signalling rate, a ‘signal barred’ lamp is extinguished at the end of pulsing.

The interdigital pauses are the minimum necessary for correct functioning of the exchange equipment, whereas with a conventional dial, the user’s reaction and operating times are both added to the basic interval provided by the mechanism, and the effective inter-digital pause is substantially longer than the minimum. Pressetel moreover eliminates the superfluous pause before the first digit, an unavoidable feature of dial operation.

To supplement the safeguard afforded by the ‘signal barred’ lamp indication, it is arranged that the press buttons are ineffective whilst this lamp is lit. It is impossible therefore to mutilate a digit by operating the next button prematurely.

The logic is confined to that necessary for the storage and pulsing out of one digit at a time and, in consequence, the circuitry is relatively simple.

To summarize the operation; information corresponding to the required digit is ‘written in’ to the store when a button is pressed. The store is set to the empty or ‘home’ condition on lifting the handset and also after each digit has been pulsed out.
When a digit is stored, a gate controlled by the store is closed and a switching circuit associated with the gate removes an inhibit condition from the pulsing device, an astable multivibrator. This free-runs, pulsing an ‘A’ relay to transmit pulse signals to line. The multivibrator also feeds pulses at the same rate back to the store which counts down to the ‘home’ condition. This is reached when the number of pulses transmitted is numerically equal to the digit stored. The gate controlled by the store now opens and re-imposes the inhibit condition on the multivibrator.

During pulsing, off-normal relays are operated by the switching circuit associated with the gate. One of these (ONA) causes the ‘Signal Barred’ lamp to glow.

At the end of each pulse train a timing device, the Inter Digital Pause (IDP) Generator, is operated to place a second inhibit condition on the multivibrator, independent of that from the store-controlled gate. Therefore, although the next digit may be stored at any time after the end of pulsing, the multivibrator will not free run until the IDP inhibit is removed. A fixed minimum delay is thus obtained between the pulse trains.

The store consists of a chain of four complemented bistables of the type shown in figure 1, these being interconnected in the manner indicated to form a binary counter. Conventionally, a bistable is said to be in the ‘0’ state when Xl is ‘on’ (conducting) and X2 ‘off’, and in the ‘1’ state when the reverse situation applies.

Considering the bistable in figure 1 as the first in the chain, the waveform from the multivibrator is fed in at point A. This waveform is differentiated to produce positive and negative pulses which are applied to the junction of diodes Dl, D2 and D3. Only the positive pulses are effective because of diode blocking action. There is one such effective pulse, termed an input pulse, per line pulse.

Each input pulse is steered by Dl or D2 to the collector of whichever transistor happens to be ‘off’, causing this transistor to be turned ‘on’ and the bistable to change state from ‘0’ to ‘1’ or from ‘1’ to ‘0’.

Figure 1
The basic bistable circuit 
Figure 2
Simplified pulsing and control circuit

It will be seen that transistor Xl goes from an ‘off’ to an ‘on’ condition with alternate changes of state. The resultant positive-going potential step at Xl collector is fed to point A of bistable No. 2. This bistable thus changes state upon every second input pulse to the store. Similarly bistable No. 3 changes state upon every fourth pulse and bistable No. 4 changes state upon every eighth pulse.

If the store were initially reset to the ‘0000’ (home) condition, it would reach a ‘1111’ state on the 15th pulse and revert to the ‘0000’ state on the 16th. Each intermediate state is unique, i.e. involving a different combination of ‘l”s and ‘0”s from any other.

The most convenient way to store a digit ‘x’ is to set the store to the state it would have reached if (16-x) pulses had been fed in. Then ‘x’ actual pulses will return it to the ‘0000’ or ‘home’ condition. This method of storage is adopted in Pressetel. To control the multivibrator the gate associated with the store need only distinguish between the ‘0000’ state and any other; a relatively simple requirement.

The gate is the AND device shown in figure 2. The negative leads of the four diodes are taken to the collectors of the right-hand transistors X2 (figure 1) in the bistables.

Figure 3
Write-in circuit showing diode matrices for digits 1 and 7

When the store is in the home condition all of these transistors are ‘off’ and a negative output is provided from the gate to the transistor switch. This switch is as a result ‘on’, which causes the control transistor to be switched ‘off’ and the multivibrator to be prevented from free running; the ‘A’ relay is held operated.

When a digit is stored, i.e. one or more bistables are in the ‘1’ state, the diodes associated with these bistables conduct more heavily and the gate output potential is now such as to turn the transistor switch ‘off’, provided that no inhibit is being applied by the IDP generator. The control transistor is accordingly switched ‘on’, and the multivibrator free-runs.

Interdigital Pause
Whatever the digit stored, bistable No. 4 changes state from a ‘1’ to a ‘0’ condition once only during emptying of the store, that is, at the conclusion of pulsing.

This event is used to initiate the IDP. A pulse developed by the bistable triggers the IDP generator, which is a monostable multivibrator, to the quasistable state. This state, of duration equal to the required IDP, is marked by a reversal of conditions in the monostable; the normally ‘off’transistor is ‘on’ and the normally ‘on’ transistor ‘off’. The resulting output potential from the collector of the latter is applied to the transistor switch (figure 2). Its effect is to override the control exercised on this switch by the gate. Thus, irrespective of whether a new digit has been stored or not, the multivibrator cannot free-run until the IDP has expired.

As already implied, the function of the press button contacts is to set selected bistables in the store to the ‘1’ state, the configuration being appropriate to the digit required. That for digit 7, for instance, would be obtained by expressing (16-7)=9 in binary form, i.e. 1001; to store this digit, therefore, bistables Nos. 1 and 4 are set to the ‘1’ state.

Setting a given bistable requires the application of a positive pulse on the ‘set’ lead (figure 1) thus switching the left-hand transistor ‘off’ and the right-hand transistor ‘on’.

Figure 3 shows the press button contacts and associated circuitry for the digits 1 and 7. The diode matrix is a simple and reliable means of deriving a number of effective make contact actions (in this case a maximum of four) from the single pair of contacts associated with a given press button.

Prevention of Mutilated Digits
The common supply lead to the press buttons, which requires a potential at or near supply positive for the press buttons to be effective, is returned to a point in the circuit of the transistor switch (figure 2). This point is in the effective potential range only when the switch is ‘on’, i.e. the multivibrator is not running. Therefore, if a button is operated prematurely, there will be no effect on the digit being pulsed out.

The effect of any contact bounce in the press button springsets is masked by components C, R and D (figure 3).

A second off-normal relay (ONB) operates in series with ONA during pulsing out. Contacts of this relay short circuit the telephone transmission elements to establish a metallic ‘dialling’ 1oop during pulsing out and to prevent acoustic shocks in the receiver.

A ‘reset’ relay (not shown) operates during the charging time of a capacitor upon power being applied to the circuit by handset removal. Contacts of this relay momentarily connect the ‘reset’ points of the bistables (figure 1) to supply positive, ensuring that the store is in the home state and ready for insertion of the first digit.

Pulsing Speed and Ratio
Preset adjustments in the multivibrator circuit allow for make and break times to be varied independently, giving pulse speeds from less than 10 pps to more than 2opps, with make :break ratios between 30% :70% and
40% :60% at any speed within this range.

The above adjustments will cater for all normal Strowger and crossbar exchange systems. For exchanges requiring faster operation the pulse speed, pulse ratio and IDP would be modified accordingly.

The basis of construction is the Etelphone with a modified layout of transmission components and ringer, and a steel baseplate in place of the normal high-impact plastic. Figure 4 is a general view of the instrument with cover removed.

Figure 4
Instrument with cover removed

The majority of the logic-circuit components are assembled on two printed cards seen parallel to the base of the instrument and towards the front. The lower card is secured by screws to the baseplate and the upper card, which carries the two preset controls for make/break time adjustment, similarly affixed to brackets projecting downwards from the press button assembly. The latter can be readily detached by withdrawing screws securing it to the baseplate and instrument bracket, and the whole assembly swings aside to allow either the upper or lower card to be inspected or, if necessary, removed.

A third, smaller card is mounted immediately under the press button set and in a plane parallel to it. This card carries the matrix diodes and associated components and, like the other cards, may be inspected or removed by detaching the press button set from the instrument.

The remaining components associated with the logic circuits are the four relays and the ‘signal barred’ lamp, the latter being visible in figure 4. The relays, of a recently developed miniature-reed type, are secured to a transverse mounting strip between the sides of the instrument bracket.

The plunger-type press buttons, which are arranged in the 2 rows of 5 configuration at present used by the BPO, have a suitably light operating pressure and feature pre-tensioned contact springs. Electrically, operation occurs when a press button is approximately half-way to the limit of its travel. A step on the plunger stem controls motion of the moving spring as far as this point; beyond it, the step separates from the spring, and contact pressure thus depends only on the pre-tensioning. The moulded ridges in the instrument case between the buttons (seen in the photograph at the head of this article) prevent accidental operation of two adjacent buttons in the same row due to inaccuracies of ‘aim’.

The ‘signal barred’ lamp is provided with a clear lens in the instrument body, and forward intensity is adequate in the brightest ambient lighting.

The instrument requires a supply of 12, 24 or 50V d.c. voltage, selection being by strapping on the terminal block seen at the rear of the instrument in figure 4.
A 12V supply would normally be obtained from a small a.c. mains-operated rectifier unit mounted at some convenient point near the instrument. Supplies of 24V, or more usually 50V, could, for instance, be obtained over an additional pair of wires from the exchange.
The transistors and electrolytic capacitors are fully protected against damage from the accidental reversal of supply polarity.

Pressetel is a simple economic and reliable means of providing press button dialling facilities to subscribers on any exchange which accepts loop/disconnect pulsing and where local or administrational approval is forthcoming.


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Last revised: December 31, 2003