Computers and Telephone Control
Some telephones can be connected to a computer, sometimes via an RS-232 serial port, allowing information about calls to be sent to the computer and enabling the machine to take control of the telephone. The computer can answer calls like an answering machine or operate as a speakerphone.
- Some G3-based Macs can only sample sounds at 44.1 kHz, which can make them incompatible with speakerphone software.
The Telephony Application Programming Interface, also known as Telephony API or TAPI, is built into Windows 95 and later systems, giving appropriate applications access to a connected telephone. Other operating systems often don’t include such a mechanism, although you may be able to use Java TAPI or JTAPI with selected applications on your machine.
Telephones in Detail
The information in this section is provided purely for reference.
Telephone Basics
A telephone allows speech to be conveyed over a pair of wires, the latter being of the same form as those used for older telegraph signals. Traditionally, an analogue signal is used, although digital technology is used in newer systems. The following diagram shows one very early form of telephone:-
Each carbon microphone consists of a capsule containing carbon granules that are attached to a diaphragm. When the latter vibrates with the sound of a speaker’s voice, the electrical resistance of the granules changes, causing variations in the electrical current flowing around the circuit. These variations can then be heard in both of the moving-coil earpieces.
The speaker can also hear his or her own voice, an effect known as sidetone. This is essential, since it confirms that the phone is working. However, in the simple circuit shown above, each speaker is almost deafened by the sidetone.
A later development is the field telephone, as shown below:-
This uses an anti-sidetone induction coil (ASTIC) to reduce sidetone. When someone speaks into the phone, the voltage across the resistor opposes that produced by the ASTIC. However, when signals are received from the line the two voltages help each other. Modern phones work in the same way, but employ more sophisticated technology in place of an ASTIC.
The field telephone uses a battery in both instruments, ensuring communication in at least one direction. For convenience, modern telephone exchanges contain a central battery, with telephones wired as shown in this simplified diagram:-
The retard coils prevent any speech signals from reaching the battery, whilst the pair of capacitors allow such signals to reach their destination via the exchange’s switching system. The exchange also sends a ringing signal to the phone as an alternating current (AC) signal, typically at 80 volts (V). In the UK, this is at a frequency of 17 Hz, often generated by a syncycle, which divides the domestic 50 Hz mains frequency by one-third.
Older telephone equipment uses pulse-dialling, involving repeated shorting-out of the phone line. The earliest devices have a rotary dial, although more modern instruments employ electronic circuitry. The diagram below shows the circuit of a standard rotary-dialling phone in simplified form:-
As you can see, the phone’s cradle switch disconnects much of the instrument when it’s not in use, whilst the dialler off-normal (DON) contacts prevent any unpleasant noises in the earpiece during dialling. The dialling contact is normally closed, but during dialling it opens for around 67 milliseconds (ms) with intervening opening periods of about 33 ms. As shown, the circuit used by the AC ringer is isolated from the direct current (DC)used for speech by means of a capacitor. The latter is usually fitted in the master socket (see below) and is shared by all the instruments on a line.
Fortunately, some of these complications are obviated by modern telephones and exchanges that support tone-dialling, where special tones are sent to the exchange for each number that’s dialled.
The following table shows the impedance of typical telephone components:-
| Component | Impedance (Ω) |
|---|---|
| Line Holding Coil | 750 |
| Speech Retard Coil | 80 + 80 (Twin Coil) • |
| Internal Ringer or Indicator | 500 |
| External Ringer or Indicator | 500 + 500 (Twin Coil) |
| Hand-ringing Generator | 500 |
• Slit nickel-ferrite sleeve wrapped around coils, ensuring high impedance at speech frequencies
Telephone Connections in Detail
The 6-way modular connector that’s used in the UK is shown below:-
The instrument cable fitted to such a plug is wired as shown below:-
| Pin | Wire Colour | Circuit |
|---|---|---|
| 1 | Black | Business Telephones only |
| 2 | White | Line (B) |
| 3 | Green | Earth (for recall button) |
| 4 | Blue | Ringing (anti-tinkle) |
| 5 | Red | Line (A) |
| 6 | Orange | Business Telephones only |
although most devices are only wired onto pins 2 to 5. The corresponding socket should be connected to permanent cables, which are usually wired as follows:-
| Pin | Wire Colour | Circuit |
|---|---|---|
| 1 | Green with White rings | Business Telephones only |
| 2 | Blue with White rings | Line (B) |
| 3 | Orange with White rings | Earth (for recall button) |
| 4 | White with Orange rings | Ringing (anti-tinkle) |
| 5 | White with Blue rings | Line (A) |
| 6 | White with Green rings | Business Telephones only |
Once again, pins 1 and 6 are frequently left unconnected.
The original Post Office Series 700 telephone, as used in the UK, contains two rows of connection tags; an upper row containing tags 1 to 9 and a lower row containing 10 to 19. These should be wired to the incoming cable as shown below:-
| Wire Colour | Circuit | Tags |
|---|---|---|
| White | Line (B) | 16, 17, 18 and 19 |
| Green | Earth (for recall button) | 12 |
| Blue | Ringing (anti-tinkle) | 5 and 6 • |
| Red | Line (A) | 8 |
• 3.3 kΩ resistor should be fitted between tags 4 and 5. No other links should be fitted.
Master and Slave Sockets
The incoming circuit to your premises is usually provided at a master socket or a special connection box. Usually this is owned by and installed by the telephone company, and is fitted with spark suppressors to give limited protection from the effects of lightning. In the UK, it also incorporates a capacitor and a resistor for the anti-tinkle feature.
Only one master socket should be fitted to a line. All other sockets should be secondary sockets, also known as slave sockets, that lack the extra components found in a master socket. The wiring from the master socket to all of the slave sockets should be made up using two-pair (four-core) or three-pair (six-core) unscreened twisted-pair cable.
- Master and slave sockets come in single or twin versions in flush or surface-mounting styles.
- A type 10/3A splitter allows two telephone instruments to share a single socket. To connect an answering machine or another device thatintercepts the phone line you can use a plug-and-socket device known as a Keats adaptor.
- The number of instruments connected to a phone line is limited by the ringer equivalent number (REN) of all the devices. To find out your total REN simply get the REN for each device (often printed underneath the instrument or in its manual) and then add up the numbers for all the instruments. In the UK this number shouldn’t exceed 4. If your figure comes to more than this, your phones may not ring properly or their performance may be compromised.
