8.1 - The SLT, POT or Analogue Telephone

All equipment which is designed for use on a single analogue exchange line is designated as a simple telephone. These may variously be referred to as Simple Telephones, Single Line Telephones (SLTs), Plain Ordinary Telephones (POTs) or if connected to an inherently digital system Analogue Telephones. All these names refer to the same class of equipment.

This type of equipment includes the likes of cordless telephones, answering machines, facsimile and modems. They all work on the same basic principles with certain characteristics enhanced to perform specific functions.

8.2 - The Black Box View

The basics of a simple telephone device are shown in Figure 4.

Figure 5 The Basic Units Of A Simple Telephone

In Figure 4 the essential components of a simple telephone are separated to show their relation to each other and allow easier discussion. The handset usually contains the Microphone (M) and Earpiece speaker (S). The Base section contains the Hook switch (H), Ringer detection and signalling (R) and Dialling circuits (D). Connection to the line is via a single pair wire.

The functions of microphone and earpiece require no further explanation when referring to a speech device. In a data device they would be replaced by transmit and receive circuits. In an answering machine the microphone is usually absent and the earpiece feed directly to the answering circuit.

Power is fed from the line to operate these basic modules. However more sophisticated equipment will have additional power supplies to power their more complex functions.

The hook switch turns the telephone on and off. It is often a spring loaded micro switch held in the open position by the handset resting on a lever connected to its contacts on the most basic type of simple telephone. Automatic equipment uses and electronic switch which may be controlled via the handset being on the base or the logic designed into the telephone, E.g. Answering machine or cordless telephone. Problems can arise from electromechanical switch types as the contacts wear with use and send spurious signal to line when the switch does not open and close cleanly. This will manifest itself in the form of ringback caused by jitter or may disable the telephone altogether by not breaking and leaving a permanent busy state to the line. Devices, which use a solid state electronic hook switch, do not suffer from these problems since there is no component wear to the switch device.

The ringer module will detect an incoming ring signal and trigger a sounder or bell to alert the user that a call is being received. This will only operate with the hook switch in the open position i.e. the telephone is in the idle state. The ringer will consume current from the line and the level of this current is used to derive the Ringing Equivalence Number (REN) of the device. A domestic telephone line can normally support a REN of 4. Most modern equipment will have a REN of 1, so upto 4 can be connected before the REN is exceeded. Should the particular telephone have an external power supply, it may require less than 1 REN of power, yet be rated as REN1 since this is the lowest rating on the REN scale. In such cases it can sometimes be possible to connect more than 4 devices, providing the total does not exceed REN 4. If the REN is exceeded ringing may become erratic or stop altogether. Care should be taken when connecting older equipment as these units can often have a REN of 3 or more.

The last essential element is the dialling circuit. This creates and sends the dialled digits to the line in response to the dial pad or rotary dial wheel. Modern equipment will allow switching between pulse and tone dialling modes so that the dialling type can be matched to the line. It is unusual to require pulse dialling today. In an answering machine the dialling circuit is absent since there is no requirement to make calls.

These elements are found to a greater or lesser extent in all simple telephone equipment. However the design of the electronics often makes differentiation between the functions impossible for anyone but trained technical staff.