Call Line Identification, or Caller ID technology has been available in Australia only since June 1998. Since then, the market for products that utilize this technology has increased enormously. The information is sent between the first and second rings of a telephone call. Before any product is legally allowed to connect to the telephone lines it must be approved by the Australian Communications Authority, the regulatory body that controls the Australian Standards for telephone network connection. As the generation of the Caller ID signals occurs at the exchange, testing prototypes of products that use the information without connecting to the public telephone network is difficult.
This thesis will emulate a telephone line with Caller ID, distinctive ring and the capability to send DTMF tones to enable design and construction of Caller ID capable devices for the Australian marketplace. To correctly operate any consumer equipment that may be connected for testing, all line voltages must be emulated and the correct timing must be used for all ring signals.
Several products exist that currently perform similar functions, however they are all aimed at overseas markets. The most comprehensive of these products, the Rochelle Communications 3400 series, is a PC based card, which severely restricts portability and increases price dramatically. Telephone line simulators have also been used for educational purposes, however this is not the aim of this thesis.
The device consists of seven functional units: the system controller, Caller ID generator, DTMF send, signal switching multiplexor, line voltage power supply and the line interface device. The Caller ID generator produces a 1200/2200 Hz frequency shift keyed 1200 baud, 8 data, no parity, 1 stop bit signal. This is accomplished by switching two signal waveforms with the aid of a serial bit bashed output port. The system controller performs these functions and controls the remainder of the hardware consists of a Motorola 68HC11E2 microprocessor and an Altera EPM7128JC EPLD.
The DTMF send functionality is performed by a Phillips PCD3311 DTMF generator. This device requires a six bit parallel input code, and is controlled via the EPLD by a 16 key matrix keypad. The DTMF signals and the Caller ID signals are sent through a signal select multiplexor, which amplifies the signals to a 5 volt level and selects between the two signals.
The ring generator and line voltage power supply both consist of a push pull transformer arrangement driven by two MOSFET devices. All power circuitry is isolated from the digital section via optoisolaters and from the telephone line side via transformers that meet the Australian Communications Authority isolation standards. The output of the line power transformer is rectified, clipped to a 43V maximum level and smoothed to create a DC source of 20mA, emulating a standard telephone line. The ring generation circuitry is coupled to the telephone line using a capacitor and clipped to 120Vpp to produce the 30Hz, 120Vpp AC ring signal.
The line interface device isolates the low level analog signals (the Caller ID and DTMF) from the telephone line. It is a 2-4 wire hybrid device, that is it converts the 2 line telephone line ‘ring’ and ‘tip’ lines to two receive and two transmit lines. It also has the capability to detect ring signals, take the line off hook, and several other functions not relevant to the thesis.
The system was designed in a modular manner, with an emphasis on expandability and flexibility. The expected market for the device includes design engineers and the University market, where it could become a valuable testing tool for design students.