This C-language program initializes and monitors a serial port for the arrival of a Caller-ID data stream, returning a formatted data string in printable format when a call arrives. This code was developed and tested under RedHat Linux 6.1. The serial port control segments were derived from the Linux Serial-Programming-HOWTO document.

Description

Caller-ID information is transmitted by the telephone company between the first and second rings of an incoming call. Information is sent using Bell202 (FSK) modulation and formatted as asynchronous (start/stop) data at 1200 bits per second. If a suitable demodulator is connected to the telephone line, the data stream can be read directly by a standard serial port.

This program initializes the PC serial port and waits for a valid Caller-ID data transmission. When a phone call arrives, the data is assembled in printable form and returned. A typical return string is shown below.

Example Caller-ID Data Stream in Printable Form The original data is a stream of octets with no formatting. Information is packaged as a series of octets within a frame beginning with 0x80 and a length octet and ending with an 8-bit checksum. The frame can carry a number of fields, each headed by ID and length octets. The date and time field (ID=01) is always present. In the example below, the caller name (ID=07) and number (ID=03) are also present. Non-printable values are expressed in hexadecimal within carets (e.g. <86>), while printables are shown directly (e.g. the quotation mark is ASCII 34 (decimal)). In this format, the data fields are human readable and easily parsed by a higher level Perl script.

A higher level routine may call this program and further process the information.

What Exactly Are The Data Fields?

A compact description of the caller ID format may be found at: