isr.co.ro : glossary Modem A device that changes computer data back and forth into signal formats that can be more easily transmitted by telephone or other communications link. Technically, it modulates and demodulates a carrier signal, such as an audio tone or set of tones. That is, a modem adds and decodes variations representing the data. The type mostly commonly used for personal computers connects an RS-232 line, that is, a serial port, to the telephone network. Other similar units that connect computers and phone lines plug directly into the bus of a desktop computer or a PC card slot on a portable computer. The interface between an ISDN telephone line and a computer is often referred to as an ISDN modem, although more precisely it should be referred to as a terminal adapter.