Do You Know What is Modem?

This article gives the information of MODE concept. MODEM (MODulation/DEModulation) is an input and output device that converts digital data from a computer to analog data for transmission over the telephone lines by modulating it into waves; at the other end the modem the converts analog data back to digital form so that it can read by the computer. A MODEM is communication equipment used for long distance data transmission through telephone line. A pair of modems is necessary to link the two ends of communication.

There are two types of MODEM:

1.      Internal modem
2.      External modem
 
Internal Modem:

The internal modem is a standalone board that plugs directly into an ISA or PCI expansion slot. The internal modem contains its own universal synchronous receiver/transmitter (UART). The “UART” work here logically as serial port. When installing an internal modem car should be taken that the IRQ number and I/O address port number do not conflict with inbuilt motherboard serial ports. (i.e. COM 1 and COM 2). A modulator circuit converts the serial data from the computer into analog signals to be transmitted over telephone line. The analog signal is then coupled to the telephone line using a circuit very similar to that used by ordinary telephone to couple voice signals. Then this analog signals passes to the telephone through telephone jack (RJ 11 connector socket). On receiver side serial signal received from telephone line. The telephone interface separate received signals and passes them to demodulator. The demodulator converts analog signals into digital form and send this serial data to UART. The UART converts serial bit data into parallel byte and placed on the system data bus.

When a remote modem dials in the telephone interface defects the incoming signals and alerts the UART to begin negotiating a connection. Finally the telephone interface drives a small speaker during find stags of modem operation. The speaker is used to hear a dial tone dialing signals and audio negotiation between the two modems. Once a connection is establish the speaker is usually disabled. A controller circuit manages the overall operation of the modem. Generally it is used to manage modem between in control and data operation modes. The NVRAM is non volatile RAM it is used to store modem parameter.

External Modem:
External modem is a standalone device connected to serial port (COM 1 OR COM 2) of system. Working of External modem is as same as internal modem. The difference is that it uses standard serial interface RS 232C, so that it uses motherboard UART for serial bit to parallel byte or parallel byte to serial bit conversion.

Advantages:
Installation modem is easy.
Not required/needed to open the system.
No worry about IRQ and I/O address conflicts.
Easy to configure.
Front panel LEDs of external modems shows status of communication.

Disadvantages:
Externally powered.
Serial interfacing cable generation problem.

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