Modem lights are simply the lights that are found on external modems. Depending on whether the lights are dark, activated and steady, or flashing, each modem light will provide an indication of the activity that is currently taking place between the modem and the connected computer hard drive. The current status of the modem lights helps the end user to know whether a connection is established, if the connection is active, or if the connection has dropped for some reason.
The activity of modem lights is usually referred to as handshaking. Essentially, when the modem is in use, the device is interacting with the computer. The modem receives input from the computer and in turn provides the hard drive with an influx of data as well. Both devices confirm receipt of data received and commands issued by the other. In this sense, the handshake is not unlike a gesture between two individuals who have agreed to an exchange or a plan of action.
This interaction or handshake that is signified by modem lights is made possible by the presence of the Universal Asynchronous Receiver/Transmitter that is found on the hard drive of the computer. The UART microchip is what actually allows the computer to talk to the modem, and to process data received from the modem. Without the presence of the UART, it is impossible to establish rapport with an external modem.
There are several types of modem lights found on the standard external modem. The AA, or auto answer light, signifies that the modem is engaged and ready to receive an inbound call. The CD, or carrier detector light, acknowledges that the computer and the modem have achieved a connection and communication back and forth is now possible. The HS, or high speed light, will be lit if the modem is prepared to transfer data at a relatively quick pace. Finally, the MR, or modem ready light, allows the end user to know that the modem is set for operation.
The series of modem lights found on a standard external modem help the end user to be able to monitor the current status of the connection at all times. In the event that the connection between the computer and the modem is broken, it normally takes nothing more than a quick reset of the modem and using a key, switch, or button on the modem to begin the process of re-establishing the connection.