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A terminal adapter (TA) is a device that connects a computer to an integrated services digital network (ISDN). The terminal adapter transmits digital data directly from the computer to the ISDN line without having to modulate and demodulate between analog and digital signals.
Some manufacturers refer to a terminal adapter as an ISDN modem. This term is partially misleading because the modem does not perform the modulating and demodulating functions.
A TA may be an internal or built-in device for a computer, or it may be an external device connected to an RS-232 serial port or USB port.
There are devices that combine the functions of a modem with those of a terminal adapter. These allow ISDN connections and also function as modems by modulating analog signals to transmit digital data and demodulating analog signals to receive digital data.
Some TAs may contain an interface and codec (a device or software for encoding and decoding digital data) for one or more telephone lines. These TAs enable the upgrade of a regular telephone service to ISDN without the purchase of new telephones.
In mobile networks, a TA is used by terminal equipment to allow mobile termination (when a mobile phone user terminates a call originating in another network and paid for by the originator in another network). With 2G equipment, the TA is optional, but with 3G equipment it is mandatory and part of the mobile termination.
In the automation industry, a TA is a passive device that changes a regular connector such as an RJ-45 modular jack into a terminal block (a strip of terminal connection points) to accommodate proper wiring.