Data terminal equipment is any digital device that can interpret data messages from the end-user. Examples of data terminal equipment are fax machines, a computer capable of accessing the web or a network, and even printers. Furthermore, these data terminal equipment are connected to data communications equipment which represent the means for one entity such as a client to connect to another entity such as a host. In between data terminal equipment and data communications equipment are devices that handle serial data interfacing of which the most common is UART (Universal Asynchronous Receiver/Transmitter).

UART provides asynchronous data transmission, which means clock timing is different between the DTE and DCE. Furthermore, its primary functions are as follows: (1) parallel-to-serial data conversion in the transmitter and series-to-parallel for the receivers; (2) error detection by inserting parity bits in the transmitter and checking parity bits in the receiver; (3) insert start and stop bits; (4) format data; (5) provide status updates to central processing unit; (6) voltage level conversion; and (7) provide means of achieving bit an character synchronization. Functionally, a UART has two sections: transmitter and receiver.

A UART transmitter operates by first receiving the desired mode instruction word. This is done by the central processing until in the data terminal equipment, which applies the mode instruction word to the control word bus and then activates the control-register strobe. After which, data can now be exchanged with control signals managing the end of characters, bit rates and transmit serial output.

A UART receiver monitors the line regularly and watches out for the start bit of an incoming data stream. When a start bit is verified, the data character is clocked into the receive shift register. After checking parity and loading data character into the shift register, an acknowledgment signal, receive data available (RDA) flag is set in the status word register. The CPU acknowledges this flag with a signal that resets the RDA pin and another character is sent.

Posted 2010-12-14 and updated on Dec 14, 2010 1:38pm by crisd

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