The  8251A  is  the  industry  standard  Universal  Synchronous/Asynchronous  Receiver/Transmitter  (USART) designed for data communications with microprocessor families such as MCS-48, 80, 85, and iAPX 86, 88. The 8251A is used as a peripheral device and is programmed by the CPU to operate using virtually any serial data transmission technique presently in use (including IBM “bi-sync”). The USART accepts data characters from the CPU in parallel format and then converts them into a continuous serial data stream for transmission.
Simultaneously, it can receive serial data streams and convert them into parallel data characters for the CPU. The USART will signal the CPU whenever it can accept a new character for transmission or whenever it has received a character for the CPU. The CPU can read the complete status of the USART at any time. These include data transmission errors and control signals such as SYNDET, TxEMPTY. The chip is fabricated using Intel’s high performance HMOS technology.