the serial port adapter because Intel chip had no internal baudrate generator, and the adapter had to be more complex and expensive. Later IBM implemented Syncronous Communication Adapter, but it was not popular. References External links Serial and UART Tutorial archive.pcjs.org: National Semiconductor PC16450C/NS16450, PC8250A/INS8250A Universal Asynchronous Receiver Transmitter (datasheet, dated 1990) Input/output integrated circuits