UARTs and Data Buffering

UARTs, or Universal Asynchronous Receiver/Transmitters, are the method by which computers send information to a serial device, such as a modem. A UART is an integrated circuit (chip) that converts parallel input into serial output. The CPU communicates with the serial device by writing in the UART's registers. UARTs have FIFO, or First-In, First-Out buffers through which this communication occurs. First-In First-Out means that the first data to enter the buffer is the first to leave. Without the FIFO, information would be scrambled when sent by a modem.

The first UART was National Semiconductor's INS-8250. This was soon upgraded to the faster 16450. Both the 8250 and 16450 only had a 1-byte FIFO, however. This meant that if information was traveling too fast for the CPU to handle it, it would be overwritten. It is nearly impossible for either of these UARTS to handle the transfer speeds of today's modems, most notable under Desqview and Windows, where the processor is kept busy, allowing data in the FIFO to be overwritten. The solution to this is the modern 16550A UART, which has fixed a bug in the original 16550. The 16550 has a 16-byte FIFO. Thus, up to 16 bytes can be written to the UART's registers before anything is deleted. This allows the CPU to catch up if it has been busy dealing with other tasks. 16550 UARTs are necessary for high speed communications under Windows or for DOS applications running in a Windows shell.