Xerox Paris, a document system Support & Software 用户指南

The UART chip

Every serial port uses a UART chip to send and receive data. Many machines
still use either the 8250 or the 16450 UART chip. The 8250 was the original
UART to ship with the IBM Personal Computer. This UART and the improved
16450 version are limited to one input register that holds one byte at a time.

These UART’s are not normally suited for speeds greater than 9600 bits per
second because of the possible input data overruns that occur, if a character is
left in the input register when the next byte is received.

The newer 16550A UART allows reliable data transfer at high speeds with its
16 byte FIFO (first in, first out) input register. The FIFO feature can buffer up
to 16 bytes at a time, which greatly improves serial communications speeds.

Other causes of hardware overrun error messages

Although the baud rate is the most likely cause of problems, other factors also
need to be considered. These include:-

C P U   s p e e d

Faster CPU’s allow faster throughput with fewer errors. This is because they
allow the communications driver to service character interrupts faster
(preventing interrupt overflow) and allow applications to handle transfer
protocols better.

System overhead

The more applications, TSR’s and device drivers that are running the busier the
operating system is. This reduces overall execution time for everything in the
system.

Windows 3.0 support of UART

Windows 3.0 does not support the advanced first-in first-out (FIFO) buffer
feature of the 16550A UART. It supports the 16550 and 16550A as a standard
8250 UART.