Xerox Paris, a document system Support & Software User Guide
Job Definitions
The Paris Spooler Technical Manual
79
UART Signals an Error
The hardware overrun error indicates an overrun error of the receive buffer in the
Universal Asynchronous Receiver Transmitter (UART). The Comm driver obtains
this error by reading the Line Status Register (LSR) of the UART, of which bit 1 is
set when an overrun occurs.
Universal Asynchronous Receiver Transmitter (UART). The Comm driver obtains
this error by reading the Line Status Register (LSR) of the UART, of which bit 1 is
set when an overrun occurs.
When the Comm driver cannot service a Received Data Available interrupt, before
it receives the next transmitted character, the UART signals an error because at
least one character has been lost.
it receives the next transmitted character, the UART signals an error because at
least one character has been lost.
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.
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.
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.
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:-
need to be considered. These include:-
CPU speed
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.
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.
operating system is. This reduces overall execution time for everything in the
system.