Q119579: Windows Support of the 16550 UART
Article: Q119579
Product(s): Microsoft Windows 95.x Retail Product
Version(s): WINDOWS:3.0,3.0a,3.1,3.11
Operating System(s):
Keyword(s):
Last Modified: 21-SEP-1999
3.00 3.00a 3.10 3.11
WINDOWS
kbref kbhw kbdocerr
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The information in this article applies to:
- Microsoft Windows versions 3.0, 3.0a, 3.1, 3.11
- Microsoft Windows for Workgroups versions 3.1, 3.11
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SUMMARY
=======
The universal asynchronous receiver-transmitter (UART) is an integrated circuit
that contains the software programming control of the PC serial port. 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 only one
byte at a time. These UARTs are not usually suitable for modem speeds greater
than 9600 bits per second (BPS) because of possible input data overruns that can
occur if a character is left in the input register when the next byte is
received.
The newer 16550 UART allows reliable data transfer at high speeds with its
16-byte first in, first out (FIFO) input register. The FIFO feature can buffer
up to 16 bytes at a time, which improves serial communications by preventing
data overruns in applications that are 16550 aware.
This article outlines the use and history of support for the 16550 in Microsoft
Windows.
MORE INFORMATION
================
Windows 3.0 and Earlier Versions
--------------------------------
Windows 3.0 and earlier versions do not support the FIFO feature of the 16550
UART. Instead, the UART remains in the 8250 UART compatibility mode, allowing
one byte to be received at a time. If the incoming character isn't read fast
enough by the computer, the byte is lost. Lost or "dropped" characters are
likely to occur at speeds faster than 9600 bits per second (BPS).
There are no UART-specific switches in the SYSTEM.INI file with Windows 3.0.
NOTE: Third-party communications drivers (such as Turbocom by Pacific Commware)
are available to add support for the 16550 FIFO feature in Windows 3.0.
Windows 3.1, 3.11, and Windows for Workgroups 3.1
-------------------------------------------------
Windows 3.1 was the first version of Windows to support the FIFO feature of the
16550 UART for Windows-based applications. (MS-DOS-based applications run under
Windows 3.1, 3.11, or Windows for Workgroups 3.1 do not support the FIFO
feature.) The receive buffer (RX) is set by the serial communications driver
(COMM.DRV) to 14 bytes. The transmit buffer (TX) is not enabled.
[386Enh] Section SYSTEM.INI Setting:
COM1FIFO=0 | 1
COM2FIFO=0 | 1
COM3FIFO=0 | 1
COM4FIFO=0 | 1
Default: Enable the FIFO if detected and the setting is not in the SYSTEM.INI
file.
Purpose: Specifies whether the FIFO buffer of a communication (COM) port 16550
UART should be enabled (1) or disabled (0). If a serial port does not have a
16550 UART, this setting is ignored. If you place a numerical value other than
"1" or "0" (for example, COMxFIFO=2 ), the setting is ignored, and the FIFO is
enabled if detected. Using TRUE, FALSE, or any other non- numerical value sets
the switch to "0" and therefore disables the FIFO.
Example:
The COMxFIFO= setting is not fully Boolean aware. To properly use these switches,
use the following syntax
COM<x>FIFO=1
-or-
COM<x>FIFO=0
where <x> is the number of the COM port you want to set.
The SYSINI.WRI file from the Microsoft Windows Resource Kit for version 3.1
incorrectly identifies TRUE and FALSE as functioning with this switch. Page 196
of the Windows Resource Kit manual incorrectly identifies "On" and "Off" as
functioning with this switch. Again, using any non-numerical value sets the
switch to "0" and therefore disables the FIFO.
These values are used by Windows for both standard and 386 enhanced modes.
NOTE: Third-party communications drivers (such as Turbocom by Pacific Commware
and KingComm by OTC Corporation) enable FIFO support for MS-DOS- based
applications running under Windows.
Windows for Workgroups 3.11
---------------------------
Windows for Workgroups 3.11 is the first version of Windows to enable 16550 FIFO
support for MS-DOS-based applications running under Windows. In addition to
Windows-based applications, your MS-DOS-based applications can now use FIFO to
prevent data overruns without using a third-party communications driver. The
Windows for Workgroups communications driver (SERIAL.386) can also use the
transmit buffer (TX) of the 16550 UART. Previous versions of the communications
driver use the receive buffer (RX) only.
[386Enh] section SYSTEM.INI settings:
In addition to the COMxFIFO setting outlined above, Windows for Workgroups 3.11
introduces two new settings, RXTRIGGER and TXFIFO.
RXFIFO Setting:
COM1RxTRIGGER=1 | 4 | 8 | 14
COM2RxTRIGGER=1 | 4 | 8 | 14
COM3RxTRIGGER=1 | 4 | 8 | 14
COM4RxTRIGGER=1 | 4 | 8 | 14
Default: 8
Purpose: Sets the number of bytes to enable for the receive FIFO buffer (RX) on
the 16550 UART.
Example:
COM<x>RXTRIGGER=8
where <x> is the number of the COM port you want to set.
NOTE: In general, you should not change the RXFIFO value. Increasing the value of
RXFIFO to 14 causes fewer interrupts to be generated, but it also decreases the
space in the remainder of the buffer to 2 characters. On a busy system, which
may have an increase in interrupt latency (the time that it takes for an
interrupt to be serviced by the CPU), this may cause characters to overrun the
buffer. Likewise, decreasing the value for RXFIFO increases the number of
interrupts generated, which could lead to interrupt saturation, negating the
usefulness of the FIFO on a multitasking system.
TXFIFO Setting:
COM1TXFIFO=0 | 1
COM2TXFIFO=0 | 1
COM3TXFIFO=0 | 1
COM4TXFIFO=0 | 1
Default: 0
Purpose: Enables the transmit buffer (TX) on the 16550 UART. If enabled, 16 bytes
are sent to the UART with each empty transmit interrupt generated.
Example:
COM<x>TXFIFO=0
where <x> equals the number of the COM port you want to set.
NOTE: Enabling the transmit buffer (TX) may result in better system performance
during a high-speed file upload. It does not affect downloads.
Additional query words: 3.00 3.0 3.0a 3.1 3.11 3.10 50Z PS/2 modems
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Keywords :
Technology : kbAudDeveloper kbWin3xSearch kbWFWSearch kbZNotKeyword3 kbWin300 kbWin300a kbWin310 kbWin311 kbWFW310 kbWFW311
Version : WINDOWS:3.0,3.0a,3.1,3.11
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