Windows Support of the 16550 UART

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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.


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
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.


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.

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.


Article ID: 119579 - Last Review: February 27, 2014 - Revision: 2.1
  • Microsoft Windows 3.0 Standard Edition
  • Microsoft Windows 3.0a
  • Microsoft Windows 3.1 Standard Edition
  • Microsoft Windows 3.11 Standard Edition
  • Microsoft Windows for Workgroups 3.1
  • Microsoft Windows for Workgroups 3.11
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