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MT2834BLR Owner’s Manual
3.5.1 How Can You Identify Your UART Type?
If you have MS-DOS 6.0 or later, you can find your UART type from a diagnostic program called
MSD. To use it, type MSD at the DOS prompt. After the opening screen, select COM Ports.... The
last line of the report tells you what type UART you have for each COM port. MSD does not
distinguish between the 8250 and the 8250A. However, if you have an IBM AT or newer computer,
you likely have an 8250A or 16450 UART installed, both of which are reliable to 19,200 bps. If you
would like more detailed information about your UART than MSD can provide, you can download
shareware UART identification programs from the IBM Communications Forum on CompuServe, or
from a local BBS.
3.5.2 The 16550 UART and Windows 3.1
Windows 3.1 may cause a loss of data when communicating at high speed, even with a 16550 UART
installed. Because Windows is multitasking—it switches between several programs running at the
same time—it can make the serial port wait briefly while Windows performs other tasks. Meanwhile,
incoming data can be lost. The older 8250A and 16450 UARTs can store data in a one-byte first in-
first out (FIFO) buffer (a memory area in the UART in which the first bit in is the first out), but once
the buffer is full, the data is lost. The 16550 has a 16-byte FIFO buffer, which accounts for much of its
high speed. However, in Windows 3.1, the 16550’s FIFO buffer is disabled by default. To enable the
buffer, use Notepad to open the SYSTEM.INI file in your Windows directory, and add the following
line to the [386Enh] section:
COMnFIFO=1
where n is the number of the COM port the 16550 is installed on. For example, if the 16550 is
installed on COM2, the line should be:
COM2FIFO=1
After you save SYSTEM.INI you must restart Windows for the change to take effect.
However, there is another problem when the FIFO buffer is enabled: Windows cannot recognize the
COM port because of a bug in the Windows 3.1 serial port driver, COMM.DRV. The only way around
this problem is to replace the COMM.DRV file in the WINDOWS\SYSTEM directory with another
driver. Several drivers are available commercially or on BBSs that will solve this problem. A freeware
driver called CyberCom is available as CYBERC.ZIP on CompuServe in the IBM Communications
Forum, Library 0. CyberCom will enable Windows communications at speeds up to 115,200 bps if
you have a 16550 UART installed.