MORE INFORMATION
OpenComm(), by default, sets the baud rate of a communications port to
BuildCommDCB() can be used to set the baud rate; however,
BuildCommDCB() accepts only the following baud rates: 110, 150, 300, 600,
1200, 2400, 4800, 9600, and 19200. If none of these baud rates are used,
BuildCommDCB() returns an error.
The SetCommState() function can be used to set the baud rate to values
between 2 and 115,200. The following CBR_ constants are defined in
WINDOWS.H:
CBR_110 CBR_14400
CBR_300 CBR_19200
CBR_600 CBR_38400
CBR_1200 CBR_56000
CBR_2400 CBR_128000
CBR_4800 CBR_256000
CBR_9600
NOTE: These are not the same constants as a listed in the Windows 3.1 SDK
documentation for the DCB structure.
NOTE: The CBR_56000 constant actually corresponds to a baud rate of 57,600.
NOTE: The baud rate divisor is incorrect for CBR_14400.
SetCommState() allows either the CBR_ constant or the actual baud rate in
the BaudRate field of the DCB. SetCommState() calls into COMM.DRV to apply
the DCB settings to the communications port. Although CBR_128000 and
CBR_256000 are defined, they are not used in the communications driver
(COMM.DRV).
When setting the baud rate, COMM.DRV first determines if the baud rate is a
CBR_ constant; if so, COMM.DRV looks up the baud rate divisor in a table.
If the baud rate is not in the CBR_ constant table and the UART is 8250,
16540, or 16550, the baud rate is set using 115,200 as the divisor and is
divided by the user specified baud rate. The subsequent integer value from
this division is used to set the baud rate (the remainder from the division
is not used). For example, if the user specified a baud rate of 60000, the
calculation would be 115200/60000, which is 1.
Therefore, the baud rate can be set to 115,200 by placing any value in
BaudRate that is greater than CBR_56000 (57,600) and less than CBR_110,
because this will always equal a baud rate of 1 (using the above formula)
and signal the UART to operate at it's highest possible speed (115,200). A
115,200 baud rate cannot be specified directly in the DCB structure because
the baud rate variable is an unsigned integer and cannot be larger than 64k
in the 16 bit world.
Third-party replacement communications drivers may set baud rates
differently. If using a replacement, check with the software manufacturer
to determine how to correctly set the baud rate.