Setting Baud Rates in Windows 3.1

Under Windows 3.1, baud rates are specified in the BaudRate field of the communications port's DCB structure, and are set using SetCommState(). Baud rates can be specified by using the CBR_ constants listed in WINDOWS.H, or by using the actual baud rate.

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: 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.