Q: Why can’t we change to a 76800 baud rate on a PC but are able to on some devices?
A: The reason for this is that the the standard 16C550 PC UART is too slow to keep up with the signals coming in and out. 76800 is not a standard multiple of the UART clock frequency.
Devices can be customized and therefore allow baud rates of 76800. If your PC has a customizable COM port card where you can program the bit clock then you would be able to set this value.
There is no fix for this, but if you find a converter that can change 76800 to a higher UART this could be a work around.
The standard frequencies are: