Hi Andy,

it depends from many things, e.g. how many other tasks and interrupts executed.

E.g. on the AT89C4051 at 24MHz you should not be faster than 9600 baud (full duplex) or 38400 baud (half duplex).
Otherwise too many CPU time can be needed.

Also you should use the hardware UART for the faster baud rate.
And all other interrupts should have the lower priority.


Peter