On 06/09/2016 09:43, Fritz Mueller wrote:
  But thinking on it, I bet USB has something to do
 with it here ? perhaps buffer latency through the USB drivers is
 keeping software flow-control (xon/xoff) from functioning effectively
 at higher baud rates 
I've had that problem with a printer and a double-buffered UART.  As
near as I could determine, the printer waited to send XOFF until there
was only one or maybe two characters free in its software buffer, but by
that time there was one char in transit, one more in the printer's
receiver buffer register and another in the computer's UART's
transmitter buffer register.  That's at least one too many :-)
--
Pete
Pete Turnbull