Peter,

Was the byte lost on the receiver side of your notebook or x51 board? You'd better guess whether it is transmitting or receiving error, and on which side was it occurred?

1. Is the error from the notebook?

To find it, link your notebook to the desktop with null modem by running HyperTerminal to transfer files across. This way if the bytes are still missing, it must be the notebook's problem.

Most of the motherboards allow you to adjust the CPU speed by jumper or BIOS setup screen. If your notebook allows this, lower its speed and try again. I suspect that some of the I/O chips are so cheap that they don't function right at high speed.

2. Is the error from the x51?

From my point of view, x51 should handle the high-speed 115.2K rate with no problem, as long as the peer end has the same baud rate. Try to disable all the other interrupts to see if the result is still the same. Also connect two of your x51 boards to see if the byte are still missing.

3. Is the error from the communication?

Use a very short (< 1 foot) cable to link both ends and try again. If the result is fine, you can conclude that it is the H/W driver's capability problem of the notebook. Because notebooks are made to save power, its TX/RX line voltage/current may not match that of the desktop. Hook up the scope and examine the waveform carefully. The missing byte might be caught by the scope if you are lucky.

I would really like to see what went wrong.

Regards,
Oliver