You've been given some excellent software solutions so I thought I'd mention a hardware one. It's not too difficult to make (you may even be able to buy these) a "tee" cable that you can use to attach between your computer's serial port and the device such that all I/O is available to another serial port. You can then spy on your device's I/O without interfering at all with normal communications.

duff

In reply to Re: Listening to I/O on serial port... by duff
in thread Listening to I/O on serial port... by Anonymous Monk

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