>Do you know how to read a half duplex line and know, by hardware,
>how to tell who is sending data, if master or slave ??

I think you are approaching this the wrong way. To look at the protocol you
have to know what it should be. Then use an oscilloscope to verify that the
voltage levels being transmitted are correct, and the smallest bit time,
high or low, is correct for the baud rate. Then use the com port of a PC to
eavesdrop on the line to watch the data going past, use two com ports if
necessary, but the impression I get is that you have a single line that
transmits data both directions, in which case a single com port should be
enough. This will display the data for you, and allow you to see where the
protocol is going wrong.

If you are trying to reverse engineer someone else's equipment, then you are
in a whole different ball court, and you need to just watch what happens,
make your own hypothesis about what you saw, and try to confirm that using
more observations.

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.