TXT Buffer. This is needed for compatibility of "Frame Swapping"
and "Retransmitt limit".

If frame swapping is enabled then if retransmitt occurs, then
change of the source buffer indicates that retransmit error counter
should be erased, since other frame is being transmitted!
In case that buffer does not change in retransmitt, the counter
must be kept the same.
In case that frame is transmitted succesfully and no retransmitt
occurs ,the PRotocol control erases the the counter by itself.