tty driver strange problem

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I am writing a tty driver for a multiport intelligent serial card under SCO
XENIX SYSTE. My test case for transmit is very simple. I try to "cat" a file
of 1 Mb size and redirect the output to the 8 ports at the same time in the
background "cat filename > /dev/port[1-8] &". The problem is an elusive bug.
Every now and then the processes go to sleep on some channel and the whole
test freezes up. I have increased the number of system clists from 100 to 1000.
That did not solve the problem either. The xon/xoff handshaking is done automatically by the board and the driver does not have to handle that. Does anybody 
know any debugging hints to trace the problem. I would like to know if I can
find out what those channel numbers which te processes are sleeping mean? 
Does this look like a driver or a hardware problem ? Any comments or help
is very appreciated.     Thanks.