TTY DRIVER PROBLEM UNDER XENIX SYSTEM V

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I have an elusive bug with a tty driver for a multiport intelligent serial
card under SCO XENIX SYSTEM V. My hardware platform is an EVEREX 386 PC.
My test case is simple. I send a i Mb file to all 8 ports at the same time
in the background "cat filename > /dev/dev[1-8] &". The problem is that 
every now and then the whole test case freezes up, the processes go to sleep
on some channel. The xon/xoff handshaking is automatically done by the
board itself. I have increased the number of system clists from 100 to 1000 and
that did not seem to have solved the problem either. I need some debugging 
hints to pinpoint the problem. Is there any way to tell what are the channels
that the processes are sleeping on? Is this a hardware, xenix, or driver 
problem? My speed on all ports is 9600 bits/second. Any coment or help is
greatly appreciated.    Thanks.