Inexplicable loss of DTR on async port

[John Stewart]

I have a telebit modem connected directly to an async port on our Sun
4/380 machine.  The port is configured for bidirectional uucp connections.
Up until recently it has functioned flawlessly.

On several occasions in the last two weeks, the port has got into a state
in which DTR is not present.  When this happens, the modem of course
refuses to answer incoming calls.  uucp is still able to initiate outgoing
calls.

It's a real disaster when this happens because some of my uucp connections
are unidirectional (ie. they call me and I cannot call them) and a backlog
quickly develops.  In this state, getty is still running and restarting
getty does not fix the problem.  If stty is used to look at the port
parameters, the stty process hangs before printing anything and cannot be
aborted with ^C.

The only solution I have found is to reboot the machine, which isn't too
popular with users running long jobs.

I'd sure appreciate any hints as to what is causing this problem and how I
can fix it without having to reboot the machine.