DCD Problems

[Mike McNally]

Lynx has an Integrated Solutions 68K VME system, running 4.2 BSD.  The
IS port of 4.2 (maybe other's; I don't know) uses one of the local mode
word bits of the tty driver, the LNOMDM bit, to control whether an open()
request to the port will require Data Carrier Detect to be asserted before
completing.  The default is (of course) that it DOES need to be asserted,
and as a result almost all of our cables to the thing have DCD wired active.
An exception is with the dial-in modem.  The modem itself asserts DCD
when it answers and connects.  The getty at that port just sits around,
hung in open() until the modem connects.  Great.  What we'd like to be
able to do, however, is use one line for both directions of modem 
traffic.

I'm rambling.  The big question is this:  somehow, through a miracle of
technology, tip seems able to break through the DCD barrier.  In other
words, if I plug a modem that is not asserting DCD straight into the
machine without a funny connector, I can actually talk to the modem
with tip!  Strangely, it takes a long time for tip to connect; we suspect
that it's waiting for a timeout signal and then proceeding to do some
secret operation.

Does anybody know anything about the DCD mystery?