We suspect the cable may have a pin jumpered in it that may be causing this
problem when the mainframe polls the Macintosh in the evening. The
possibility is that pins 6 and 8 (jumpered), which should go to 20 on the
DG, may be pulled down signal-wise and responding as if the connection is
gone. This closes the line for security at the mainframe.
When the cable is disconnected from the Macintosh, those signals are
cancelled, because there is not a continuous loop inside the serial port
and the mainframe can tell that. When the cable is connected, it goes
through the serial port and incorrect cabling may respond as if the
connection has dropped.
We suggest you verify that the cable you are using is the correct
configuration and not having any extra pins jumpered inside. When the
Macintosh is off, nothing happens with the serial port (no power, no
activity).
We cannot tell you if there is a way to have the mainframe discontinue the
polling overnight. That may be one workaround besides verification of the
cable.