[comp.sys.mac.hardware] How do you poll the carrier-detect line on the serial port?

midkiff@portia.Stanford.EDU (Neil Midkiff) (07/21/90)

On the Mac SE and II, pin 7 of the serial ports is the
"general-purpose input" or GPi, connected to the serial controller's
Data Carrier Detect input for that port.  The DCD input to the SCC can
be polled by software or used to generate an interrupt.
(Above summarized from p. 362 of _Guide to the Mac Family Hardware,
2nd edition_.)

My question is HOW?  Inside Mac volumes 4 and 5 are apparently silent
on the matter, and since this input didn't exist on the Plus or
before, it can't be in an earlier volume.  (At least I couldn't find
it!)
-Neil