From The Free On-line Dictionary of Computing (8 July 2008) [foldoc]:
comm mode
<chat> (Or "com mode") An ITS feature supporting interactive
on-line chat.
[Jargon File]
(1998-01-18)
From Jargon File (4.4.4, 14 Aug 2003) [jargon]:
comm mode
/kom mohd/, n.
[ITS: from the feature supporting on-line chat; the first word may be
spelled with one or two m's] Syn. for talk mode.