from the (irix) manpage: o2/octane/onyx2/origin systems have a pc-style db9.
earlier-generation sgi db9 (challenge/onyx) have a nonstandard pinout:
2	txd
3	rxd
4	rts
5	cts
7	gnd
8	dcd
9	dtr
sgi din8 is also prestandard, txd/rxd are common to the mac serial cable but
all the flow control is different. 4d pinout viz:
1	dtr
2	cts
3	txd
4	gnd
5	rxd
6	rts
7	dcd
8	gnd
the powerseries-vintage db9/din8 keyboard adaptor converts neatly between
sgi's usage of those connectors (eg, you get a challenge-style db9 from
the din8, not a pc-style one).
iirc, the din8 modem cable is sgi X5-25TO8. the ideal part here would be a
din8-rj45 buildit, but i don't know of such a beast. i typically cut the
molded minidin8 off something and solder the db9/db25 end, the minidins are
a pain.