The Peripheral Interface Guide (page 5.8) states that the Apple Modem 300/1200
590-0197 cable works with the 590-0341 (M01089 Beige) or 590-0553 (M0199
Platinum) on an Apple IIGS, Macintosh Plus, Macintosh SE, or a Macintosh II.
You can also build a cable using the pinouts listed below.
Apple IIGS (Mini-8) 30/1200 Modem (DB-9)
HSKO 2 2 DSR
TD- 3 9 TD
Gnd 4 8 GND
RD- 5 5 RD
TD+ 6 3 SGND
RD+ 8 8 SGND