For an introduction to SSM2 see this RomRaider page. Does not cover advanced details though.
Tutorial shows working with a J2534 capable device using free generic software tool: Manual Serial Connection Tutorial.
SSM2 works with serial (K-) line. This protocol looks like modified ISO 14230/KWP2000. Seems to be used by Subaru only.
SSM2 needs bidirectional serial communication on a single line called the K-line (pin 7 on the OBD-II-connector). L-Line (pin 15) ist not being used, as wakeup apparently is not required on SSM2 control units.
OBD-II (J1962 spec) female connector, often found below steering wheel:
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