If you're using an SBS Octal Serial IndustryPack module and a Motorola 5100
serial processor, you can't use serial channel 7 (numbering: 0...7).  There
is a poorly understood interrupt problem that is fixed with version 2.8 of
the ipac module, but that version is not compatible with epics 3.13, so
the fix is unavailable to synApps 4.6 users.