Wow Jake - I guess there's always a first ;)
EFIS-D10A and EFIS-D100 have only one serial input, so that must be shared between a GPS/SL30 and the PC connection. Thus, the GPS/SL30 MUST be disconnected to use the PC connection.
If you have an EMS-D10 or EMS-D120 (but not an HS34), those have...