The following is a list of Roland pads and triggers that are compatible with the SPD-SX's external trigger inputs:
KD-series kick triggers
PD - series pads
PDX - series pads
CY - series pads (dual zone triggering only)
BT-1 Bar Trigger pads
RT-30 series acoustic triggers
RT-10 series acoustic triggers
Note: When splitting the trigger inputs (as described on page 27 in the SPD-SX manual), only single trigger results can be produced from each pad.
Additional note: None of the "FD-series" hihat controller pedals are compatible with the SPD-SX.