On the TD-50X, connect the analog snare pad to one of the AUX 1–4 jacks and assign a snare instrument to it.
Note that the AUX-connected pad does not support cross-stick playing. If a pad is also connected to the TRIGGER IN (2 SNARE) jack while the PD-140DS is assigned to snare on a DIGITAL TRIGGER IN port, the PD-140DS takes priority and the pad on the SNARE jack will not sound.