malfunction in MIDI

[nikydvt]

I work on Pro Tools 10 (S.O. Windows 8). My audio interface is a RME Fireface UCX. I am searching to play a clip on a midi track that have as output a synthesizer set on Midistudio setup menù, that pours is sound on an input port of the Fireface UCX interface; now if I use as port to whom connect the synthesizer an ESI M8U_XL MIDI interface (that also is recognized from Pro Tools), no sound is product. Otherwise if I use the MIDI Port of the RME Fireface UCX interface everything goes OK. Why not with ESI M8U_XL MIDI interface that also is always offered as possible choice output next to RME Fireface UCX in MIDI tracks? Thanks niky