Yamaha DTXpress/DTXplorer/DTXtreme

I am working "long distance" with a drummer on some home studio 
projects.  He uses the DTXpress.  We are having a problem.

The set up

DTX is patched into a PC running SONAR. 

When we run DTX MIDI out to Computer MIDI in, we get MIDI data input 
and it records fine. That is just what we would expect

When we run Computer MIDI out to DTX MIDI in , we get MIDI data 
output and it plays just fine. Also what we would expect.

Now - - - - when we patch both ways  (DTX MIDI out to Computer MIDI 
in AND Computer MIDI out to DTX MIDI in)  we get an uncotrollable 
MIDI loop.  The only way to stop it is to unplug a MIDI cable.

I am sure there is a way to turn off local echo of MIDI data, but due 
to the long distance nature of this "jam", I can not read the manual 
to find out what to do.

Any help would be greatly appreciated

Tom N