Psychtoolbox and MEG

Greetings,
I have been having difficulties receiving triggers from an MEG system
using psychserial commands and a CMU button box connected to the serial
port. Triggers get sent from the MEG system when the subject
responds by pressing a button. Sending triggers using psychserial
commands works great. When the trigger from the MEG system is sent,
psychserial reads "something" but it is not consistent what it reads even
if the same button is pressed. Therefore, I do not know which button the
subject has pressed. Knowing this is critical for the timing of my
paradigm. Has anyone else had similar troubles and been successful in
resolving them?

Your help is appreciated.

Thanks,
Dan Wehner