restart when i connect to Diecimila

hi i bought this Diecimila board and it is running whith an external power supply. its running a program and i clock timer on it, and some times i have to connect to it with my pc. but when i open the "com" (USB) port it resets. why does it do that and is there anything i mayby can do?
thanks, Peter

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1204641836

:slight_smile: