Hi Dennis, Thank you for your reply. I developed the control program with VC++. I want the talker/listener to run on the same pc with the control program, so it can intercept the GPIB commands sent to its virtual GPIB address by the control program, do some special functions and send the result back to the control program. The control program will be finally replaced by a commercial software, that's why I cannot add these special functions to the control program directly.