Solution:

On the QSU-200/300 you have to externally jump the pins on the DB9 port. Pin 2 to Pin 4 = positive Pin 7 to Pin 8 = negative Pin 3 = ground On the QSU2-400 you do not have to jump the pins but you have to set it for 2 wire in Device manager. Pin 3 = positive Pin 4 = negative Ping 5 = ground Refer to Advanced Options using Device Manager (page 15 on link below) for details on software-selectable advanced options for setting the unit for 2 wire. http://www.bb-elec.com/Products/Manuals/USB2-User-Manual.pdf