f both networks are on the same PC then make sure you are using the provided USB extension cable with the U10.
If the networks are on different PCs then most likely the problem is due to OpenLDV (the low level driver) was installed while the U10 was plugged in. If you install OpenLDV while the U10 is plugged in then OpenLDV will install but the driver may not fully work. Perform the following workaround to fix this issue. OpenLDV is installed either by itself or installed automatically when installing a LNS or OpenLDV application (e.g., LonMaker).
Problem: The U10/U20 (or PCC-10 and PCLTA) network interface must be un-pluged from the PC when installing OpenLDV.
Workaround: un-plug the U10 on the PC, un-install OpenLDV (from the Windows Control Panel), re-install OpenLDV, reboot your PC, plug-in the U10 and try your test again.