VUSB / +5V selector to safely power KNX project
From the datasheet, there is no risk to the TP-UART if you power through the USB when there's no external power (I assume KNX is P1). Relevant information is in Section 1.1.1:

This tells you that there is no damage if \$V_\mathrm{VCC} &gt; V_\mathrm{VSP}\$ and when \$V_\mathrm{IF} \geq V_\mathrm{VCC}\$ respectively. These are the conditions that would be met if you were using \$V_\mathrm{USB}\$ to power the device.