good morning,
in this case i guess my experience could help to avoid damage &:
for controlling a model-railway-switch i wired Ardi-Nano with L293D (motor-controlling similar to a h-bridge),
not beeing aware that the bridge could be defect (delivered just before)
as the Input pin of the L293D had motor-voltage (15V instead of a low level
for input) the connected pin at Ardi got this high level and Ardi seems to be damaged &: no uploading possible and recommended loopback-test fails.
the additional delivered items of the bridge seem ok after testing and can control the switch.
so I learned to check delivered items on breadboard before using them 
guess to wire diodes between ardi and additional items or use optic-coupling 