Solution for S8500, USB Interface not found when phone in Download Mode
I’m stuck in the same situation for couple hour and finally successes.
Here is solution:
1. Power on phone.
2. Type *#197328640# -> common -> Service configuration -> Data Service config -> USB ACM AP
3. Connect USB cable, setup drivers.
4. On the phone select USB debugging.
5. Read info.
6. Select flash file.
7. Update firmware.
The phone will go in to download mode automatically.