My Galaxy Ace doesn't get recognised by my HP laptop with USB connection. When I tried to connect it with my friend's cable on his Dell laptop, it notified CONNECTION UNSUCCESSFUL - NO DRIVER FOUND. My phone still gets charged though. When I tried the *#7284# solution it wrote:

DIAG CONFIG
[1]USB[*]
[2]UART[]
[3]DBG MSG ON[]
[4]DBG MSG OFF[*]

I selected [1]USB[*] and it gave me:

USB_Diag Selected
Change Complete.
IF you change Uart or USB diag
Please reboot the mobile.

I'm not able to find MODEM->PDA mode.