Hi Kirill,
I tried your project on my HP3478A and it doesn't do anything when I press SRQ. Can you check the fuses configuration on the Atmega? I've checked the connections twice.
Thanks,
Miti
Edit: I'm not using the USB to serial adapter. I understand that part is optional if you want to send commands to the meter, or am I missing something? Should it work only with the mini pro?