@madires:
Okay, it is not too much communication happening between those two components.
U4, Pin 7 ---> ATmega PD1 / TXD0
U4, Pin 8 ---> ATmega PD2 / RXD1
I attached the communication i sniffed using one of those Saleae clones.
The comm. at a normal starup is built up of three parts:
1) The first one is on PD2 and is somehow optional, it sometimes is there and sometimes is not - don't know why
2) The second one is the one which was already observed in
https://www.eevblog.com/forum/testgear/$20-lcr-esr-transistor-checker-project/msg1553443/#msg1553443 (first on PD1 then PD2), it's exactly the same here.
3) As soon as the detection is finished, there is a last bit at PD1
If you long press the button to shutdown the tester, then there is again some sort of comm. on PD1.
I attached the sniffed data in Salea's format, as csv and pictures.