Avast free and conflict with program

Avast Free Antivirus / Premium Security
1

Windows 10 : I have a BT90 EPC mulitmeter which connects via USB Prolific serial driver PL2303 and PC-Link program in Win 10.
With no third party antivirus software installed, the PC-link program communicates with the multimeter.

Once Avast free ( 2024) is installed, communication fails.

2

It would probably be considered an untrusted network connection being blocked either by the Network Inspector or Firewall components (settings).

Check your settings there and see if you can’t assign it as a trusted network. Obviously you would have to be certain it can be trusted.