- UNI-T UT61B/C/D: Press the "REL/RS232/USB" button for roughly 1 second.
- UNI-T UT71x: Press the "SEND/-/MAXMIN" button for roughly 1 second.
Briefly pressing the "EXIT" button leaves this mode again.
+ - UNI-T UT181A: In the "SETUP" menu set "Communication" to "ON".
- UNI-T UT325: Briefly press the "SEND" button (as per manual). However, it
appears that in practice you don't have to press the button (at least on
some versions of the device), simply connect the device via USB.