To my knowledge, there is no official protocoll documentation published by the manufacturer. So the protocol and data formats described below were reverse engineered. By sniffing the communication between the instrument and the original vendor software.
If you are interested in the revering process you can read about it in my blog (sorry, German only).
The instrument speaks USB-HID via the integrated CP2110 USB UART bridge. Commands and data records are binary.
The instrument continuously streams measurement data at a rate of about 3 readings per second. This happens automatically – no specific polling command is required.
The following commands (given in hex) are supported and do exactly what the corresponding press of a physical button would do:
abcd04460001c2 # Hold
abcd04470001c3 # Back light
abcd04480001c4 # Select
abcd04490001c5 # Manual Range
abcd044a0001c6 # Auto Range
abcd044b0001c7 # Min/Max
abcd044c0001c8 # Exit Min/max
abcd044d0001c9 # Rel
abcd044e0001ca # D Value
abcd044f0001cb # Q Value
abcd04500001cc # Exit DQR
abcd04510001cd # R value
abcd04580001d4 # request device-ID
The original vendor software (Windows only) always sends some kind of
confirmation package after each command package (abcd045a0001d6). However, it
is unclear what this really does because all command packages I tested seem to
work fine without the extra package.