Voltcraft 4080
The Voltcraft Plus 4080 LCR Meter Dual Display comes with an optically coupled serial interface.
Protocol
You can access the device e.g. with cutecom. The protocol is basically human readable.
The description of the commands you find at Conrad.
When the device does not respond it might be in setup mode. You can see that in the display. But the software can not. So it might be a good idea to do an initialization with [BXXXXXX] to stop the setup mode.
Some findings not mentioned in the protocol definition above; without waranty:
disabling keys
sending
A
responds with
REMOTE READY..
Keys on device are disabled
enabling keys
sending
B
responds with
REMOTE STOP...
Keys on device are enabled
Images
Device complete with rubber bumper and optical/serial cable:
Screenshot of a short terminal session using CuteCom: