I am interested in the OEM version for field testing because it has flat surfaces for mounting on my equipment.
I can program it before visiting sites so I would like a simple on/off button to run the programs.
Can you explain how to start and stop the OEM version, it is not obvious from the literature.

No problem. You can Start/Stop the unit with a simple Push Button or compatible TTL Gate.
The 2 needed control pins are available on the top of the unit from the 2x3 Pin Connector (2,54mm Pitch).