Waking up a serial port

While looking around inside a 12 volt to 120 volt inverter charger
I noticed a pin header labeled "5v Rx Tx Gnd" near the edge of the
main circuit board. This piqued my curiosity, so I connect a 'scope
to the Tx pin and Gnd, then tried power-cycling the inverter a few
times. Several power cycles later I could find no hint of a signal
anywhere near the usual 9600 baud rate that I believed customary.

Are there any customary tricks to making a diagnostic serial port
active? Most of the silkscreen is in English, so there might be
an enable/disable jumper position if I could recognize it.

The manufacturer is actively unhelpful on sincere questions, so
this kind of inquiry won't be taken kindly. Some sources indicate
the inverter brand (ampinvt) is a knockoff of a Sigineer design.
I was unable to locate technical information of either of them. 

The inverter-charger works acceptably but sometimes behaves oddly,
I'd like to explore the control parameters if it's possible.

Thanks for reading, and any ideas.

bob prohaska