I would venture to assume that the signal ground would be at the same same ground potential as the shield, mine was not.
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Flexing my MS paint skills here, I tried to capture the DB9 connector terminal, I was able to read around 1.5VAC to ground at the RS232 DB9 connector.
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This is specifically drawn to show case grounding only, the conduits that I have between my inverters, cable gutter and battery rack and sub panel are all PVC/ PVC flex. The inverters have their #6 THHN wiring to ground that I am not showing on purpose. Curious what others are measuring on their DB9 connectors and if jumpering them to ground stops the power cycling. This might have nothing to do with anything, just ruling things out at this point.