Re: The big red button
APC can have an optional module that isn't RS-232 but is 9-pin d-sub. Typically used by IBM AS400s which should therefore be the *only* thing plugged in to the UPS, as the design spec is that the system monitors UPS battery level until a certain level is reached then shuts down the server *and the UPS* so the batteries will recharge faster when the power is restored. It was a design that worked well when the AS400s had their own internal UPSes but didn't translate to the bigger world of "other devices".
For the terminally curious: https://www.apc.com/bm/en/faqs/FA159551/