"No UPS Found" Smart-UPS

[Anonymous user]

SURTD5000XLI.
Originally shipped with AP9619 NMC. 

With no NMC installed, I can access the UPS console for configuration via its RJ50 serial port.

With the NMC installed, this RJ50 appears to be redirected to the AP9619's console port, presumably while the AP9619 now communicates with the UPS console.

The NMC reports "No UPS found", despite latest (v3.x.x) sumx firmware installed. 

Installed an AP9631 NMC. "No UPS found" - latest (v6.x.x) sumx firmware installed.

With AP9631 installed, UPS RJ50 console port ceases to function - must connect to 2.5mm console port on NMC. I presume this is normal.

Both NMCs have been tested in my SMT3000 with no issue.

I presume this is a fault with the transistor array near the optocouplers on the main control board, based on my total guess that these are responsible for redirecting the console lines when an NMC is installed.

Given the age of this unit, is there any possibility I might be able to get a schematic/service guide for the main control board?

I understand these are "not user-serviceable" but this thing is well out of warranty and I am in the electronics engineering field, so it would be nice to find the faulty component and replace it rather than trying to source a replacement control board.

On a side note: I notice the firmware on these SURTD "Legacy" units is considered to be not field-servicable, but they do support XMODEM for transfer of new firmware.

Is XMODEM not considered something that can be done in the field?

Any help appreciated!