Just a quick update for my small part of this thread: I got the UPS working fine after I took the time to re-read
the single source of truth which says to use the 940-0024c cable with the APC SC1500 UPS. FreeNAS connected to the UPS straightaway and power-cut shutdown is working fine.
However, this little part of my FreeNAS journey has shown me that the SC1500 is not a suitable UPS for this NAS. The redundant 620w PSUs with 24 disk load will tax the UPS beyond its spec and in the event of excessive current draw, the SC units cut power. I wish I could find the article about redundant PSUs ans UPS compatibility, but I cannot.
The SC1500 is also not a pure sine wave UPS either.
So I will be buying a nice refurbed 3U SmartUPS 3000VA UPS instead. And use SNMP :)
Summary: for those with a serial-connected APC UPS, use the official 940-0024c cable.
