APC UPS control system - Manual (Page 144)

Note 2: the same as note 1 except that the line is normally closed,
and opens when the line voltage fails.
The Back-UPS Office 500 signals
The Back-UPS Office UPS has a telephone type jack as output, which looks
like the following:
Looking at the end of the connector:
6 5 4 3 2 1
_____________
| . . . . . . |
|
|
| |----------|
|__|
It appears that the signals work as follows:
UPS Signal meaning
1 (brown) <- Shutdown when set by computer for 1-5 seconds.
2 (black) -> On battery power
3 (blue) -> Low battery
4 (red) Signal ground
5 (yellow) <- Begin signalling on other pins
6 (none) none
Analyses of APC Cables
940-0020B Cable Wiring:
This diagram is for informational purposes and is not complete. Although
we do not know what the black box semi-conductor contains, we believe that
we understand its operation (many thanks to Lazar M. Fleysher for working
this out).
This cable can only be used on voltage-signalling UPSes, and provides the
On Battery signal as well as kill UPS power. Most recent evidence (Lazar’s
143