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APC UPSes have a port on the back for UPS monitoring (serial, USB, or RJ45). Before you can use these ports, there is a variety of things to be aware of – things that are not all in one place.

Firstly, the USB port is not a standard USB port and should not be used as such; it requires a special cable. If you use the USB port on an APC UPS, then you will have to cycle the power on the unit to start using the serial port instead.

Secondly, the serial port also requires a special cable (the example here uses the SmartUPS 2200XL). There are two types of cables: simple signaling and smart signaling. Simple signaling cables (usually gray) work on all APC UPSes, but BackUPS units only support simple signaling. The simple signaling cables provide very few capabilities (only three settings: Battery On, Battery Low, and UPS Off) – all based on measuring pin levels on the serial cable itself. Smart signaling cables (normally black) are used for more powerful units (such as SmartUPS and others). The smart signaling cable connects the monitoring system to the UPS and provides a character interface to the unit with a large number of commands.

The Linux UPS monitoring tool nut supports smart signaling cables; you should use smart signaling cables whenever you can because of the added capabilities they allow.