3. Smart and dumb UPSs.

UPSs fall into two categories, which I'll call ``smart'' and ``dumb''. The difference between the two is the amount of information one can get from the UPS and the amount of control one can exert over the UPS.

Dumb UPS

• Connects to the computer via serial port.
• Uses modem control lines to communicate with the computer.
• Can signal whether or not the power is out.
• Typically can signal whether or not the battery is low.
• The computer can usually signal the UPS to turn itself off.

Smart UPS

• Connects to the computer via serial port.
• Communicates with the computer via normal data transfer through the serial port.
• Typically has some sort of command language that the computer can use to get various pieces of information from the UPS, to set various operating parameters for the UPS, and to control the UPS (such as turning it off).

Usually smart UPSs can be operated in dumb mode. This is useful because as far as I know, the company which manufactures the most popular smart UPS (namely APC) will only disclose the communication protocol for their UPSs to people who sign a non-disclosure agreement.

As far as I know, the only smart UPS available which is easy to communicate with under Linux are those made by Best. Furthermore, BEST fully documents the smart mode (and the dumb mode) of their UPSs. BEST also supplies source code for programs which can communicate with their UPSs.

All the packages listed in section Software will communicate with a UPS in dumb mode. This is all you really need. The ones specifically for the APC UPSs make various claims as to being usable in smart mode, but I don't know exactly what they permit. A full implementation would give you a pop-up window with all sorts of fun gauges displaying various statistics for the UPS, such as load, internal temperature, fault history, input voltage, output voltage, etc. It seems like the smupsd-0.9-1.i386.rpm package (section Software) approaches this. I'm not sure about the others.

The rest of this document is pretty much confined to configuring your system to work with a dumb UPS. The general idea is about the same with a smart UPS, but the details of how powerd would need to work and what kind of cable you need are different for a smart UPS.