Monitoring Your UPS With apcupsd

Computers, like any other kind of electronic device, are very sensitive to utility power quality. According to American Power Conversion definitions (see Table 1) power anomalies can be classified as sags (better known as brownouts), blackouts, spikes, surges and noise. These power events may damage your electronic equipment, or cause loss of data if allowed to pass through the power supply to sensitive devices.

Table 1. Power Anomalies

The solution for all or part of these power anomalies is to connect an Uninterruptible Power Supply (UPS) to sensitive devices. Key to the UPS' function is a battery, used like a buffer, where power is accumulated when the utility power is normal, and released during low-voltage or blackout events. For high-voltage events, the UPS is usually equipped with filtering electronics that are capable of reducing the voltage. UPS output power is guaranteed to be sinusoidal alternating current kept within utility power specifications.

While a UPS alone can solve the immediate problem of utility power anomalies, when a blackout occurs the UPS' battery is used continuously, and its discharge time is faster in proportion to the load applied on the UPS output. Obviously, if a blackout is long enough, the battery charge will eventually be completely exhausted and cease to deliver power to the connected devices.

If a computer is connected to an exhausted UPS, it will suffer a system crash due to lack of power, as if the utility power suffered a blackout event. To overcome this problem, most UPSes on the market today have a built-in interface to communicate their status and to receive commands from a computer. This interface has traditionally been a serial port, but UPSes are now being shipped with USB interfaces.

Because this article is about apcupsd, we will focus on American Power Conversion (APC) brand UPSes. UPSes from other manufacturers share the same general behavior but, to date, apcupsd does not communicate with non-APC UPSes.

A UPS protocol must accomplish two main tasks. The first and most important task is to asynchronously notify when power events happen. Second, and perhaps less important, is to allow the computer to query the UPS for status information.

Table 2 lists the UPS events that are sent by APC UPSes to the computer through the serial line. Looking at this table, we find that some of these events are paramount for computer integrity during utility power anomalies.

Table 2. UPS Events

Now let's see the most important events at work. In Figure 1, the state machine of a UPS is described in a simplified form. When the utility power is present, the UPS monitors it for failures. If a failure is detected, the UPS sends a Line Fail alert to the computer and switches itself to battery power. At this time, the computer is powered by the UPS' battery. The UPS starts monitoring for battery power failures. If the utility power returns, the UPS sends a Return from Line Fail alert and switches back to utility power. If the battery power goes down, below a security level, the UPS sends an About to Shutdown alert and, after a defined amount of time, switches itself off.

Figure 1. UPS State Machine

On the computer side, the alerts sent by the UPS are very important because they can be used to decide the actions to be executed during utility power anomalies. When the UPS switches to batteries, it could be wise to deny user logins. When the UPS is about to switch off, it is very important to shut down the computer operating system. The UPS will usually switch off after a defined delay that in some cases can even be configured, and this delay is usually long enough to allow the computer to shut down itself.

It is clear now why having an UPS does not mean, by itself, that our computer is safe. To make it really safe, the computer must constantly monitor the UPS and must be able to take actions in response to UPS alerts.

APC, in its catalog, offers a number of different monitoring software packages for its UPSes. Unfortunately, at the time we needed it in 1996 and 1997, no APC software for Linux was available. Nowadays there are at least four Linux products that can be downloaded from the APC ftp server including the one described here, apcupsd.