.TH POWERD 8 "Feb 14, 1994" "" "Linux System Administrator's Manual"
.\" powerd \(em monitor a serial line connected to an UPS.
powerd -- monitor a serial line connected to an UPS.
.RB " serial-device "
is a daemon process that sits in the background and monitors the state
of the DCD line of the serial device. This line is meant to be
connected to a UPS (Uninterruptible Power Supply) so that \fBpowerd\fP knows
about the state of the UPS. As soon as \fBpowerd\fP senses that the
power is failing (it sees that DCD goes low) it notifies \fBinit\fP(8),
and \fBinit\fP then executes the \fBpowerwait\fP and \fBpowerfail\fP entries.
If \fBpowerd\fP senses that the power has been restored, it notifies \fBinit\fP
again and \fBinit\fP will execute the \fBpowerokwait\fP entries.
Some serial port that is not being used by some other device, and does not
share an interrupt with any other serial port.
\fBPowerd\fP regularly checks the \fBDSR\fP line to see if it's high.
\fBDSR\fP should be directly connected to \fBDTR\fP and \fBpowerd\fP
keeps that line high, so if \fBDSR\fP is low then something is wrong
with the connection. \fBPowerd\fP will notify you about this fact every
two minutes. When it sees that the connection has been restored it
will say so.
It's pretty simple to connect your UPS to the Linux machine. The steps
Make sure you have an UPS with a simple relay output: it should
close its connections (make) if the power is gone, and it should
open its connections (break) if the power is good.
Buy a serial plug. Connect the DTR line to the DSR line directly.
Connect the DTR line and the DCD line with a \fB10 kilo ohm\fP
resistor. Now connect the relay output of the UPS to GROUND
and the DCD line. If you don't know what pins DSR, DTR, DCD and
GROUND are you can always ask at the store where you bought the plug.
You're all set.
Well, not a real bug but \fBpowerd\fP should be able to do a broadcast or
something on the ethernet in case more Linux-boxes are connected to
the same UPS and only one of them is connected to the UPS status line.
.SH SEE ALSO
.BR shutdown (8),
.BR init (8),
.BR inittab (5)
Miquel van Smoorenburg, email@example.com