Post by Harvey J. Stei » Sun, 30 Apr 1995 04:00:00

Archive-name: linux/howto/ups
Last-modified: 13 Apr 95

   By Harvey J. Stein (
   Copyright (c) 1994 by Harvey J. Stein.  You may use this document
   as you see fit, as long as it remains intact.  In particular, this
   notice (along with the contributions below) must remain untouched.

With contributions by (in order of appearance):
   Miquel van Smoorenburg (
   Danny ter Haar (
   Hennus Bergman (
   Tom Webster (
   Marek Michalkiewicz (
   Christian G. Holtje (
   Ben Galliart (
   Lam Dang (

Version 1.2 (last edited Sun Dec 11 1994)


         0. The usual nonsense
         1. Introduction
         2. What you need to do (summary)
         3. How it's supposed to work
         4. Where to get the appropriate software
         5. How to set things up
         6. User Enhancements
         7. How to make a cable
         8. Serial port pin assignments
         9. Ioctl bit numbers corresponding to RS232 control lines
         10. Info on selected UPSs
         10.0. General Experiences
         10.1. Advice 1200 A
         10.2. GPS
         10.3. TrippLite BC750LAN (Standby UPS)
         10.4. APC Backup-UPS
         10.4.1. A message of caution
         10.4.2. BUPS-HOWTO
         10.5. APC Smart-UPS, Model 600
         11. Reverse-engineering cables & hacking powerd.c
         12. How to shutdown other machines on the same UPS
         12.1. UPS status port method
         12.2. Broadcast method
         12.3. Dummy login method

0. The usual nonsense

I really can't guarantee that any of this will work for you.
Connecting a UPS to a computer can be a tricky business.  One or the
other or both might burn out, blow up, catch fire, or start World War
Three.  Furthermore, I only have direct experience with the Advice
1200 A UPS, and I didn't have to make a cable.  So, BE CAREFUL.

On the other hand, I managed to get everything working with my UPSes,
without much information from the manufacturer, and without burning
out anything, so it is possible.

1. Introduction

This HOWTO covers connecting a UPS to a PC running Linux in such a way
that Linux can shutdown cleanly when the power goes out.  To a large
extent it is reduntant, because all the basic info is contained in the
powerd man page that comes with the SysVinit package.  None the less,
there seems to periodically be alot of discussion on the net regarding
connecting Linux PCs to UPSs, (and the versions of Linux that I
installed didn't come with a powerd man page).  I figured having a
HOWTO would be a good idea because:

   -A second source of information might help to understand how
    to connect Linux to a UPS, even if it's just the same information
    written differently.
   -The HOWTO can serve as a repository for UPS specific data.
   -The HOWTO contains additional details that aren't in the powerd
    man page.

None the less, this does not replace the powerd man page.  Hopefully,
after reading both, people will be able to deal with UPSs.

2. What you need to do (summary)

  -Plug the PC into the UPS.
  -Connect the PC's serial port to the UPS with a special cable.
  -Run powerd on the PC.
  -Setup your initd to do something reasonable on powerfail & powerok
   events (like start a shutdown & kill any currently running
   shutdowns, respectively, for example).

3. How it's supposed to work

   UPS's job:
      When the power goes out, the UPS continues to power the PC &
      signals that the power went out by throwing a relay or turning
      on an opticoupler on it's control port.

   Cable's job:
      The cable is designed so that when the UPS throws said relay,
      this causes a particular serial port control line (typically
      DCD) to go high.

   Powerd's job:
      Powerd monitors the serial port.  Keeps raised/lowered
      whatever serial port control lines the UPS needs to have
      raised/lowered (typically, DTR must be kept high & whatever line
      shuts off the UPS must be kept low).  When powerd sees the UPS control
      line go high, it writes "FAIL" to /etc/powerfail & sends the
      initd process a SIGPWR signal.  When the control line goes low
      again, it writes "OK" to /etc/powerfail & sends initd a SIGPWR

   Initd's job (aside from everything else it does):
      When it receives a SIGPWR, it looks at /etc/powerfail.  If it
      contains "FAIL" it runs the powerfail entry from /etc/inittab.
      If it contains "OK" it runs the powerokwait entry from inittab.

4. Where to get the appropriate software

Pick up /pub/Linux/system/Daemons/SysVinit-2.50.tgz from or a mirror.  It includes a copy of powerd.c,
shutdown.c, an initd that understands what to do with SIGPWR, & can
handle powerfail & powerokwait entries in the inittab file.

5. How to set things up

   -Edit /etc/inittab.  Put in something like this:

# What to do when power fails (Halt system & drain battery :):
pf::powerfail:/etc/powerfailscript +5

# If power is back before shutdown, cancel the running shutdown.

   -Write scripts /etc/powerfailscript & /etc/powerokscript to
    shutdown in 5 minutes (or whatever's appropriate) & kill any
    existing shutdown, respectively.  Depending on the version of
    shutdown that you're using, this will be either so trivial that
    you'll dispense with the scripts, or be a 1 line bash script,
    something along the lines of:

       kill `ps -aux | grep "shutdown" | grep -v grep | awk '{print $2}'`

    and you'll keep the scripts.

   -Tell initd to re-process the inittab file with the command:

       telinit q

   -Edit rc.local so that powerd gets run upon startup.  The syntax
         powerd <line>

    Replace <line> with the serial port that the modem is connected,
    such as /dev/cua1.

   -Connect PC's serial port to UPS's serial port.  DO NOT PLUG PC

   -Plug a light into the UPS.

   -Turn on the UPS & the light.

   -Run powerd.

   -Test the setup:
      -Yank the UPS's plug.
         -Check that the light stays on.
         -Check that /etc/powerfailscript is running.
         -Check that shutdown is running.
      -Plug the UPS back in.
         -Check that the light stays on.
         -Check that /etc/powerfailscript is no longer running.
         -Check that shutdown is no longer running.
      -Yank the UPS's plug again.  Leave it out & make sure that the
       PC shuts down properly in the proper amount of time.

      -After everything seems to be proper, powerdown the PC & plug it
       into the UPS.  Run a script that sync's the hard disk every
       second or so.  Simultaneously run a second script that keeps
       doing a find over your entire hard disk.  The first is to make
       this a little safer & the second is to help draw lots of power.
       Now, pull the plug on the UPS, check again that shutdown is
       running & wait.  Make sure that the PC shuts down cleanly
       before the battery on the UPS gives out.

Congratulations!  You now have a Linux PC that's protected by a UPS
and will shutdown cleanly when the power goes out!

6. User Enhancements

   -Hack powerd.c to monitor the line indicating that the batteries
    are low.  When the batteries get low, do an *immediate* shutdown.
   -Modify shutdown procedure so that if it's shutting down in a
    powerfail situation, then it turns off the UPS after doing
    everything necessary.

7. How to make a cable

This section is just from messages I've seen on the net.  I haven't
done it so I can't write from experience.  If anyone has, please write
this section for me :).  See also the message about the GPS1000
contained in section 10.2.

   >From Wed Jul 21 14:26:33 1993
   Newsgroups: comp.os.linux
   Subject: Re: UPS interface for Linux?
   From: (Miquel van Smoorenburg)
   Date: Sat, 17 Jul 93 18:03:37
   Distribution: world
   Organization: Cistron Electronics.

   In article <1993Jul15.184450.5...@excaliber.uucp> (Joel M. Hoffman) writes:
   >I'm in the process of buying a UPS (Uninteruptable Power Supply), and
   >notice that some of them have interfaces for LAN's to signal the LAN
   >when the power fails.
   >Is there such an interface for Linux?

   When I worked on the last versioon of SysVinit (Now version 2.4),
   I temporarily had a UPS on my computer, so I added support for it.
   You might have seen that in the latest <signal.h> header files there
   is a #define SIGPWR 30 now :-). Anyway, I did not have such a special
   interface but the output of most UPS's is just a relais that makes or breaks
   on power interrupt. I thought up a simple way to connect this to the
   DCD line of the serial port. In the SysVinit package there is a daemon
   called 'powerd' that keeps an eye on that serial line and sends SIGPWR
   to init when the status changes, so that init can do something (such as
   bringing the system down within 5 minutes). How to connect the UPS to
   the serial line is described in the source "powerd.c", but I will
   draw it here for explanation:

                        +------------------------o  DTR
                      |   | resistor
                      |   | 10 kilo-Ohm
                      |   |
                      +---+                                To serial port.
          +-----o-------+------------------------o  DCD
          |             |
          o  UPS        |
        \    relais     |
         \              |
          |             |
          +-----o-------+------------------------o  GND

   Nice drawing eh?

   Hope this helps.
   SysVinit can be found on sunsite (and tsx-11 probably) as



   Miquel van Smoorenburg, <> cannot open CONFIG.SYS: file handle broke off.

   >From Wed Jul 21 14:27:04 1993
   Newsgroups: comp.os.linux
   Subject: Re: UPS interface for Linux?
   From: (Danny ter Haar)
   Date: Mon, 19 Jul 93 11:02:14
   Distribution: world
   Organization: Cistron Electronics.

   In article <> (Miquel van Smoorenburg) writes:
   >How to connect the UPS to the serial line is described in the source
   >"powerd.c", but I will draw it here for explanation:

   The drawing wasn't really clear, please use this one in stead !
   >                     +------------------------o  DTR
   >                     |
   >                   +---+
   >                   |   | resistor
   >                   |   | 10 kilo-Ohm
   >                   |   |
   >                   +---+                                To serial port.
   >                     |
   >       +-----o-------+------------------------o  DCD
   >       |
   >       o  UPS
   >     \    relais
   >      \
   >       |
   >       +-----o--------------------------------o  GND

   The DTR is kept high, when the UPS's power input is gone it
   will close the relais . The computer is monitoring
   the DCD input port to go LOW . When this happens it will start a
   shutdown sequence...


   Danny ter Haar  <> or <>
   Robins law #103: 'a couple of lightyears can't part good friends'

8. Serial port pin assignments

(The following is from David Tal's <GSRG...@TECHNION.BITNET>
'Frequently Used Cables and Connectors' document).

   Pin Assignment for the Serial Port (RS-232C), 25-pin and 9-pin
   DB-25   DB-9
   Pin #   Pin #   Name    EIA     CCITT   DTE-DCE Description
   -----   -----   -----   -----   -----   ------- -------------------
    1              FG      AA       101    ----    Frame Ground/Chassis GND
    2      3       TD      BA       103    --->    Transmitted Data, TxD
    3      2       RD      BB       104    <---    Received Data, RxD
    4      7       RTS     CA       105    --->    Request To Send
    5      8       CTS     CB       106    <---    Clear To Send
    6      6       DSR     CC       107    <---    Data Set Ready
    7      5       SG      AB       102    ----    Signal Ground, GND
    8      1       DCD     CF       109    <---    Data Carrier Detect
    9              --      --        -       -     Positive DC test voltage
   10              --      --        -       -     Negative DC test voltage
   11              QM      --        -     <---    Equalizer mode
   12              SDCD    SCF      122    <---    Secondary Data Carrier Detect
   13              SCTS    SCB      121    <---    Secondary Clear To Send
   14              STD     SBA      118    --->    Secondary Transmitted Data
   15              TC      DB       114    <---    Transmitter (signal) Clock
   16              SRD     SBB      119    <---    Secondary Receiver Clock
   17              RC      DD       115    --->    Receiver (signal) Clock
   18              DCR     --        -     <---    Divided Clock Receiver
   19              SRTS    SCA      120    --->    Secondary Request To Send
   20      4       DTR     CD       108.2  --->    Data Terminal Ready
   21              SQ      CG       110    <---    Signal Quality Detect
   22      9       RI      CE       125    <---    Ring Indicator
   23              --      CH       111    --->    Data rate selector
   24              --      CI       112    <---    Data rate selector
   25              TC      DA       113    <---    Transmitted Clock

        1                         13         1         5
      _______________________________      _______________
      \  . . . . . . . . . . . . .  /      \  . . . . .  /    RS232-connectors
       \  . . . . . . . . . . . .  /        \  . . . .  /     seen from outside
        ---------------------------          -----------      of computer.
        14                      25            6       9

   DTE : Data Terminal Equipment (i.e. computer)
   DCE : Data Communications Equipment (i.e. modem)
   RxD : Data received; 1 is transmitted "low", 0 as "high"
   TxD : Data sent; 1 is transmitted "low", 0 as "high"
   DTR : DTE announces that it is powered up and ready to communicate
   DSR : DCE announces that it is ready to communicate; low=modem hangup
   RTS : DTE asks DCE for permission to send data
   CTS : DCE agrees on RTS
   RI  : DCE signals the DTE that an establishment of a connection is attempted
   DCD : DCE announces that a connection is established

9. Ioctl bit numbers corresponding to RS232 control lines
(taken from /usr/include/linux/termios.h)

/* modem lines */
#define TIOCM_LE        0x001
#define TIOCM_DTR       0x002
#define TIOCM_RTS       0x004
#define TIOCM_ST        0x008
#define TIOCM_SR        0x010
#define TIOCM_CTS       0x020
#define TIOCM_CAR       0x040
#define TIOCM_RNG       0x080
#define TIOCM_DSR       0x100
#define TIOCM_CD        TIOCM_CAR
#define TIOCM_RI        TIOCM_RNG

Note that the 3rd column is in Hex.

10. Info on selected UPSs

   **** Please send them to me for inclusion here. ****

10.0. General Experiences.

I've been saving peoples comments, but haven't gotten permission yet
to include them here.  Here's a general summary of what I've heard
from people.

   Won't release info on their "smart" mode without your signature on
   a non-disclosure agreement.  Thus, people are forced to run their
   "Smart" UPSes in the "dumb" mode as outlined above.

Tripp Lite:
   One person reported that Tripp lite won't release info either.

   One person reported that Upsonic has discussed technical details
   over the phone, answered questions via faz & are generally helpful.

10.1. Advice 1200 A

   UPS from Advice Electronics, Tel Aviv Israel (they stick their own
   name on the things).

   UPS Control Port

         2 - Power Fail.
         5 - Battery Low.
         6 - Shut Down UPS.
         4 - Common ground for pin 2, 5, 6.

   They also gave me the following picture which didn't help me, but
   may help you if you want to build a cable yourself:

         2 ----------+
                    \/     (<--- The "\/" here indicates the type of
                    |             this transister.  I forget what
                    |             denotes what, but this one points
                 +-----+          away from the center line.)
                /  /  /

         5 ----------+
                /  /  /

              10K    |/
         6 --\/\/\/--|
                   /  /  /

         4 ----------+
                 /  /  /

   Cable supplied
   They first game me a cable that was part of a DOS UPS control
   package called RUPS.  I used this for testing.  When I was
   satisfied, they gave me a cable they use for Netware servers
   connected to UPSs.  It functioned identically.  Here are the

      DTR - Powers cable (keep high).
      CTS - Power out (stays high & goes low when power goes out).
      DSR - Battery low (stays high & goes low when battery does).
      RTS - Turns off UPS (keep low & set high to turn off UPS).

   (The powerd.c that comes with SysVinit set or left RTS high,
   causing the UPS to shut off immediately when powerd was started

10.2. GPS1000 from ACCODATA

   >From Thu Mar 10 15:10:22 1994
   Subject: Re: auto-shutdown with UPS
   From: (Hennus Bergman)
   Date: Tue, 1 Mar 1994 22:17:45 GMT
   Distribution: world
   Organization: The Organization For Removal Of On-Screen Logos

   In article <>,
   Colin Owen Rafferty <> wrote:
   >I am about to buy an Uninterruptable Power Supply for my machine, and
   >I would like to get one that has the "auto-shutdown" feature.
   I just got one of those real cheap :-)
   It's a GPS1000 by ACCODATA. Anybody know how good the output
   signal of these things is? [Don't have a scope myself :-(]

   >I assume that these each have some kind of serial connection that
   >tells the system information about it.
   I took it apart to find out how it worked. There were three optocouplers
   (two output, one input) connected to a 9 pin connector at the back.
   One turns on when the power fails, and goes off again when the power
   returns. While the power is off, you can use the `input' to shut the
   battery off. [It releases the power-relay.] The third one is some kind
   of feedback to tell that it did accepted the `shut-down command'.
   I think the interface for my UPS was designed to be connected to TTL-level
   signals, but with some resistors it could be connected to serial port.
   It's wired in such a way that using a RS-232 port you cannot use both
   output optocouplers; but the shutdown feedback is not necessary anyway,
   just use the important one. ;-)
   [Note that it is possible to blow the transistor part in optocouplers
   with RS-232 levels if you wire it the wrong way round ;-)]

   I was hoping I would be able to connect it to my unused game port,
   but that doesn't have an output, does it?
   I'll probably end up getting an extra printer port for this.

   Not all UPS' use optocouplers, some use simple relays, which are
   less critical to connect, but of course not as `nice'.

   >Has anyone written a package that watches the UPS and does a shutdown
   >(or something) when the power is off?
   SysVinit-2.4 (and probably 2.5 as well) has a `powerd' daemon that
   continually watches a serial port for presence of the CD (Carrier
   Detect) line and signals init when it drops. Init then activates
   shutdown with a time delay. If the power returns within a few minutes
   the shutdown is cancelled. Very Nice.
   The only problem I had with it is that it doesn't actually tell the
   UPS to turn off when the shutdown is complete. It just sits there with
   a root prompt. I'll probably write a small program to shut it down
   >from /etc/brc. RSN.

   > Colin Rafferty, Lehman Brothers <>

   Hennus Bergman

10.3 TrippLite BC750LAN (Standby UPS)

>From Mon Aug  8 22:30:14 1994

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From: Tom Webster <>
To: hjst...@MATH.HUJI.AC.IL (Harvey J. Stein)
Subject: Re: Help - Powerd & UPS - Help
Date: Mon, 8 Aug 1994 12:26:09 -0700 (PDT)


First off, let me say that I enjoyed reading your HOWTO.  It is
about what I'd hoped my document might grow into.  I wrote my
pseudo-HOWTO late on night because I kept seeing the "Can I
hook a UPS up to Linux...." message, about once a month on
c.o.l.*.  Mine deals specifically with hooking one vendor's
model of UPS (Tripplite's BCxxx/LAN series) to a Linux box and
making powerd work with it.

It is in need of some upkeep, things have been a little hectic.  
Now that I have posted it three or four time in response to
questions, I'm finally getting some feedback (which catches
these errors).  The problems that I know it has so far are:

The proper version of SysVinit is 2.4, not 2.04.

There is some argument about whether one or more resistors are
needed in my cable.  The only place I really see that it might
be needed is in the inverter shutoff, to make sure that I don't
send too much voltage to the UPS.  For the sensor circuit I
don't see why a DTE device can't stand to have its signal
looped back to it.  All that I'm doing is connecting a line
that is held high to the Carrier Detect line.

I should be a little clearer about how the UPS acts when it
goes into powerfail mode.  The Tripplites provide both an open
and a closed circuit on powerfail, two different pins and a
common negative pin.  Thus all I have to do is wire a pin that
it held high to the carrier detect line and route this through
the UPS's open on powerfail circuit, to cause the carrier
detect to drop.  I think that this confuses some people who
read Miquel van Smoorenburg's description of a UPS that only
provides only a closed circuit on powerfail, and requires a
much more complex cable.

Well that is all I can think of for now.  I'm planning on
seeing if my cable will still operate if I insert resistors
into all of the circuits.  If it does then I'll make the
changes to my document, should find out this weekend.



Of Linux and Uninterruptible Power Supplies
[or How to connect a TrippLite BCXXXLAN UPS]

by Tom Webster <>
   05/20/94 (Version 1.0)

1.0 Introduction
I struggled through connecting a TrippLite BC750LAN (Standby UPS) to
my Linux box about six months ago.  Since then I've seen several
requests for information on this subject, so I'm putting it in a
relatively stable format so I can just send this out when the
question reappears.

1.1 The Results
When the power fails in my apartment, several things happen:

   1.  The UPS switches its inverter on and the computer starts
       drawing off of the UPS.  The warning beeper also starts going
       off.  If the power comes back on, the UPS shuts the inverter
       down and switches back to line power.  Nothing else happens,
       other than the beeper turns off.

   2.  If the power is off for ~15-30 seconds, the system will send
       a message to the users (via wall) and initiate a shutdown
       (to halt) in five minutes.  If the power comes back on, the
       shutdown is canceled and a message stating this is sent
       to the users (via wall) stating that the shutdown has been

   3.  While the system is shutting itself down, its dying act is to
       shut off the inverter on the UPS, killing power to the system.
       This is done after the disks are unmounted, and is done to
       prevent the halted system from draining power from the UPS.

   4.  When the line power comes back on, the system restarts normally.

The BC750 has enough juice to keep my system going for quite some time,
so why do I only run it off of the UPS for five minutes?  The answer
rests in a couple of 'rules-of-thumb' (your mileage may vary):

   a.  If the power browns out or blacks out, 90% of the time it will
       be out for 0-2 minutes.

   b.  If it is out for longer, it will be out for .5-3+ hours.

   c.  If it is out for a while (see b), the power will yo-yo at least
       once while the power company is working on it.  (The power
       will come up for 5-45 seconds, and then fail again.)

So, I set my system up to cover the majority of the power outages I
get, without trying to cover the really long ones.  I also keep plenty
of reserve in the UPS to handle yo-yo situations.

1.2  Disclaimer
I make no warranties or guarantees as to the suitability or sanity of
following my advice.  This is how my system is setup, and as far as
I can tell it works fine for me.  Your setup may need to be different
to fit your needs, especially if you are using different UPS hardware.

2.0  Hardware
In the case of my UPS, I thought that the RS-232 interface was something
of a misnomer.  I was expecting the UPS to send and receive data, like
talking to your modem with Hayes "AT" commands.  This was not the case.
It seems that it is called an RS-232 interface because it stays within
the voltage and signal limits of the RS-232  spec.  To communicate with
the UPS, you need to be able to sense changes in state on certain lines
and change the state of other lines.  The fact that these lines may have
nothing in common to the lines your system might expect to use, if it
were talking to say a modem or printer, is probably why the UPS
needs special cables to allow software (including the manufacturer's)
to communicate with the UPS.

Through trial and error with a RS-232 patch panel, I was able to come
up with this cable diagram for the cable between the UPS and the
computer.  Please note that I did this without looking at the official
TrippLite cable and it may be different.

          UPS                System
         DB-25               DB-25
           1 <-------------->  1       Ground

           2 <-------------->  4       Power Fail
           8 <-------------->  8       Sensing Circuit

           3 <-------------->  2       Inverter Shutdown
          20 <--------------> 22       Circuit

Once you have the cable patched together, just hook the UPS side to
the UPS and the System side to a free serial port on your Linux box.
You will probably have to mess around with 9->25 and 25->9 adapters
to get your cable to fit, but you and a good computer store should
be able to handle this.

3.0 Software
The software that I use is all available to Linux users and comes
with most distributions (SLS and Slackware at least).  This setup
has worked for me through Kernels .99.9, .99.14, and 1.00.

3.1 System V Init
This package is needed to make the whole thing work.  If you are
still using the "Simple Init" package, perhaps it is time you looked
at upgrading.  The version I am using is 2.04, and I believe that
Miquel van Smoorenburg is the author of the package.

3.2 powerd
powerd is the power daemon, by default is sits and watches for a
change in state on the DCD line and reports these changes to the
system via the signal mechanism.  The source for powerd is provided
in the System V Init package.  Compile it, move it to a binary
directory (I put it in /sbin on my system), and alter your rc.local
script to start the daemon.  The relevant part of my rc.local looks
like this:

    ----- snip -----
    # Add support for the UPS
    echo "Starting powerd daemon..."
    if [ -x /sbin/powerd ]; then
       /sbin/powerd /dev/cua4
    ----- snip -----

3.3 inittab
Your inittab needs to be modified to properly handle the signals
that powerd will send if there is a power failure.  The relevant
lines of my inittab look like this:

   ----- snip -----
   # What to do when power fails (shutdown to single user).
   pf::powerfail:/sbin/shutdown -f +5 "THE POWER IS FAILING"

   # If power is back before shutdown, cancel the running shutdown.
   pg:0123456:powerokwait:/sbin/shutdown -c "THE POWER IS BACK"

   # If power comes back in single user mode, return to multi user mode.
   ps:S:powerokwait:/sbin/init 2
   ----- snip -----

3.4 rc.0 (brc)
Depending on how your system is setup either rc.0 or the brc script
is executed immediately prior to shutdown.  These scripts take care
of things like unmounting disks and any other last minute clean-up.

The inverter shutdown circuit, is designed to signal an inverter
shutdown when data is sent out over the DTR line.  In my case,
I just cat a short file to the serial port (/etc/passwd - since I
know it will always be there).  My rc.0 is as follows, please note
that it is overly conservative, the sync can be removed and the
sleep times can probably be tightened, but it works so I haven't
messed with it.

  ----- snip -----
  #! /bin/sh
  # brc       This file is executed by init(8) when the system is being
  #           shutdown (i.e. set to run at level 0).  It usually takes
  #           care of un-mounting all unneeded file systems.
  # Version:  @(#)/etc/brc            2.01    02/17/93
  # Authors:  Miquel van Smoorenburg, <>
  #           Fred N. van Kempen, <>
  # Modified: 01/15/94 - Inverter shutdown support added.
  #           Tom Webster <>

  echo Unmounting file systems.....
  umount -a
  sleep 2
  cat /etc/passwd > /dev/cua4
  sleep 5
  echo Done.
  ----- snip -----

(On my UPS the inverter is only running when the line power is off,
so there is no harm in sending the shutdown signal at every shutdown.)  

4.0 Conclusion
Well that's how I hooked my TrippLite UPS up to my Linux box.
Feel free to drop me a line with the results of you attempts,
especially if you have any improvements.  :->

From: Tom Webster <>
To: hjst...@MATH.HUJI.AC.IL (Harvey J. Stein)
Subject: Re: Help - Powerd & UPS - Help
Date: Thu, 11 Aug 1994 12:20:50 -0700 (PDT)


> Like I tried to say, powerd can just run shutdown directly in the
> event of a low battery, so that init doesn't need to deal with it &
> doesn't need to be hacked - no new signal necessary.  Although this
> violates the nice separation of labor between powerd & init, it's
> easier than adding another signal.  Or do you just mean another
> command?  Something like having powerok/powerfail/powerfailnow as
> commands in inittab which execute when SIGPWR is received &
> /etc/powerfail contains OK/FAIL/LOWBATT (respectively).  This would be
> cleanest, but having powerd execute shutdown -r now is trivial to do
> now, and might as well be done - the logic will be the same regardless
> of the action that powerd takes when it senses a low battery.  For now
> it can just run shutdown, & when init gets hacked it can write LOWBATT
> to /etc/powerfail (or whatever the hell the file is called) & give
> init a SIGPWR.

I'd like to add the LOWBATT command, it would be the cleaner way to do
it.  I just need to take a look at the code and see how hard it would
be to add it.  Also need to look at my wiring, guess this may mean I've
got to run the system all the way down for a final test (once I think I
have it working.


From: Tom Webster <>
To: hjst...@MATH.HUJI.AC.IL (Harvey J. Stein)
Subject: Re: Help - Powerd & UPS - Help
Date: Mon, 15 Aug 1994 09:46:06 -0700 (PDT)


Well, I messed about all weekend taking readings with my multi-
tester and comparing it to the scant documentation that I have for
the UPS.  The only conclusions I came to were:

1.  My system has been working for about 8-9 months now.  If I was
going to fry anything, it should have happened by now.

2.  If my success is based on my serial hardware doing odd things
(I'm using an STB 4COM board), there is no way I'm going to be
able to find out on my system.  I'll have to leave that to other
poor souls on the net.

3.  As far as I can tell the only reason the 10kohm resistor was
in Miquel's diagram, was to keep the line higher than DCD, even
after the circuit had been shunted to ground.

4.  The only circuit that is expected to do anything other that be
switched by the UPS (the inverter shutdown circuit) has a 40kohm
resistor built into it (inside the UPS).  That should take care of
any worries there.

5.  Miquel's circuit will work for power fail sensing, and might
be extrapolated to include the low battery circuit.  If my circuit
proves unviable for others, it would just require more soldering
than I'd care to deal with in a cable.

In other related news, I broke down this morning and ordered a
cable from Tripplite.  I ordered the LanTastic/LAN Manager/Win NT
cable, it's just a cable (9M-9F, no software).  This will set me
back about $40 (the PC UNIX cable (w/ software) is about $140).  
The motivation for doing this was three part.  

(1)  Within a year I'll probably be running Win 4.0, or WinNT 3.5
(I beta'ed 3.1) and both should support UPS monitoring (I know NT
does and saw the power management icon on a Win4.0 desktop in one
of the computer mags).  

(2)  My current cable isn't going to be compatible with any other
monitoring software for other OS's (except by random chance),
working to a known (and presumably soon to be common?) cable via a
hacked powerd, should widen the audience.  

(3)  I can always reverse engineer the cable to see if Tripplite
is indeed building any safety into their cables.

It should be here in a week or so.  In the mean time, I'll start
looking into hacking powerd for LOWBATT.


10.4 APC Backup-UPS

There seems to be some controversy as to the accuracy of the
information here on APC Back-UPSes.  So, please be careful.  I'm
prefacing this section with one message of caution I received.  It
might not make alot of sense before the rest of this section is read,
but this way, at least you're more likely to see it.  And again, since
I don't have any APC UPS units, I can't verify the accruacy of either
of these messages.

10.4.1. A message of caution

>From Sun Oct  9 11:00:42 1994

Newsgroups: comp.os.linux.admin
Subject: BUPS-HOWTO warning
From: (Marek Michalkiewicz)
Date: 6 Oct 1994 18:38:15 GMT
Organization: Technical Univeristy of Wroclaw
X-Newsreader: TIN [version 1.2 PL2]

If you want to connect the APC Back-UPS to your Linux box, this might
be of interest to you.

There is a good BUPS-HOWTO which describes how to do this. But it has
one "bug".

The RTS serial port signal is used to shut down the UPS. The UPS will
shut down only if it operates from its battery. The manual says that
the shutdown signal must be high for at least 0.5s. But few milliseconds
is enough, at least for my APC Back-UPS 600.

Using RTS to shut down the UPS can be dangerous, because the RTS goes
high when the serial device is opened. The backupsd program then turns
RTS off, but it is on (high) for a moment. This kills the power when
backupsd is first started and there is a power failure at this time.
This can happen for example when the UPS is shut down, unattended,
and the power comes back for a while.

Either start backupsd before mounting any filesystems for read-write,
or (better) use TX (pin 3) instead of RTS (pin 7) to shut down the
UPS (pin numbers are for 9-pin plug). Use ioctl(fd, TCSBRKP, 10);
to make TX high for one second, for example. Using TX should be safe.
Maybe I will post the diffs if time permits...

-- Marek Michalkiewicz

10.4.2. BUPS-HOWTO

                      Luminated Software Group Presents

                         HOWTO use Back-UPS (by APC)
                     (to keep your linux box from frying)

                                Version: 1.01  BETA

             Document by: Christian G. Holtje <>
        Cabling info and help: Ben Galliart <>

        This document, under one condition, is placed in Public Domain. The
one condition is that credit is given where credit is due.  Modify this as
much as you want, just give some credit to us who worked.

        I, nor any of us who have written or helped with this document, make
and guarantees or claims for this text/source/hints.  If anything is damaged,
we take NO RESPONSIBILITY!  This works to the BEST OF OUR KNOWLEDGE, but
we may have made mistakes.  So be careful!

        Al right, you just bought (or are going to buy) a Back-UPS from APC.
(Other brands might be able to use this info, with little or no modification,
but we don't know)  You've looked at the price of the Power-Chute
software/cabling, and just are not sure it's worth the price.  Well, I made my
own cable, and my own software and am using it to automatically shut off the
power to my linux box when a power failure hits.  Guess what?  You can too!

*** The Cable ***

        This was the hardest part to figure out (I know little about hardware,
so Ben did the most work for this).  To build one, you need to buy from your
local radio shack (or other part supplier) this stuff:
        1 9-Position Male D-Subminature Connector (solder-type)
                [Radio Shack cat. no. 276-1537c]
        1 9-Position Female D-Subminature Connector (solder-type)
                [Radio Shack cat. no. 276-1538c]
        2 casings for the above plugs (usually sold separately)
        Some stranded wire (wire made of strands, not solid wire)

You also need, but may be able to borrow:
        1 soldering iron

Okay...this is how you connect it up!

These diagrams are looking into the REVERSE SIDE (the side where you solder
the wire onto the plugs)  The letters G, R, and B represent the colors of the
wires I used, and help to distinguish one line from the next.
(NOTE:  I'm use standard rs-232 (as near as we can tell) numbering.  The APC
book uses different numbers.  Ignore them!  Use ours...I already changed the
numbers for you!)

   ---------------------     Male Side! (This goes into the UPS)
    \  B   R  *  *  * /    
      \  *  *  *  G  /

   ---------------------     Female Side! (This goes into your COM port)
    \  R   *  *  *  G /
      \  *  B  *  *  /

For those who like the numbers better:
        Male            Female
        1               7               Black
        2               1               Red
        9               5               Green

---------Aside:  What the rs-232 pins are for!-----------
Since we had to dig this info up anyway:

>From the REAR (the soldering side) the pins are numbered so:

    \  1   2  3  4  5 /
      \  6  7  8  9  /

The pins mean:
        Number  Name                    Abbr. (Sometimes written with D prefix)
        1       Carrier Detect          CD
        2       Receive Data            RD
        3       Transmit Data           TD(?)
        4       Data Terminal Ready     DTR
        5       Signal Ground           Gnd
        6       Data Set Ready          DSR
        7       Request to Send         RTS(?)
        8       Clear to Send           CS
        9       Ring Indicator          RI

What we did is connect the UPS's RS-232 Line Fail Output to the CD, the UPS's
chassis to Gnd, and the UPS's RS-232 Shut Down Input to RTS.
Easy now that we told you, no?

I have no idea if the software below will work, if you purchase the cable
from APC.  It might, and it might not.

*** The Software ***

        Okay, I use the SysVInit package by Miquel van Smoorenburg for Linux.
(see end for file locations, credits, email addresses, etc.)  I don't know
what would have to be changed to use someone elses init, but I know this code
(following) will work with Miquel's stuff.
        Just so I give credit where credit's due.  I looked at Miquel's code
to figure out how ioctl()'s worked.  If I didn't have that example, I'd have
been in trouble.  I also used the powerfail() routine (verbatim, I think),
since it must interact with his init, I thought that he should know best.
        The .c file is at the end of this document, and just needs to be
clipped off.  To clip the file, edit away and extra '.sigs' and junk.  This
document should end on the line /* End of File */.....cut the rest.

        This program can either be run as a daemon to check the status of the
UPS and report it to init, or it can be run to send the kill-power command
to the UPS.  The power will only be killed if there is a power problem, and
the UPS is running off the battery.  Once the power is restored, it turns back

        To run as a daemon, just type:
                backupsd /dev/backups

/dev/backups is a link to /dev/cua0 at the moment (COM 1, for you DOSers).
The niceness of the link is that I can just re-link the device if I change
to com 2 or 3.

Then, if the power dies init will run the commands for the powerwait.
An example (This is from my /etc/inittab):

# Here are the actions for powerfailure.
pf::powerwait:/etc/rc.d/rc.power start
po::powerokwait:/etc/rc.d/rc.power stop

The powerwait will run, if the power goes down, and powerokwait will
run if the power comes back up.

Here is my entire rc.power:
#! /bin/sh
# rc.power      This file is executed by init when there is a powerfailure.
# Version:      @(#)/etc/rc.d/rc.power   1.50    1994-08-10
# Author:       Christian Holtje, <>

  # Set the path.

  # Find out how we were called.
  case "$1" in
                echo "Warning there is Power problems."        | wall
                # Save current Run Level
                ps | gawk '{ if (($5 == "init") && ($1 == "1")) print $6 }' \
                         | cut -f2 -d[ | cut -f1 -d] \
                         > /tmp/run.level.power
                /sbin/shutdown -h +1m
                echo "Power is back up.  Attempting to halt shutdown." | wall
                shutdown -c
                echo "Usage:  $0 [start|stop]"
                exit 1
#End of File

Pretty nifty, no?  Actually, there is a problem here...I haven't had
time to figure it out...If there is a 'sh' wizard out there....

There is one little detail left, that is having the UPS turn off the power if
it was halted with the power out.  This is accomplished by adding this line
into the end of your halt script:

  /sbin/backupsd /dev/backups killpower

This will only kill the power if there is no power being supplied to your

*** Testing the stuff ***

        This is just a short section saying this:

        BE CAREFUL!

        I recommend backing up your linux partitions, syncing several times
before testing and just being careful in general.  Of course, I'm just
recommending this.  I wasn't careful at all, and had to clean my partition
several times testing my config.  But it works.  :)

*** Where to Get It ***

        Miquel van Smoorenburg's SysVInit can be gotten at:

        and a fix for some bash shells is right next-door as:

        As to getting this HOWTO, you can email me.  with the subject saying 'request'
        and the keyword 'backups' in body of the letter.
        (I may automate this, and other stuff)

*** Credit Where Credit's Due Dept. ***

        Thanks to Miquel van Smoorenburg <>
for his wonderful SysVInit package and his powerd.c which helped me very much.

        Christian Holtje <>
                backupsd.c (what wasn't Miquel's)

        Ben Galliart <>
                The cable
                Information for the RS-232 standard
                Lousy Jokes (none quoted here)

------------------>8-------------CUT HERE--------8<---------------------------

/*  backupsd.c -- Simple Daemon to catch power failure signals from a
 *                Back-UPS (from APC).
 *  Parts of the code are from Miquel van Smoorenburg's powerd.c
 *  Other parts are original from Christian Holtje <>
 *  I believe that it is okay to say that this is Public Domain, just
 *  give credit, where credit is due.
 *  Disclaimer:  We make NO claims to this software, and take no
 *               resposibility for it's use/misuse.

#include <sys/types.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <signal.h>

/* This is the file needed by SysVInit */
#define PWRSTAT         "/etc/powerstatus"

void powerfail(int fail);

/* Main program. */
int main(int argc, char **argv)
  int fd;
  int killpwr_bit = TIOCM_RTS;
  int flags;
  int status, oldstat = -1;
  int count = 0;

  if (argc < 2) {
        fprintf(stderr, "Usage: %s <device> [killpower]\n", argv[0]);

  /* Open the the device */
  if ((fd = open(argv[1], O_RDWR | O_NDELAY)) < 0) {
        fprintf(stderr, "%s: %s: %s\n", argv[0], argv[1], sys_errlist[errno]);

  if ( argc >= 3  && (strcmp(argv[2], "killpower")==0) )
          /* Let's kill the power! */
          fprintf(stderr, "%s: Attempting to kill the power!\n",argv[0] );
          ioctl(fd, TIOCMBIS, &killpwr_bit);
          /* Hmmm..... If you have a power outtage, you won't make it! */
      /* Since we don't want to kill the power, clear the RTS. (killpwr_bit) */
      ioctl(fd, TIOCMBIC, &killpwr_bit);

/* Become a daemon. */
  switch(fork()) {
  case 0: /* I am the child. */
  case -1: /* Failed to become daemon. */
                fprintf(stderr, "%s: can't fork.\n", argv[0]);
  default: /* I am the parent. */

  /* Now sample the DCD line. */
  while(1) {
      ioctl(fd, TIOCMGET, &flags);
      status = (flags & TIOCM_CD);
      /* Did DCD jumps to high? Then the power has failed. */
      if (oldstat == 0 && status != 0) {
          if (count > 3) powerfail(0);
          else { sleep(1); continue; }
      /* Did DCD go down again? Then the power is back. */
      if (oldstat > 0 && status == 0) {
          if (count > 3) powerfail(1);
          else { sleep(1); continue; }
      /* Reset count, remember status and sleep 2 seconds. */
      count = 0;
      oldstat = status;
  /* Error! (shouldn't happen) */


/* Tell init the power has either gone or is back. */
void powerfail(ok)
int ok;
  int fd;

  /* Create an info file needed by init to shutdown/cancel shutdown */
  if ((fd = open(PWRSTAT, O_CREAT|O_WRONLY, 0644)) >= 0) {
        if (ok)
                write(fd, "OK\n", 3);
                write(fd, "FAIL\n", 5);
  kill(1, SIGPWR);


/* End of File */
 LocalWords:  rc

10.5. APC Smart-UPS, Model 600

Many people have APC Smart UPSes.  To the best of my knowledge, no one
can run them in "smart" mode under Linux.  This is because APC refuses
to release the protocol for the "smart" mode without a non-disclosure
agreement.  Not very smart of them, I'd say :).

The general consensus is to buy from a brand which does release the
information.  Best is one such brand.

If you are stuck with an APC Smart-UPS, you can still use it, but only
in a dumb mode like all the other UPSes & as outlined above.

Here's some info on how to make a cable for doing such.  You'll
probably have to hack powerd.c as outlined in section 11.

>From Mon Aug 22 10:16:23 1994

Newsgroups: comp.os.linux.misc
Subject: UPS Monitoring Cable For APC
From: (Lam Dang)
Date: Fri, 19 Aug 1994 11:56:28 GMT
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
X-Newsreader: TIN [version 1.2 PL1]

[Didn't make it the first time.]

A few netters have asked about UPS monitoring cables.  This is what I
found when I made one for my APC Smart-UPS, Model 600.  A disclaimer is in
order.  This is just an experimenter's report; use it at your own risks.
Please read the User's Manual first, especially Section 6.4, Computer
Interface Port.

The cable is to run between a 9-pin female connector on the UPS and a
25-pin male connector on the PC.  Since I cut off one end of a 9-pin
cable and replaced it with a 25-pin connector, I had to be VERY
CAREFUL ABOUT PIN NUMBERS.  The 25-pin hood is big enough to contain a
voltage regulator and two resistors.  I got all the materials (listed
below) from Radio Shack for less than 10 bucks.  As required by Windows NT
Advanced Server 3.5 (Beta 2), the "interface" between the UPS connector
and the PC connector is as follows:

        UPS (9-pin)              PC (25-pin)

        1 (Shutdown)             20 (DTR)
        3 (Line Fail)             5 (CTS)
        4 (Common)                7 (GND)
        5 (Low Battery)           8 (DCD)
        9 (Chassis Ground)        1 (Chassis Ground)

This is pretty straightforward.  You can use UPS pin 6 instead of 3
(they're the inverse of each other).  The complication is in pulling up
UPS open collector pins 3 (or 6) and 5.

This APC model provides an unregulated output of 24 Vdc at UPS pin 8. The
output voltage is available all the time (at least until some time after
Low Battery has been signalled).  The supply is limited to 40 mA.  To
pull up, UPS pin 8 is input to a +5 Vdc voltage regulator.  The output of
the regulator goes into two 4.7K resistors.  The other end of one
resistor connects both UPS pin 3 (Line Fail) and PC pin 5 (CTS).  That
of the other resistor connects both UPS pin 5 (Low Battery) and PC pin 8
(DCD).  The two resistors draw about 2 mA when closed.

Test your cable without connecting it to the PC.  When the UPS is on
line, pins 5 (CTS) and 8 (DCD) at the PC end of the cable should be very
close to 5 Vdc, and applying a high to pin 20 (DTR) for 5 seconds should
have no effect.  Now pull the power plug to put the UPS on battery.  Pin
5 (CTS) should go down to zero Vdc, pin 8 (DCD) should stay the same at 5
Vdc, and applying a high to pin 20 (DTR), e.g., by shorting pins 8 and 20,
should shut down the UPS after about 15 seconds.

Keep the UPS on battery until Low Battery is lighted on its front panel.
Now pin 8 (DCD) should go down to zero Vdc too.  Wait until the UPS
battery is recharged.  Then connect your cable to the PC, disable the UPS
option switches by turning all of them ON, and run your favorite UPS
monitoring software.

For those who want to run it with Windows NT Advanced Server, the UPS
interface voltages are NEGATIVE for both power failure (using UPS pin 3)
and low battery conditions, and POSITIVE for remote shutdown.  Serial
line parameters such as baud rate don't matter.

I haven't tested my cable with Linux powerd.  When you do, please let us
know.  I run NT as often as Linux on the same PC.  I must conform to NT's
UPS scheme.  Perhaps somebody can modify powerd to work with it and post
the source code here.

List of materials:

        1 shielded D-sub connector hood (Radio Shack 276-1510)
        1 25-pin female D-sub crimp-type connector (276-1430)
        1 7805 +5Vdc voltage regulator (276-1770)
        2 4.7K resistors
        1 component perfboard (276-148)
        1 cable with at least one 9-pin male connector.

You'll need a multimeter, a soldering iron, and a couple of hours.

Hope this helps.


Lam Dang

11. Reverse-engineering cables & hacking powerd.c

Try to get documentation for the cables that your UPS seller supplies.
In particular find out:

   -What lines need to be kept high.
   -What line(s) turn off the UPS.
   -What lines the UPS toggles to indicate that:
      -Power is out.
      -Battery is low.

You then need to hack powerd.c appropriately.

If you have trouble getting the above information (or just want to
check it) the following program (upscheck.c) might help.  It's a
hacked version of powerd.c.  It allows you to set the necessary port
flags from the command line & then monitors the port, displaying the
control lines every second.  I used it as "upscheck /dev/cua1 2" (for
example) to set the 2nd bit (DTR) & to clear the other bits.  The
number base 2 indicates which bits to set, so for example to set bits
1, 2 & 3, (& clear the others) use 7.  See the code for details.

Here's the (untested) upscheck.c program.  It's untested because I
edited the version I originally used to make it clearer, and can't
test the new version at the moment.

 --------- Begin upscheck.c ------------------
 * upscheck     Check how UPS & computer communicate.
 * Usage:       upscheck <device> <bits to set>
 *              For example, upscheck /dev/cua4 4 to set bit 3 &
 *              monitor /dev/cua4.
 * Author:      Harvey J. Stein <>
 *              (but really just a minor modification of Miquel van
 *              Smoorenburg's <> powerd.c
 * Version:     1.0 19940802
#include <sys/types.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <signal.h>

/* Main program. */
int main(int argc, char **argv)
  int fd;

/*  These TIOCM_* parameters are defined in <linux/termios.h>, which  */
/*  is indirectly included here.                                      */
  int dtr_bit = TIOCM_DTR;
  int rts_bit = TIOCM_RTS;
  int set_bits;
  int flags;
  int status, oldstat = -1;
  int count = 0;
  int pc;

  if (argc < 2) {
        fprintf(stderr, "Usage: upscheck <device> <bits-to-set>\n");

  /* Open monitor device. */
  if ((fd = open(argv[1], O_RDWR | O_NDELAY)) < 0) {
    fprintf(stderr, "upscheck: %s: %s\n", argv[1], sys_errlist[errno]);

  /* Line is opened, so DTR is high. Force it anyway to be sure. */
  /*    ioctl(fd, TIOCMBIS, &dtr_bit); */
  /*    The above line was from the original powerd.c, but it turned off */
  /*    my UPS!  So, I changed it to the line below which clears the DTR */
  /*    instead of setting the DTR bit & that worked for me.  However,   */
  /*    it might not work for you, so I commented it out too.            */
  /*    ioctl(fd, TIOCMBIC, &dtr_bit); */

  /* Get the bits to set from the command line. */
  sscanf(argv[2], "%d", &set_bits);

  while (1) {
    /* Set the command line specified bits (& only the command line */
    /* specified bits).                                             */
    ioctl(fd, TIOCMSET, &set_bits);
    fprintf(stderr, "Setting %o.\n", set_bits);


    /* Get the current line bits */
    ioctl(fd, TIOCMGET, &flags);
    fprintf(stderr, "Flags are %o.\n", flags);

/*  Fiddle here by changing TIOCM_CTS to some other TIOCM until    */
/*  this program detects that the power goes out when you yank     */
/*  the plug on the UPS.  Then you'll know how to modify powerd.c. */
    if (flags & TIOCM_CTS)
        pc = 0 ;
        fprintf(stderr, "power is up.\n");
        pc = pc + 1 ;
        fprintf(stderr, "power is down.\n");



 ----------- End upscheck.c ---------------------------

12. How to shutdown other machines on the same UPS

Some people (myself included) have several Linux PCs connected to one
UPS.  One PC monitors the UPS & needs to get the other PCs to shut
down when the power goes out.  There are a number of ways to do this,
all are do-it-yourself currently, and most are just hypothetical.

We assume the PCs can communicate over a network.  Call the PC that
monitors the UPS the master & the other PCs the slaves.

12.1. UPS status port method

Set up a port on the master which, when connected to, either sends
"OK", "FAIL", or "BATLOW", the first when the power is ok, the second
when the power has failed, and the third when the battery is low.
Model this on port 13 (the time port) which one can telnet to &
receive the local time.

Have the slaves run versions of powerd that look at this port instead
of checking a serial line.

The only down side I can see to this method is the network load due to
checking this port.  One would want to check this port often to
quickly catch the BATLOW message & shut down before the battery dies.

12.2. Broadcast method

Same as 12.1 except send an ethernet broadcast message that the power
has just gone down.

This might have security implications.

12.3. Dummy login method

Set up dummy logins on the slaves with login names "powerok" &
"powerfail", both with the same UID.  Make /etc/powerokscript the
shell of the powerok user, & make /etc/powerfailscript the shell of
the powerfail user.  On the master, have the /etc/powerokscript rlogin
to each slave as user powerok, & have the /etc/powerfailscript rlogin
to each slave as user powerfail.  Put a .rhosts file on each slave in
the home directory of powerok & powerfail to allow root from the
master to login as user powerok & powerfail to each slave.

This is the system I'm currently using.

This might also have security implications.