A serial line computer shutdown button and LED

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original in en Guido Socher 

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Guido is a long time Linux fan. Most of the time he develops software or works for _LF_, but from time to time he likes to design hardware parts.

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In this article we design a little push button to connect to your server on the serial line. You can use it to shut the machine down when needed.

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A real server machine does normally not need any keyboard, monitor or mouse. It needs a good operating system, like Linux, and a network card. External devices like mouse and keyboard disturb only. With Linux running on the machine, remote administration is the best and most convenient solution.

When I buy a computer I always verify that it has an BIOS option to not check for keyboard at startup. All good motherboard manufactures have that option today. Some that still think people run only Windows on their machines require a keyboard at startup but that is a different story and I don't recommend to buy such hardware.

Such a computer without keyboard and monitor is the perfect computer for a firewall, a web-server, a NFS file server, a mail-server, an ADSL gateway computer etc. It does not have to be a powerful machine. An old pentium 100Mhz with enough RAM will easily serve a hundred and more other machines.

Having no keyboard connected is nice and convenient because the server is then just a small box that fits somewhere in the corner or on a shelf in the cellar. Sometimes you would however like to have a button to properly shut it down without logging first in and running the shutdown command. It could be that you are already in the server room and the next workstation is 100 meters away or you have somebody who has physical access to the machine but no root password and that person needs to shutdown the computer before taking away the power supply.

Just powering it off is no good idea because you might loose data and the disk check during start up takes then very long time.

The solution is the small and inexpensive serial device presented in this article.

The device

We will build a small device with 2 LEDs and one pushbutton. A green LED will indicate when the pushbutton is active and a slowly blinking yellow LED indicates that the server is working or still in the process of shutting down. To shut down the server you press the pushbutton for 3 seconds.

When you press the pushbutton then the green LED will go off indicating the shutdown is in progress. The command "/sbin/shutdown -t2 -h now" is now executing. The yellow LED continues blinking while the server is still shutting down. When the yellow LED stops blinking then you should still wait 10 seconds before you power off the computer. The yellow LED stops when all processes are killed and the disks are being unmounted.
If your computer supports automatic power off then you do not have to worry about the 10 seconds. Just wait until it has switched off it self.

The serial line

The interesting thing about of the serial interface is that it has a number of pins which you can control via ioctl system calls. This means that you do not need to modify anything in the kernel to control a LED connected to the serial line or to read out the state of a push button. The serial line has just enough power to drive 2 low power LEDs. We will use the RTS and DTR pins to connect the LEDs to and the Carrier Detect line will be used to read the state of the push button.

9 PIN D-SUB MALE at the Computer.

9 PIN-connector 25 PIN-connector Name Dir Description
1 8 CD input Carrier Detect
2 3 RXD input Receive Data
3 2 TXD output Transmit Data
4 20 DTR output Data Terminal Ready
5 7 GND -- System Ground
6 6 DSR input Data Set Ready
7 4 RTS output Request to Send
8 5 CTS input Clear to Send
9 22 RI input Ring Indicator

The circuit

You need the following parts to build this shutdown button with LEDs.

circuit diagram The parts are connected together as shown in the circuit diagram. The voltage levels on the serial lines is +/- 10Volts. The 1.5K Ohm resistors are exactly right for low power LEDs but normal LEDs will also do. I don't recommend to change resistors or use external power supply since the serial line control chips are part of the motherboard's chip set. If you burn them then you will need to change the entire motherboard.


The above image shows the etched circuit board. I don't have an exact board layout for you because I prefer to do small circuits as free hand drawing directly on the board with etch resistant ink.

The final circuit with all parts is shown on the picture at the top of this article. You can carefully drill 3 holes into the front panel of your server. Two 3mm holes for the LEDs and one to screw in the push button. The whole circuit is very light and the screw of the push button will be enough to hold it.

The software

The software is a small daemon process. Which can be started via a script from /etc/rc.d/rc3.d/. It uses the ioctl(fd, TIOCMSET,...) and ioctl(fd, TIOCMGET, ...) functions to read from and write to the serial port. You can download sled-0.1.tar.gz. A makefile and installation instructions are included.

Happy soldering