Play it Cool: Solving Thermal Problems, Page 3

Implementing a Script To Monitor the Button

Many methods can be used to monitor the signals on a serial line. Most telecom programs have built-in scripting to monitor the signals and perform actions accordingly. If the scripting language cannot invoke the audio script directly, it could write a log file that another script could monitor and act on accordingly.

However, sticking with our lightweight, modular design, I chose to use a little utility called statserial, which simply displays the status of a specified serial port. For example, running statserial with no parameters displays the status of serial port 2, /dev/ttyS1, as shown in Listing 2.

Listing 2: The default statserial display

Device: /dev/ttyS1

Signal  Pin  Pin  Direction  Status  Full
Name    (25) (9)  (computer)         Name
-----   ---  ---  ---------  ------  -----
FG       1    -      -           -   Frame Ground
TxD      2    3      out         -   Transmit Data
RxD      3    2      in          -   Receive Data
RTS      4    7      out         1   Request To Send
CTS      5    8      in          0   Clear To Send
DSR      6    6      in          0   Data Set Ready
GND      7    5      -           -   Signal Ground
DCD      8    1      in          0   Data Carrier Detect
DTR     20    4      out         1   Data Terminal Ready
RI      22    9      in          0   Ring Indicator

This display is updated every second. The statserial utility also has an option to display the status of the port in decimal form, coded using the scheme shown in Listing 3.

Listing 3: The encoding used by statserial for displaying the status of the serial port.

+---+---+---+---+---+---+---+---+---+
| 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
+---+---+---+---+---+---+---+---+---+
|DSR|RI |DCD|CTS|XXX|XXX|RTS|DTR|XXX|
+---+---+---+---+---+---+---+---+---+

The script will only have to check bit 8 of the result (DSR). When it's set high, the script runs the audio script, and the temperature is read aloud. The final shell script is shown in Listing 4.

Listing 4: The final script to monitor the analog button (buttonsay.sh).

#!/bin/sh

DIR=/home/sschafer/officetemp

chkDSR() {
  LINESTAT=`statserial -d /dev/ttyS1`
  # If bit 8 is set, DSR is on
  #   (button pressed)
  if [ $(( $LINESTAT & 256 )) -eq 256 ]; then
    DSR=1
  else
    DSR=0
  fi
}

# Repeat endlessly
while [ true ]
do

chkDSR

# If DSR is high, run audio script
if [ $DSR == 1 ]; then

  $DIR/saytemp.sh >/dev/null 2>&1

  # Loop while line remains hot
  while [ $DSR -eq 1 ]
  do
    chkDSR
    sleep 2
  done

fi

The script is executed in the background every time the machine is rebooted. Standard script control signals can be used to stop and kill the script as necessary.

Next Time

This article showed how the serial communication between the computer and the temperature sensor was improved by using a dedicated script, and an analog button was added for on-demand actions.

The last article in this series will show how the data can be sent to various reporting applications so it can be charted and trended appropriately.

About the Author

Freelance consultant Steve Schafer has written multiple technology books and articles. In the business world, he most recently worked in-house as COO/CFO of Progeny Linux Systems in Indianapolis. Serving as president and CEO in the company's startup years, Steve led Progeny's business and financial turnaround during the tech crash of the early 2000s. Prior to joining Progeny, he was a senior title manager for Macmillan Digital USA, overseeing the operating system and entertainment software products and tripling Macmillan's Linux product line revenue. He partnered Macmillan with Mandrake, bringing Mandrake's Linux distribution to the competitive retail market.