currentcost.rb

#! /usr/bin/env ruby
# Read current cost info. Output in a form suitble for cacti.

# On ubuntu, place in the following sudoers to allow running from cacti: 
# www-data ALL=NOPASSWD: /usr/share/cacti/site/scripts/currentcost.rb

# To create device specific names, create:
# /etc/udev/rules.d/70-persistent-currentcost.rules containing: 
# KERNEL=="ttyUSB*", SUBSYSTEM=="tty", ATTRS{manufacturer}=="Prolific Technology Inc.", SYMLINK+="currentcost%n"

require 'rubygems'
require 'serialport'
require 'rexml/document'

# Extract and print specified data.
def parse_to_string(line)
  xml = REXML::Document.new(line)
  output = $data.each.map do |key, value|
    "#{key}:#{xml.elements[value].text.to_f}"
  end
  output.join(' ')
end

def print_forever(port)
 while line = port.readline
   STDERR.puts line if $DEBUG
   parse_and_print(line)
 end
end

# Names and values to output
$data = {
#  'src'         => '//src',
#  'dsb'         => '//dsb',
#  'time'        => '//time',
#  'sensor'      => '//sensor',
#  'id'          => '//id',
#  'type'        => '//type',
  'temperature' => '//tmpr',
  'power'       => '//ch1/watts',
}

port = ARGV.shift || '/dev/ttyUSB0'

# Open a serial port, and read a single line.
SerialPort::open(port , 57600, 8, 1, SerialPort::NONE) do |port|
  print parse_to_string(port.readline)
end

__END__
# Example output from the device.
<msg><src>CC128-v0.12</src><dsb>00003</dsb><time>09:11:56</time><tmpr>19.0</tmpr><sensor>0</sensor><id>00077</id><type>1</type><ch1><watts>00380</watts></ch1></msg>

If you're using the udev rule above, you can easily find all current costs devices using:

Dir['/dev/currentcost*']

Note: all prolific usb-serial adapters will probably show up in this glob / udev rule, so if you have more, this might need editing.