Class: Rdm6300

Inherits:
Object
  • Object
show all
Defined in:
lib/rdm6300.rb

Overview

This class reads 125kHz tags over an RS-232 serial port using the RDM6300 module

Constant Summary collapse

START_CODE =

RDM6300 uses 0x02 to indicate the start of a tag

0x02
END_CODE =

RDM6300 uses 0x03 to indicate the end of a tag

0x03
VALID_LENGTH =

Valid tag data will always have a total length of 12 bytes over RS-232

12

Instance Method Summary collapse

Constructor Details

#initialize(serial_port_dev = '/dev/ttyAMA0', debug: false, flush_delay: 10) ⇒ Rdm6300

The default serial port is /dev/ttyAMA0 since this is the primary serial port on a Raspberry Pi

You will need to stop the Linux kernel using /dev/ttyAMA0 if you want to use it with the RDM6300

On a Raspberry Pi 4 and Raspberry Pi OS, this can be done by adding enable_uart=1 and dtoverlay=disable-bt to /boot/config.txt and removing console=serial0,115200 from /boot/cmdline.txt

The flush delay handles the situation where rogue RF triggers the RDM6300. Genuine tag data takes less than a second to transmit to the RDM6300. If there is tag data that takes longer than flush_delay to arrive, the input buffer is flushed. This improves the reliability of a long-running tag reader.



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# File 'lib/rdm6300.rb', line 32

def initialize(serial_port_dev = '/dev/ttyAMA0', debug: false, flush_delay: 10)
  @serial_port_dev = serial_port_dev
  @debug = debug
  @flush_delay = flush_delay
  @serial_port = SerialPort.new(@serial_port_dev, 9600, 8, 1, SerialPort::NONE)
  @serial_port.read_timeout = 0
end

Instance Method Details

#checksum_is_valid?(received_bytes) ⇒ Boolean

Checks if the tag checksum is valid

Returns:

  • (Boolean) 


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# File 'lib/rdm6300.rb', line 124

def checksum_is_valid?(received_bytes)
  # Format is 10 data bytes then two checksum bytes
  # Group the received bytes into pairs, convert from ascii to hex, then XOR them
  pairs = []
  received_bytes[0..9].each_with_index do |char, index|
    next if index % 2 == 1
    pairs << char + received_bytes[index + 1]
  end
  checksum = pairs.map { |pair| pair.to_i(16) }.inject(:^)

  # Then compare to [10..12] of received_checksum
  received_checksum = (received_bytes[10] + received_bytes[11]).to_i(16)
  return checksum == received_checksum
end

#close ⇒ Object

Closes the serial port



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# File 'lib/rdm6300.rb', line 41

def close
  @serial_port.close
end

#get_tag ⇒ Object

Waits for a tag to be presented and returns a zero-padded string with the tag id



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# File 'lib/rdm6300.rb', line 46

def get_tag()
  @received_bytes = []
  @time_of_last_byte = Time.now
  @receive_started = false

  # Flush input buffer if partial data was received
  @timeout_active = true
  Thread.new do
    while @timeout_active
      sleep 1
      if @receive_started and (Time.now - @time_of_last_byte) > @flush_delay
        STDERR.puts "Received_bytes not empty after #{@flush_delay}s" if @debug
        @time_of_last_byte = Time.now
        @received_bytes = []
        @receive_started = false
      end
    end
  end

  while true
    byte = nil
    begin
      byte = @serial_port.readbyte
    rescue EOFError => e
      # This shouldn't happen if @serial_port.read_timeout == 0
      STDERR.puts "Timed out reading serial data" if @debug
      @time_of_last_byte = Time.now
      @received_bytes = []
      next
    end

    # check if this is the first byte received and it is valid. if not, reset
    if !@receive_started and byte != START_CODE
      STDERR.puts "Got initial byte but it is not #{START_CODE} (#{byte})" if @debug
      reset_serial_buffer()
    end

    if !@receive_started and byte == START_CODE
      @receive_started = true
      @time_of_last_byte = Time.now
      next
    end

    if byte > 127
      STDERR.puts "Got invalid byte #{byte}, resetting" if @debug
      reset_serial_buffer()
      next
    end

    if byte == END_CODE
      if @received_bytes.length == VALID_LENGTH
        if checksum_is_valid?(@received_bytes)
          @timeout_active = false
          return '%010d' % @received_bytes[2..9].join.to_i(16)
        else
          STDERR.puts "Failed to verify checksum for received data" if @debug
        end
      else
        STDERR.puts "Invalid number of bytes received (#{@received_bytes.length})" if @debug
      end
      reset_serial_buffer()
      next
    end

    @received_bytes << byte.chr
  end
end

#reset_serial_buffer ⇒ Object

Resets the serial buffer, called after a tag is presented



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# File 'lib/rdm6300.rb', line 115

def reset_serial_buffer()
  @receive_started = false
  @received_bytes = []
  @serial_port.close
  @serial_port = SerialPort.new(@serial_port_dev, 9600, 8, 1, SerialPort::NONE)
  @time_of_last_byte = Time.now
end