Does anyone know lua code to manipulate the transponder mode. What I have does not work?

Question

Here is my code

require ("radio")

dataref("TRANSPONDER_MODE","sim/cockpit/radios/transponder_mode","writable")

function transponder_test_mode()

  if button((6*160) + 31) ==True then

    TRANSPONDER_MODE = 3 

    XPLMSpeakString("Transponder set to off")

  end

end

do_every_frame( "transponder_test_mode ()" )

function transponder_standby_mode()

  if button((6*160) + 30) ==True then

    TRANSPONDER_MODE = 1

    XPLMSpeakString("Transponder set to standby")

  end

end 

do_every_frame( "transponder_standby_mode ()" )

function transponder_alt_mode()

  if button((6*160) + 29) ==True then

    TRANSPONDER_MODE = 2 

    XPLMSpeakString("Transponder set to active")

  end

end

do_every_frame( "transponder_alt_mode ()" )

function transponder_ground_off_mode()

  if button((6*160) + 28) ==True then

    TRANSPONDER_MODE = 0 

    XPLMSpeakString("Transponder set to ground")

  end

end

do_every_frame( "transponder_ground_off_mode ()" )