Turck modbus communication

[CristianAgata]

Hi everyone, I'm trying to communicate with a Turck gateway, I have write a simple resident script to ask cyclical a value of the power supply, it works for a n numbers of request then the Turck passes me  'nill'
Any suggest?
Best regards 

This is my script 

if not mb then
    require('luamodbus')
    mb = luamodbus.tcp()
  mb:open('192.168.0.162', 502)
  mb:connect()
  log('stato modebus ', mb)
end

-- loop while condition is met
while (1) do
  -- read from address 1000
value = mb:readinputregisters(9216)
  log('valore di value',value)
  -- wait for 1.5 seconds
os.sleep(5.5)
end