Modbus script problem - Timeout

Code:
require('luamodbus')

vent_command = grp.getvalue('1/3/13')

reset_command = grp.getvalue('34/1/4')





control_motor = true

motor_on = vent_command



speed_value_perc = grp.getvalue('34/1/2')

hz_sp = math.floor(speed_value_perc*50)



mb = luamodbus.rtu()

mb:open('/dev/RS485-2', 19200, 'E', 8, 1)

mb:connect()

mb:setslave(5)





speed_read,err= mb:readregisters(202)

--log('Speed read22',speed_read,err)



read_mode,err= mb:readregisters(8)

--log('Read mode',read_mode,err)



read_speed,err= mb:readregisters(13)

log('Read speed',read_speed,err)







read_inputs,err= mb:readregisters(214)

--log('Read inputs',read_inputs,err)



read_alarm,err= mb:readregisters(500)

--log('Read alarms',read_alarm,err)





if control_motor then

  if motor_on then

    if read_mode ~= 75 then

      res,err = mb:writeregisters(8,2)

      --log('Write_motor_to_on',res,err)

    end



    if read_speed ~=hz_sp then

      res,err =mb:writeregisters(13,hz_sp)

      --log('Write_motor speed',res,err)

    end

  else 

    if read_mode ~= 0 then

      res,err = mb:writeregisters(8,0)

      --log('Write motor off',res,err)

    end

  end

end



if reset_command then

  --log('Reseting frequency converter')

  log(mb:writeregisters(1,1))

  grp.checkupdate('34/1/4', 0)

end



mb:close()



if read_mode == 75 or read_mode == 67 then

  grp.update('34/1/3', read_speed/50)

elseif read_mode == 0 then

  grp.update('34/1/3', 0)

end