In accordance with The instructions To control The flow meter specifications, and The results show that: The Gas, The Select of The selected Gas can be changed via RS232/RS485 input To The selected Gas, enter The following commands: In Polling Mode: Address $$# & lt; Enter> (e.g. $B $# & lt; Enter>) , and then I use string constants in Labview "A $$1" as A command, but there is no show that gas flow meter has been changed, A serial port parameters that are set up, I think it's because of the & lt; Enter> On, but I don't know how to solve, PS: "A $$1" I tried on hyperterminal, just press enter key after will have A response, and returns A value, hope everybody can greatly help me to solve it, thank you!