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Could you give us a little more information? For example, when you say you've had trouble with "the commands of transmit and receive string," could you tell us what error code was returned by those commands? Common errors for those commands include:
-52 = You'll get this if the communication handle was not successfully opened first (using the command OpenOutgoingCommunicaion).
-38 or -39 = Timeout (check the LEDs on the S2 and the serial device).
-20 = "In use by other" means you need to use PAC Manager's > Tools > Inspect > Communications > Communications Port Control and make sure the port you're trying to use says "None" (the default for port 0 is PPP).
Also, do you see any LEDs blinking (either on your S2 or the serial device you're trying to connect to)?
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