I want to use a SIM900-module with a RasPi to send alarm-messages from my home-automation. I wrote a test-program for sending messages, which works without a (known to me) problem. Then I wrote a test-program for receiving SMSs, which also works. Then I tried to combine the two. I put the receive part in a thread, which read from the module every .1 seconds and also checks a sendFlag at each turn. I set this sendFlag, when a message has to be sent. But as soon as the program sends the callstring (AT+CMGS="+43myPhoneNumber" terminated with CR+LF) the SIM900 does not answerwith "OK" but with ">" to any AT-command.

Can anyone tell me why this happens and how to avoid it? If you need the code to answer, please tell me so I can prepare it. At the moment it has to much in, that is not relevant to the question (MTTQ, reading files....)

TIA, Karl