I started working on a U-blox LEON-G100 gprs modem connected to the PC terminal through UART. Whatever character I enter into the terminal gets echoed back except "AT". When I enter "AT", the modem hangs up. Then I have to reset the modem to make it respond. So, I cant even execute a single AT command. As it echoes back every character I enter until "AT" sequence, I think there is no the electric connections and the UART port settings must be correct too. What would be wrong here? I had experience with other GPRS modems, but this is the first time I am encountering this bizarre trouble.

  • \$\begingroup\$ Do you have something like auto-bauding enabled which does a poor job? \$\endgroup\$ – Tom L. Feb 4 '13 at 20:26
  • \$\begingroup\$ Its a new board based on the U-Blox modem which bought very recently. I just hooked it to a buspirate which acts as a USB to uart converter. The baudrate at the PC terminal is set at 115200 which will work perfectly according to its datasheet. \$\endgroup\$ – 0xakhil Feb 4 '13 at 20:28
  • \$\begingroup\$ If I'm not completely wrong, that module has auto-bauding enabled. Try to send "at" a few times. After a while it should figure out the baud-rate and respond. \$\endgroup\$ – Tom L. Feb 4 '13 at 20:38
  • \$\begingroup\$ My mistake.. I forgot to put it in null modem state. It had hardware flow control enabled by default. Wasted a lot of time. \$\endgroup\$ – 0xakhil Feb 4 '13 at 20:38
  • \$\begingroup\$ Hey, I figured it out. It has hardware flow control enabled by default. Shorted the CTS and RTS lines and its working fine now. Thanks for the reply. \$\endgroup\$ – 0xakhil Feb 4 '13 at 20:40

I figured it out. The modem had hardware flow control enabled by default. So I had to put it into null modem state. Its just a matter of shorting CTS and RTS lines each other. Now its working perfectly fine.


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