通过mini2440的串口扩展板将串口2接TC35i GSM模块，虽然不能显示AT、也没有OK返回，但是能够发出短信。但是将之接到SIM300 GPRS模块之后，既不能显示AT，也不能返回OK，短信，ppp联网都失败，请指点，谢谢！
[root@FriendlyARM /]# pppd
timeout set to 15 seconds
abort on (BUSY)
abort on (\nERROR\r)
abort on (\nNO ANSWER\r)
abort on (\nNO CARRIER\r)
abort on (\nNO DIALOTONE\r)
abort on (\nRINGING\r\n\r\nRINGING\r)
timeout set to 6 seconds
press CRTL-C to close the connection at any stage!
defining PDP context...
send (^MAT^M)
expect (OK)
|^?|alarm
Failed
Connect script failed
