SIM900（AT + SAPBR = 3,1，“ CONTYPE”，“ GPRS”）返回“ ERROR”

Question

我正在使用与Arduino MEGA 2560连接的SIM900 GSM模块。当我发送以下命令（AT + SAPBR = 3,1，“ CONTYPE”，“ GPRS”）来设置GPRS的连接类型时，得到了响应“错误”。

Command sequences that I send are here:


**Commands are here**         **Responses are here**

// See if the SIM900 is ready
AT                                  OK

// SIM card inserted and unlocked?
AT+CPIN?                            +CPIN: READY
                                    OK

// Is the SIM card registered?
AT+CREG?                            +CREG: 0,1
                                    OK

// Is GPRS attached?
AT+CGATT?                           +CGATT: 1
                                    OK

// Check signal strength - should be 9 or higher
AT+CSQ                              +CSQ: 14,0
                                    OK

// Set connection type to GPRS
AT+SAPBR=3,1,"Contype","GPRS"       ERROR

能否请您帮助理解问题所在？ 请注意，我检查了SMS和RING，效果很好。问题仅与提到的命令有关。

谢谢， 弗拉基米尔