首先，您需要在您的a7模块中配置串口通讯，并且确保您可以通过串口与模块进行通讯。

接下来，您需要编写代码来发送短信。以下是一个示例代码：

#include <SoftwareSerial.h>

SoftwareSerial sim900Serial(7, 8); // RX, TX

void setup() {
  // 初始化串口
  Serial.begin(9600);
  sim900Serial.begin(9600);
}

void loop() {
  // 发送AT指令检查模块是否正常工作
  sim900Serial.write("ATr");
  delay(1000);
  while (sim900Serial.available()) {
    Serial.write(sim900Serial.read()); // 打印模块返回的信息
  }
  
  // 发送短信
  sim900Serial.write("AT+CMGF=1r"); // 设置短信格式为文本模式
  delay(1000);
  sim900Serial.write("AT+CMGS="+8613xxxxxxxxx"r"); // 设置接收号码
  delay(1000);
  sim900Serial.write("Test message"); // 设置短信内容
  delay(1000);
  sim900Serial.write((byte)26); // 发送短信
  
  // 等待模块返回信息
  delay(5000);
  while (sim900Serial.available()) {
    Serial.write(sim900Serial.read()); // 打印模块返回的信息
  }
  
  // 等待一段时间，然后重新发送短信
  delay(60000);
}

请注意，以上代码仅供参考，您需要进行必要的修改来适配您的a7模块和具体的应用场景。同时，请确保您的模块已经被正确接线和供电。

通讯方式

a7模块可以通过串口通信方式发送短信，也可以通过AT指令方式发送短信，下面介绍串口通信方式。

硬件接口

a7模块的串口数据接口采用ttl电平，需要连接到ttl与rs232之间的转换电路或者电平转换芯片后才能与电脑进行串口通信。a7模块的串口波特率默认为9600 bps，数据位8位，无校验位，停止位1位，可以通过AT指令修改。

发送短信流程

1. 打开串口，并设置波特率为9600
2. 发送AT指令，等待ok回复，判断模块是否正常工作
3. 发送AT+CSCS=?指令，查询支持的字符集，一般返回”GSM”即可，如果返回多个字符集，需要根据需要设置
4. 发送AT+CMGF=1指令，设置短信格式为文本模式
5. 发送AT+CMGS=”手机号码”指令，进入短信发送模式，等待”>”符号提示
6. 发送短信内容，以Ctrl+z结束短信输入
7. 等待短信发送结果回复

代码实现

以下是C语言的示例代码：

include <stdio.h>

include <string.h>

include <windows.h>

define COM_PORT “COM3”

int main(int argc, char* argv[])
{

HANDLE hCom;
DCB dcb;
COMMTIMEOUTS timeouts;
DWORD bytesWritten, bytesReceived;
char response[512];

hCom = CreateFile(COM_PORT, GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING, 0, NULL);
if (hCom == INVALID_HANDLE_VALUE)
{
    printf("Failed to open serial port, error code %dn", GetLastError());
    return -1;
}

dcb.DCBlength = sizeof(dcb);
if (GetCommState(hCom, &dcb) == 0)
{
    printf("Failed to get serial port state, error code %dn", GetLastError());
    CloseHandle(hCom);
    return -1;
}
dcb.BaudRate = 9600;
dcb.ByteSize = 8;
dcb.Parity = NOPARITY;
dcb.StopBits = ONESTOPBIT;
if (SetCommState(hCom, &dcb) == 0)
{
    printf("Failed to set serial port state, error code %dn", GetLastError());
    CloseHandle(hCom);
    return -1;
}

timeouts.ReadIntervalTimeout = MAXDWORD;
timeouts.ReadTotalTimeoutMultiplier = 0;
timeouts.ReadTotalTimeoutConstant = 0;
timeouts.WriteTotalTimeoutMultiplier = 1000;
timeouts.WriteTotalTimeoutConstant = 1000;
if (SetCommTimeouts(hCom, &timeouts) == 0)
{
    printf("Failed to set serial port timeouts, error code %dn", GetLastError());
    CloseHandle(hCom);
    return -1;
}

Sleep(1000);

bytesWritten = 0;
WriteFile(hCom, "ATrn", strlen("ATrn"), &bytesWritten, NULL);

Sleep(1000);

bytesReceived = 0;
ReadFile(hCom, response, sizeof(response), &bytesReceived, NULL);
response[bytesReceived] = '';
printf("Response: %s", response);

bytesWritten = 0;
WriteFile(hCom, "AT+CSCS=?rn", strlen("AT+CSCS=?rn"), &bytesWritten, NULL);

Sleep(1000);

bytesReceived = 0;
ReadFile(hCom, response, sizeof(response), &bytesReceived, NULL);
response[bytesReceived] = '';
printf("Response: %s", response);

bytesWritten = 0;
WriteFile(hCom, "AT+CMGF=1rn", strlen("AT+CMGF=1rn"), &bytesWritten, NULL);

Sleep(1000);

bytesReceived = 0;
ReadFile(hCom, response, sizeof(response), &bytesReceived, NULL);
response[bytesReceived] = '';
printf("Response: %s", response);

bytesWritten = 0;
WriteFile(hCom, "AT+CMGS="136XXXXXXXX"rn", strlen("AT+CMGS="136XXXXXXXX"rn"), &bytesWritten, NULL);

Sleep(1000);

bytesReceived = 0;
ReadFile(hCom, response, sizeof(response), &bytesReceived, NULL);
response[bytesReceived] = '';
printf("Response: %s", response);

bytesWritten = 0;
WriteFile(hCom, "Hello, world!x1a", strlen("Hello, world!x1a"), &bytesWritten, NULL);

Sleep(5000);

bytesReceived = 0;
ReadFile(hCom, response, sizeof(response), &bytesReceived, NULL);
response[bytesReceived] = '';
printf("Response: %s", response);

CloseHandle(hCom);
return 0;

}