用C语言实现通过SecrueCRT向开发板发送on，按下回车后LED灯亮并返回字符”led on”；输入off按下回车后LED灯灭并返回字符”led off”，其他返回”command error”。

好的，此问题属于技术问题。以下给出通过C语言实现的代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <Windows.h>

int main() {
    HANDLE hSerial = CreateFile("COM1", GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL); //打开串口
    if (hSerial == INVALID_HANDLE_VALUE) {
        printf("无法打开串口\n");
        return 1;
    }

    DCB dcbSerialParams = { 0 };
    dcbSerialParams.DCBlength = sizeof(dcbSerialParams); //取得当前串口的配置参数
    if (!GetCommState(hSerial, &dcbSerialParams)) {
        printf("获取串口配置参数失败\n");
        CloseHandle(hSerial);
        return 1;
    }

    dcbSerialParams.BaudRate = CBR_9600; //设置波特率为9600
    dcbSerialParams.ByteSize = 8; //设置数据位为8
    dcbSerialParams.StopBits = ONESTOPBIT; //设置停止位为1
    dcbSerialParams.Parity = NOPARITY; //设置无奇偶校验
    if (!SetCommState(hSerial, &dcbSerialParams)) { //应用新的配置参数
        printf("设置串口配置参数失败\n");
        CloseHandle(hSerial);
        return 1;
    }

    char buffer[256];
    memset(buffer, 0, sizeof(buffer));

    while (1) {
        printf("请输入命令(on/off)：");
        fgets(buffer, sizeof(buffer), stdin); //从控制台读取输入命令
        buffer[strlen(buffer) - 1] = '\0'; //去掉换行符

        if (strcmp(buffer, "on") == 0 || strcmp(buffer, "off") == 0) { //判断命令是否为on或off
            DWORD dwBytesWritten = 0;
            WriteFile(hSerial, buffer, strlen(buffer), &dwBytesWritten, NULL); //将命令发送给开发板
            char receiveBuffer[256];
            memset(receiveBuffer, 0, sizeof(receiveBuffer));
            DWORD dwBytesRead = 0;
            ReadFile(hSerial, receiveBuffer, sizeof(receiveBuffer), &dwBytesRead, NULL); //接收从开发板返回的数据
            printf("%s\n", receiveBuffer);
        }
        else {
            printf("command error\n");
        }
    }

    CloseHandle(hSerial); //关闭串口
    return 0;
}

需要注意的是，COM1是开发板所连接的串口号，如果不是COM1，请修改该参数。