串口文件传输协议 - X/Y MODEM

Modem控制字符：

• NUL: 0x00
• STX: 0x02
• SOH: 0x04
• ACK: 0x06
• NAK: 0x15
• CAN: 0x18
• C: 0x43
• G: 0x47

Modem帧格式：

head	blk	255 - blk	data	check
1 byte	1byte	1byte	128/1024 byte	1byte sun / 2byte crc

Modem协议特点：

• 文件传输由Receiver首先发送控制字符触发；
• Sender数据包校验方式由收到的控制字符决定（NAK，sum校验；C，crc校验）；
• Sender数据包数据长度由包头决定（SOH，128 byte数据；STX，1024 byte数据）；
• 传输最后一包数据报文不足128/1024 byte时，需要使用NUL填充到128/1024 byte；
• 文件数据传输包允许混用128 byte（SOH）和1024 byte（STX）数据长度；
• Receiver收到正确数据包（帧格式、块号、校验均正确）回复ACK，否则回复NAK；
• Sender收到ACK发送下包报文（数据或控制字符），收到NAK需重发本包报文（数据或控制字符）；
• YModem增加了文件名、文件大小信息传输，支持一次传输多个文件；
• YModem-g允许Receiver端不发送ACK应答，以提高传输效率；

一、XModem时序

二、XModem-CRC时序

三、XModem-1K时序

四、YModem时序

五、YModem-g时序

六、Modem校验算法

• SUM校验

uint8_t em_modem_check_sum(const uint8_t *data, uint16_t length)
{
    uint8_t sum_result = 0x00;

    for (uint16_t i = 0; i < (length - 1); i++)
    {
        sum_result += data[i];
    }

    return sum_result;
}

• CRC校验

static const uint16_t EM_MODEM_CRC16_TABLE[256] = {
    0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7,
    0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad, 0xe1ce, 0xf1ef,
    0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6,
    0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de,
    0x2462, 0x3443, 0x0420, 0x1401, 0x64e6, 0x74c7, 0x44a4, 0x5485,
    0xa56a, 0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d,
    0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4,
    0xb75b, 0xa77a, 0x9719, 0x8738, 0xf7df, 0xe7fe, 0xd79d, 0xc7bc,
    0x48c4, 0x58e5, 0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823,
    0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969, 0xa90a, 0xb92b,
    0x5af5, 0x4ad4, 0x7ab7, 0x6a96, 0x1a71, 0x0a50, 0x3a33, 0x2a12,
    0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a,
    0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41,
    0xedae, 0xfd8f, 0xcdec, 0xddcd, 0xad2a, 0xbd0b, 0x8d68, 0x9d49,
    0x7e97, 0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70,
    0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78,
    0x9188, 0x81a9, 0xb1ca, 0xa1eb, 0xd10c, 0xc12d, 0xf14e, 0xe16f,
    0x1080, 0x00a1, 0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067,
    0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c, 0xe37f, 0xf35e,
    0x02b1, 0x1290, 0x22f3, 0x32d2, 0x4235, 0x5214, 0x6277, 0x7256,
    0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d,
    0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
    0xa7db, 0xb7fa, 0x8799, 0x97b8, 0xe75f, 0xf77e, 0xc71d, 0xd73c,
    0x26d3, 0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634,
    0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab,
    0x5844, 0x4865, 0x7806, 0x6827, 0x18c0, 0x08e1, 0x3882, 0x28a3,
    0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a,
    0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0, 0x2ab3, 0x3a92,
    0xfd2e, 0xed0f, 0xdd6c, 0xcd4d, 0xbdaa, 0xad8b, 0x9de8, 0x8dc9,
    0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1,
    0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8,
    0x6e17, 0x7e36, 0x4e55, 0x5e74, 0x2e93, 0x3eb2, 0x0ed1, 0x1ef0};

uint16_t em_modem_check_crc(const uint8_t *data, uint16_t length)
{
    uint16_t crc_result = 0x0000;

    while (length--)
    {
        crc_result = (crc_result << 8) ^ EM_MODEM_CRC16_TABLE[(crc_result >> 8 ^ *data++) & 0xff];
    }

    return crc_result;
}