Overview

The reference section contains a link to the Davis protocol specification. With it, you can request data and change the console settings. I'll use the WeatherLink software for the initial configuration so I only need to implement the LOOP packet to retrieve the current observations. Other packet types are available to retrieve archive, hi/lo and total data but those features are better handled downstream.

Serial I/O

There are dozens of ways to structure the code to poll the console. The best method will depend on your application so here I'll just present a simple example. First, we'll open the serial port with a function from the last section and close it again when we're done.

...

#define PROG "Example"

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

	int err;				
				
	for (err = -1 ; err ; err++) {
		if (usb_open()) {
			fprintf(stderr, "%s: Can't open USB port\r\n", PROG);
			break;
			}

			// poll console here
			...
		
		}
	usb_close();	

	return (int) (err);
}			

Polling the Console

There's a good chance the console will be asleep so before requesting data we need to wake it.

int vp2_wake() {

    int err, retry_count;
    
    for (retry_count = 0 ; retry_count < 3 ; retry_count++) {
        for (err = -1 ; err ; err++) {
            if (usb_read("\n") != 2)
                break;
            if (strncmp(buf, "\n\r", 2) != 0)
                break;
            }
        if (err == 0)
            break;
        sleep(1);
        }
    return (int) (retry_count < 3) ? (0) : (-1);
}

If the console responded, we're ready to go ahead and request a LOOP packet with the current observations. The response should be exactly 100 bytes. If it isn't we already know there's a problem so we return an error.

int vp2_loop() {

    int err;

    for (err = -1 ; err ; err++)
        if (usb_read("LOOP 1\n") != 100)
            break;

    return (int) (err);
}

If the received packet length is ok we still need to do some checking to make sure it's valid. The first five bytes are markers, the rest we check by implementing the Davis CRC calculation.

unsigned short crc_table [] = {
        0x0,  0x1021,  0x2042,  0x3063,  0x4084,  0x50a5,  0x60c6,  0x70e7,
        0x8108,  0x9129,  0xa14a,  0xb16b,  0xc18c,  0xd1ad,  0xe1ce,  0xf1ef,
        0x1231,  0x210,  0x3273,  0x2252,  0x52b5,  0x4294,  0x72f7,  0x62d6,
        0x9339,  0x8318,  0xb37b,  0xa35a,  0xd3bd,  0xc39c,  0xf3ff,  0xe3de,
        0x2462,  0x3443,  0x420,  0x1401,  0x64e6,  0x74c7,  0x44a4,  0x5485,
        0xa56a,  0xb54b,  0x8528,  0x9509,  0xe5ee,  0xf5cf,  0xc5ac,  0xd58d,
        0x3653,  0x2672,  0x1611,  0x630,  0x76d7,  0x66f6,  0x5695,  0x46b4,
        0xb75b,  0xa77a,  0x9719,  0x8738,  0xf7df,  0xe7fe,  0xd79d,  0xc7bc,
        0x48c4,  0x58e5,  0x6886,  0x78a7,  0x840,  0x1861,  0x2802,  0x3823,
        0xc9cc,  0xd9ed,  0xe98e,  0xf9af,  0x8948,  0x9969,  0xa90a,  0xb92b,
        0x5af5,  0x4ad4,  0x7ab7,  0x6a96,  0x1a71,  0xa50,  0x3a33,  0x2a12,
        0xdbfd,  0xcbdc,  0xfbbf,  0xeb9e,  0x9b79,  0x8b58,  0xbb3b,  0xab1a,
        0x6ca6,  0x7c87,  0x4ce4,  0x5cc5,  0x2c22,  0x3c03,  0xc60,  0x1c41,
        0xedae,  0xfd8f,  0xcdec,  0xddcd,  0xad2a,  0xbd0b,  0x8d68,  0x9d49,
        0x7e97,  0x6eb6,  0x5ed5,  0x4ef4,  0x3e13,  0x2e32,  0x1e51,  0xe70,
        0xff9f,  0xefbe,  0xdfdd,  0xcffc,  0xbf1b,  0xaf3a,  0x9f59,  0x8f78,
        0x9188,  0x81a9,  0xb1ca,  0xa1eb,  0xd10c,  0xc12d,  0xf14e,  0xe16f,
        0x1080,  0xa1,  0x30c2,  0x20e3,  0x5004,  0x4025,  0x7046,  0x6067,
        0x83b9,  0x9398,  0xa3fb,  0xb3da,  0xc33d,  0xd31c,  0xe37f,  0xf35e,
        0x2b1,  0x1290,  0x22f3,  0x32d2,  0x4235,  0x5214,  0x6277,  0x7256,
        0xb5ea,  0xa5cb,  0x95a8,  0x8589,  0xf56e,  0xe54f,  0xd52c,  0xc50d,
        0x34e2,  0x24c3,  0x14a0,  0x481,  0x7466,  0x6447,  0x5424,  0x4405,
        0xa7db,  0xb7fa,  0x8799,  0x97b8,  0xe75f,  0xf77e,  0xc71d,  0xd73c,
        0x26d3,  0x36f2,  0x691,  0x16b0,  0x6657,  0x7676,  0x4615,  0x5634,
        0xd94c,  0xc96d,  0xf90e,  0xe92f,  0x99c8,  0x89e9,  0xb98a,  0xa9ab,
        0x5844,  0x4865,  0x7806,  0x6827,  0x18c0,  0x8e1,  0x3882,  0x28a3,
        0xcb7d,  0xdb5c,  0xeb3f,  0xfb1e,  0x8bf9,  0x9bd8,  0xabbb,  0xbb9a,
        0x4a75,  0x5a54,  0x6a37,  0x7a16,  0xaf1,  0x1ad0,  0x2ab3,  0x3a92,
        0xfd2e,  0xed0f,  0xdd6c,  0xcd4d,  0xbdaa,  0xad8b,  0x9de8,  0x8dc9,
        0x7c26,  0x6c07,  0x5c64,  0x4c45,  0x3ca2,  0x2c83,  0x1ce0,  0xcc1,
        0xef1f,  0xff3e,  0xcf5d,  0xdf7c,  0xaf9b,  0xbfba,  0x8fd9,  0x9ff8,
        0x6e17,  0x7e36,  0x4e55,  0x5e74,  0x2e93,  0x3eb2,  0xed1,  0x1ef0,
};

int vp2_check() {

    int err, i;
    unsigned short crc = 0;
    unsigned char *ptr = (unsigned char *) buf;

    for (err = -1 ; err ; err++) {
        if (buf[0] != 0x06)             // the first byte should be ACK
            break;
        if (buf[1] != 'L')
            break;
        if (buf[2] != 'O')
            break;
        if (buf[3] != 'O')
            break;
        if (buf[5] != 0)                // packet format should be 0
            break;
        for (i = 1 ; i < 100 ; i++) {   // calculate the CRC for the packet
            crc = crc_table[(crc >> 8) ^ ptr[i]] ^ (crc << 8);
            }
        crc = ((crc >> 8) & 0xFF) | ((crc << 8) & 0xFF00);
        if (crc != 0 ) {
            err_crc++;
            break;
            }
        }
    return (int) (err);
}

If we made it this far, we're reasonably sure there's a valid packet of observations in the buffer. From here we can use the Davis protocol specification document to map the data into any format we need.