Thread: modbus rtu c programming

Dear all:

I am trying to use some function I downloaded from the internet to connect to a modbus meter, the connection is fine, but when I try to read the data off the registers, it kept returning -1, reading unsuccessful

Code:

int readState = modbus_read_holding_registers(2, 0, 2, mbusRegisters);

Meter register map:

Address	word	Item(description)	Code	Unit	Sign
0000H	2	Watt Hour	03H	WH	Unsigned
0002H	1	Watt	03H	W	Unsigned
0003H	1	Voltage	03H	V	Unsigned
0004H	1	Currant	03H	A	Unsigned

Below is the source code of the function I used (open source) :
(from Modbus master C library/code for Linux serial device)

Code:

int modbus_read_holding_registers(unsigned char slave_id, unsigned char start_address, unsigned int num_registers, unsigned short * modbus_registers)
{
    unsigned char transmit[255];

        //check return values
        unsigned char *retValue;
        int i;
        retValue = transmit;

    transmit[0] = slave_id;
//      printf("slave_id: %c", transmit[0]);
    transmit[1] = MB_READ_HOLDING_REGISTERS;
    transmit[2] = start_address >> 8;
    transmit[3] = start_address & 0xFF;
    transmit[4] = num_registers >> 8;
    transmit[5] = num_registers & 0xFF;
    short crc = CRC16(transmit, 6);
    transmit[6] = crc;
    transmit[7] = crc >> 8;

        for (i=0; i<8; i++) {
                printf("return value [%d] = %x\n", i, *(retValue+i));
        }

    if (write(fd, transmit, 8) == -1) {
        perror("Unable to write to serial device");
        mb_datagram_status = MB_ERROR;
        return -1;
    }


    mb_waiting_for_reply = 1;

    struct timespec time;
    clock_gettime(CLOCK_REALTIME, &time);
    time.tv_sec += 1;


    /* Time out here if no bytes have been received within the time out period */
    if (pthread_cond_timedwait(&datagram_start_receiving, &waiting_for_reply, &time) == ETIMEDOUT) {
 mb_datagram_status = MB_TIMEOUT;
        timeout_error_count++;
        return -1;
    }

        printf("after pthread_cond_timedwait()\n");

    pthread_cond_wait(&datagram_finished_receiving, &waiting_for_reply);

    mb_waiting_for_reply = 0;

    if (mb_datagram_status == MB_OK) {

        unsigned char byte_count = modbus_read_num_bytes(datagram);
        mb_datagram_status = MB_OK;

        unsigned char * datagram_data = datagram + 3;

        int i;
        for(i = 0; i < byte_count / 2; i++) {
            modbus_registers[i] = datagram_data[i * 2] << 8 | datagram_data[i * 2 + 1];
        }
        return byte_count / 2;

    } else {
        return mb_datagram_status;
    }

}

Am I using the function wrong, or it's a modbus thing?

Any help is appreciated, thanks.

barramundi9

Hi, Salem, thanks for the reply, I know it's been a month ...

It was me and I did try swapping the crc bytes but it still returns -1

And I did called modbus_init() first and it was connected, it dies at

Code:

 if (pthread_cond_timedwait(&datagram_start_receiving, &waiting_for_reply, &time) == ETIMEDOUT) {
             mb_datagram_status = MB_TIMEOUT;
             timeout_error_count++;
             return -1;
  }

Any ideas?

Thanks

Originally Posted by Salem

Is this you as well?
modbus starting address question

Did you try the response you got, namely swapping over the crc bytes?

Have you called modbus_init() at the start?
Have you tried putting some prints in receive_thread() to see if it actually does anything.

I decide to rewrite the code and the following is what I have done:

Code:

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

#include <fcntl.h>
#include <unistd.h>

#include <termios.h> //set baud rate

#include <sys/select.h>
#include <sys/time.h>
#include <sys/types.h>

#define device "/dev/ttyUSB0"
#define MyBaudRate 9600
#define rec_buf_wait_2s 2

int read_data_tty (int fd, char *rec_buf, int rec_wait) {
        int retval;
        fd_set rfds;
        struct timeval tv;

        int ret, pos;
        tv.tv_sec = rec_wait;
        tv.tv_usec = 0;
        pos = 0;

        while (1) {
                FD_ZERO (&rfds);
                FD_SET (fd, &rfds);

                retval = select (fd+1, &rfds, NULL, NULL, &tv);

                if (retval == -1) {
                        perror("select()");
                        break;
                } else if (retval) {
                        ret = read(fd, rec_buf+pos, 2048);
                        pos += ret;
                        if (rec_buf[pos-2] == '\r' && rec_buf[pos-1] == '\n') {
                                FD_ZERO (&rfds);
                                FD_SET (fd, &rfds);
                                retval = select (fd+1, &rfds, NULL, NULL, &tv);

                                if (!retval) {
                                        break;
                                }
                        }
                } else {
                        break;
                }
        }

        return 1;
}

int device_485_receive (int fd) {
        ssize_t ret;
        char rec_buf[1024];
        int i;
        char *send_buf = "02030202f925";

        for (i=0; i<10; i++) {
                ret = write (fd, send_buf, strlen(send_buf));
                if (ret == -1) {
                        printf("write device %s error\n", device);
                        return -1;
                }

                if (read_data_tty(fd, rec_buf, rec_buf_wait_2s)) {
                        printf("%s\n", rec_buf);
                } else {
                        printf("read_error\n");
                }

                //if ((read(fd, rec_buf, strlen(rec_buf))) == -1) {
                //      printf("error reading string\n");
                //      return -1;
                //} else {
                //      printf("%s\n", rec_buf);
        }
        return 0;
}


int main () {

        int fd;
        //open serial port
        fd = open (device, O_RDWR);
        //check if device opened properly
        if (fd == -1) {
                printf("open device %s error\n", device);
        } else {
                //set baudrate
                struct termios options;
                cfsetispeed (&options, MyBaudRate);
                cfsetospeed (&options, MyBaudRate);
                device_485_receive(fd);
        }

        if (close(fd) != 0) {
                printf("close device error\n");
        }


        return 0;
}

the send_buf string "02 03 02 02 f9 25" (space included here for clarity)is the modbus command, 02 is the slave id, 03 is function code, the following 02 is the starting address and 02 means 2 registers are to be read and f9 25 are the crc values.

When I execute the program, it keeps returning the value 4, any ideas???

Thanks.

barramundi9

Originally Posted by barramundi9

Hi, Salem, thanks for the reply, I know it's been a month ...

It was me and I did try swapping the crc bytes but it still returns -1

And I did called modbus_init() first and it was connected, it dies at

Code:

 if (pthread_cond_timedwait(&datagram_start_receiving, &waiting_for_reply, &time) == ETIMEDOUT) {
             mb_datagram_status = MB_TIMEOUT;
             timeout_error_count++;
             return -1;
  }

Any ideas?

Thanks

Hello, oogabooga

I actually tried send both and it still returns 4.

I have given up on that and use libmodbus instead.

libmodbus.org - News
Libmodbus works like a charm.

Thanks for the replies.


barramundi9

Originally Posted by oogabooga

When you say that it "keeps returning the value 4", do you mean
that this statement in device_485_receive prints 4:

Code:

printf("%s\n", rec_buf);

What do you expect it to print?


And are you supposed to be sending characters like you have here:

Code:

        char *    send_buf = "02030202f925";

or should it be (as your first post suggests) binary data, more like this:

Code:

        unsigned char *  send_buf = "\x02\x03\x00\x02\x00\x02\xf9\x25";