libmodbus - reading from many slave_ids via RTU

gistfile1.c

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <modbus.h>

#define SM4_ID  3
#define P120_ID 2

int main(int argc, char *argv[])
{
    modbus_t *ctx;
    uint16_t buf[4];
    int registers_address;

 
    ctx = modbus_new_rtu("/dev/ttyUSB0", 9600, 'N', 8, 1);
    modbus_connect(ctx);

    modbus_set_slave(ctx, SM4_ID);
    registers_address = 4000;
    memset(buf, 0, 4*sizeof(uint16_t));
    if (modbus_read_registers(ctx, registers_address, 2, buf) == -1) {
	printf("Read failed: %s\n",
	       modbus_strerror(errno));
	goto close;
    }

    usleep(500 * 1000);

    modbus_set_slave(ctx, P120_ID);
    registers_address = 7000;
    memset(buf, 0, 4*sizeof(uint16_t));
    if (modbus_read_registers(ctx, registers_address, 2, buf) == -1) {
	printf("Read failed: %s\n",
	       modbus_strerror(errno));
    }

close:
    /* Close the connection */
    modbus_close(ctx);
    modbus_free(ctx);

    return 0;
}