Simple modbus read for MorningStar SS-MPPT-15 solar charge controller.

modbus_test.c

/*
 * This is a simple way to read data from a MorningStar SunSaver solar charge controller 
 * over modbus. Data is read into the array "tab_reg" and you can manipulate as needed. 
 * Modbus spec: https://www.morningstarcorp.com/wp-content/uploads/technical-doc-sunsaver-mppt-modbus-specification-en.pdf
 * On a debian system, you need to install the libmodbus-dev package. 
 * Compile using: gcc -I/usr/include/modbus modbus_test.c -lmodbus -o modbus_test.o
 */

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

const int REMOTE_ID = 0x01;

int main(void) 
{
    modbus_t *mb;
    uint16_t tab_reg[50];
    memset(tab_reg, 0, sizeof(tab_reg));

    mb = modbus_new_rtu("/dev/ttyUSB0", 9600, 'N', 8, 2);
    if (mb == NULL)
    {   
        fprintf(stderr, "Unable to create the libmodbus context\n");
        return -1;
    }

    modbus_set_slave(mb, REMOTE_ID);

    if(modbus_connect(mb) == -1)
    {   
        fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
        modbus_free(mb);
        return -1;
    }

    // Read all 50 registers 
    if(modbus_read_registers(mb, 8, 50, tab_reg) == -1)
    {   
        fprintf(stderr, "Modbus Error: %s\n", modbus_strerror(errno));
    }
    else
    {   
        double batteryVoltage = (double)(tab_reg[0] * 100) / 32768.0;
        double arrayVoltage = (double)(tab_reg[1] * 100) / 32768.0;
        double loadVoltage = (double)(tab_reg[2] * 100) / 32768.0;
        double chargeCurrent = (double)(tab_reg[3] * 79.16) / 32768.0;
        double loadCurrent = (double)(tab_reg[4] * 79.16) / 32768.0;
        double minBattVoltage = (double)(tab_reg[43] * 100) / 32768.0;
        double maxBattVoltage = (double)(tab_reg[44] * 100) / 32768.0;

        printf("Battery: %.3f V\n", batteryVoltage);
        printf("Array: %.3f V\n", arrayVoltage);
        printf("Load: %.3f V\n", loadVoltage);
        printf("Charging: %.3f A\n", chargeCurrent);
        printf("Consuming: %.3f A\n", loadCurrent);
        printf("Heatsink temp: %d C\n", tab_reg[5]);
        printf("Ambient temp: %d C\n", tab_reg[7]);
        printf("Charge State: %d\n", tab_reg[9]);
        printf("Min Battery Voltage: %.3f V\n", minBattVoltage);
    }

    modbus_close(mb);
    modbus_free(mb);
}