Problem to read data from any server modbus

[Inatec Engenharia]

Hello,

I have to read/write data from any MODBUS TCP/IP. And I am using visual studio 2012 to make every thing.

I can write a data in address 40001-40010, but I got a return error with "rc = modbus_write_register(ctx, index, *tab_rq_registers);" function. After I trying to write one more time, I got a error like in following figure.



In case of read a data, I can not read anything even having a return error. And the error cited above, I got the same error when I try to read data by second time.

I am using the following function to read and write data.

//function to connect libmodbus
bool connect_Modbus(char *ip, int port, const char *erro){
        ctx = modbus_new_tcp(ip, port);
        if (ctx == NULL){
            success = false;
        }
        if (modbus_connect(ctx) == -1) {
            modbus_free(ctx);
            success = false;
        }else {
            success = true;
        }
    return success;   
}
//function to write data
bool writedata(int index, String^ value){
    uint16_t *tab_rq_registers;
    tab_rq_registers = (uint16_t *) malloc(1 * sizeof(uint16_t));
    memset(tab_rq_registers, 0, 1 * sizeof(uint16_t));   
   
    tab_rq_registers[0] = Convert::ToInt16(value);
    rc = modbus_write_register(ctx, index, *tab_rq_registers);
    if (rc < 0) {       
        MessageBox::Show("ERROR modbus_write_registers");       
        return false;
    } else{
        return true;
    }
}
//function to read data
uint16_t readdata(int index){   
    uint16_t *tab_rp_registers;
    tab_rp_registers = (uint16_t *) malloc(1 * sizeof(uint16_t));
    memset(tab_rp_registers, 0, 1 * sizeof(uint16_t));   

    rc = modbus_read_registers(ctx, index, 1, tab_rp_registers);
    if (rc > 0)
        return tab_rp_registers[0];
    else       
        MessageBox::Show("ERROR modbus_read_registers");   
}

Anyone can help me?

thanks

luis