modbus->crc = modbus_serial_rtu_crc(0xFFFF, &slave_addr, sizeof(slave_addr));
modbus->crc = modbus_serial_rtu_crc(modbus->crc, function_code, 1);
modbus->crc = modbus_serial_rtu_crc(0xFFFF, &slave_addr, sizeof(slave_addr));
modbus->crc = modbus_serial_rtu_crc(modbus->crc, function_code, 1);