This is usually a hardware issue. Check your cable or try a lower Baud Rate.
You will need a physical COM port or a high-quality USB-to-RS232 Serial Adapter (look for those using the FTDI chipset for the best stability). Connecting Your PC to the CNC siemens rcs 802 software download
Often bundled with the RCS functionality for 802D systems. Installation and System Requirements This is usually a hardware issue