Thank you for your information. Actually, I confirm the program in the flash memory because my program trigger some digital i/o after program start up and also I have rs232 in the same board to communicate to another board, and 232 communication is working. also I use the same USB-to-rs485 converter with ground connection to do communication. By the way, I have question for DC-COM and GND for BL4S200,I found out GND terminal on RS485 port and DC-COM from power supply are connected, and also I notice on cpu board, One pin on programming port is connected to ground ring hole on cpu board and the same as the case of ethernet. So they are grounded. Do you think in this case do I need to connect DC-COM to ground?