Dear William Scott, Bill. Thank you very much. I can send the message after stoppage of receipt. The write function is executed normally. Though read function is executed normally, my program can't receive any message. I made LED message receive/transmit checker at the other MCU. The MCU seems not to send/receive message. This serial port hang happens after 2~3 days from program running. Once this happens, the serial port does not work even though I re-run the program. If I reboot or power down/up agin, the serial port works. Thank you very much.