I'm writing an Arduino program to communicate with another module using RS485, so I need to control data flow with one pin. But when I set the pin to HIGH (line 88) it almost instantly falls (as you can see in the image, line 70). So my guess is that Arduino Serial.write() function is non-blocking, but I need it to be blocking 
so the flow control pin stays high during whole packet transmit. Please help. Thank you.
You are right: Serial.write() is non blocking: it puts the data in a
buffer and waits for an interrupt routine to send it through the UART.
If you want to wait for the transmission to complete, you can call
Serial.flush().
answered Apr 14 '16 at 8:04
{}button. It makes things easier for those who want to help you. – Edgar Bonet Apr 14 '16 at 7:59