pc_side_serial_test.py

import serial
import time

send_time = 4  # sec after starting
sent = False

START_TIME = time.monotonic()
with serial.Serial('/dev/ttyACM0', 19200, timeout=1) as ser:
    while True:
        if not sent:
            if time.monotonic() - START_TIME >= send_time:
                print("sending")
                ser.write(b'q\r\n')
                sent = True

        line = ser.readline()  # read a '\n' terminated line
        if line:
            print(line)