Send a CNC program over RS-232 with Python (DNC / drip-feed)

"""Send a CNC program over RS-232 (DNC / drip-feed style) with pyserial.

Defaults match a common FANUC setup: 4800 baud, 7 data bits, even parity,
2 stop bits, XON/XOFF flow control. Match YOUR control's I/O parameters,
and put the control in READ / tape mode BEFORE running this.

Usage:  python send_program.py O1234.nc COM3
        python send_program.py O1234.nc /dev/ttyUSB0 --baud 9600
"""

import argparse
import sys
import time

import serial  # pip install pyserial


def main():
    ap = argparse.ArgumentParser(description=__doc__)
    ap.add_argument("file", help="program file to send")
    ap.add_argument("port", help="serial port, e.g. COM3 or /dev/ttyUSB0")
    ap.add_argument("--baud", type=int, default=4800)
    ap.add_argument("--settle", type=float, default=2.0,
                    help="seconds to wait after opening the port")
    args = ap.parse_args()

    try:
        with open(args.file, "rb") as fh:
            data = fh.read()
    except OSError as e:
        sys.exit(f"Cannot read {args.file}: {e}")

    ser = serial.Serial(
        port=args.port,
        baudrate=args.baud,
        bytesize=serial.SEVENBITS,
        parity=serial.PARITY_EVEN,
        stopbits=serial.STOPBITS_TWO,
        xonxoff=True,   # the control pauses us with XOFF when its buffer fills
    )

    print(f"Port open ({args.port} @ {args.baud}). "
          f"Waiting {args.settle}s - start READ on the control now.")
    time.sleep(args.settle)

    sent = 0
    try:
        for line in data.splitlines(keepends=True):
            ser.write(line)
            sent += len(line)
            print(f"\r{sent}/{len(data)} bytes", end="", flush=True)
        ser.flush()
        print("\nDone. Let the control finish reading before you close anything.")
    finally:
        ser.close()


if __name__ == "__main__":
    main()