"""Quick diagnostic — just print what comes over serial."""
import sys
import serial

if len(sys.argv) < 2:
    print(f"Usage: python {sys.argv[0]} <COM_PORT>")
    sys.exit(1)

ser = serial.Serial(sys.argv[1], 115200, timeout=2)
print(f"Listening on {sys.argv[1]}...")

while True:
    raw = ser.readline()
    if raw:
        line = raw.decode("utf-8", errors="replace").strip()
        prefix = line[:20] if len(line) > 20 else line
        print(f"[{len(line):5d} chars] {prefix}...")
    else:
        print("(timeout — no data)")