read_patch

def read_feedback(ser):
    """Non-blocking state machine reader."""
    static = read_feedback.__dict__

    if "state" not in static:
        static["state"] = 0
        static["buf"] = []

    while ser.in_waiting:
        b = ord(ser.read(1))

        if static["state"] == 0:          # waiting for start byte
            if b == 0xAB:
                static["buf"] = []
                static["state"] = 1

        elif static["state"] == 1:        # collecting payload
            static["buf"].append(b)
            if len(static["buf"]) == 4:
                c1, c2, c3, cs = static["buf"]
                static["state"] = 0
                if (c1 ^ c2 ^ c3) == cs:
                    return (c1, c2, c3)
                else:
                    return None

    return None