Serial checksum.

[Rupponen]

# bytes
bytes_msg_without_checksum = b'0\xc6\xf7UV\xac\x03A\xbd\xc2\x8fB\x12\xae\x14Dz\x81XA\x00\x8b\xde?\x95\xe4\xe7\x04\xd2'
bytes_msg_with_checksum = b'0\xc6\xf7UV\xac\x03A\xbd\xc2\x8fB\x12\xae\x14Dz\x81XA\x00\x8b\xde?\x95\xe4\xe7\x04\xd2\x9e'
# hex strings
hex_msg_without_checksum = '30c6f75556ac0341bdc28f4212ae14447a815841008bde3f95e4e704d2'
hex_msg_with_checksum = '30c6f75556ac0341bdc28f4212ae14447a815841008bde3f95e4e704d29e'


def checksum(msg: bytes | str) -> bytes | str | None:
    """
    Calculates the 8-bit checksum of bytes or a hexadecimal string.
    :param msg: Bytes or hex string to sum
    :return: One bytes or one byte hex string of the sum, or None if msg type is not bytes or hex string
    :note: The last 256 modulo is for a message content with 0, summed as 0x100
    """
    m = 256  # 8-bit checksum modulo value
    msg_type = type(msg)
    if msg_type is bytes:
        return ((m - sum(msg) % m) % m).to_bytes(1, 'little')
    elif msg_type is str:
        return f'{((m - (sum(bytes.fromhex(msg))) % m) % m):02x}'
    else:
        return None


def main():
    print(checksum(bytes_msg_without_checksum))
    print(checksum(bytes_msg_with_checksum))
    print(checksum(hex_msg_without_checksum))
    print(checksum(hex_msg_with_checksum))
    print(checksum(1.23))


if __name__ == '__main__':
    main()

[easytarget]

By co-incedence I have a need for just such a function on something I have planned (which has a 9m serial cable running up my boats mast, checksumming that is on my ideas list) . Thank you!

[peterhinch]

It's perhaps worth noting that this type of checksum, while widely used, is not very robust. The checksum may not detect a message containing two single bit errors in the same bit position. CRC algorithms (cyclic redundancy code) are more heavy-duty.

[Rupponen]

Thanks Peter,
I am aware of this "one bit" checking, but the customer has thousands installations since 80-era, where serial stream (commands and responses) are flavored with this simple checksum.
Saluere optimum,

• Asko.

[scruss]

If you want the code in a usable form and not broken by copypasta, here it is:

# bytes
bytes_msg_without_checksum = b"0\xc6\xf7UV\xac\x03A\xbd\xc2\x8fB\x12\xae\x14Dz\x81XA\x00\x8b\xde?\x95\xe4\xe7\x04\xd2"
bytes_msg_with_checksum = b"0\xc6\xf7UV\xac\x03A\xbd\xc2\x8fB\x12\xae\x14Dz\x81XA\x00\x8b\xde?\x95\xe4\xe7\x04\xd2\x9e"

# hex strings
hex_msg_without_checksum = (
    "30c6f75556ac0341bdc28f4212ae14447a815841008bde3f95e4e704d2"
)
hex_msg_with_checksum = (
    "30c6f75556ac0341bdc28f4212ae14447a815841008bde3f95e4e704d29e"
)


def checksum(msg: bytes | str) -> bytes | str | None:
    """
    Calculates the 8-bit checksum of bytes or a hexadecimal string.
    :param msg: Bytes or hex string to sum
    :return: One bytes or one byte hex string of the sum, or None if msg type is not bytes or hex string
    :note: The last 256 modulo is for a message content with 0, summed as 0x100
    """
    m = 256  # 8-bit checksum modulo value
    msg_type = type(msg)
    if msg_type is bytes:
        return ((m - sum(msg) % m) % m).to_bytes(1, "little")
    elif msg_type is str:
        return f"{((m - (sum(bytes.fromhex(msg))) % m) % m):02x}"
    else:
        return None


def main():
    print(checksum(bytes_msg_without_checksum))
    print(checksum(bytes_msg_with_checksum))
    print(checksum(hex_msg_without_checksum))
    print(checksum(hex_msg_with_checksum))
    print(checksum(1.23))


if __name__ == "__main__":
    main()

[Rupponen]

Thanks scruss, fixed.