RS-232 Communication
====================

.. note::
   The RS-232 interface requires a common ground connection between the USBoard-USS4 and the computer.

The RS-232-interface runs at 19200 Baud.
Communication via RS-232 uses the same format as the
:ref:`us_can` except for the following addition:

Every message **from** the USBoard-USS4 begins with a start byte which must have the value ``0xFF``.
After each message, made up of eight data bytes, a 16 bit checksum is sent.
Because of this a complete message is 11 bytes long.

======== ========== ================== =================
Byte 1   Bytes 2-9  Byte 10            Byte 11
======== ========== ================== =================
``0xFF`` data bytes checksum high byte checksum low byte
======== ========== ================== =================

The checksum is calculated with an CRC-CCITT algorithm using 8 databytes.

Messages **to** the USBoard-USS4 only contain the data bytes.


Example implementation of the checksum calulation (C code):

.. literalinclude:: ../checksum.c
   :language: c