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USB2CAN Interface
=================
The `USB2CAN <http://www.8devices.com/products/usb2can>`_ is a cheap CAN interface based on an ARM7 chip (STR750FV2).
There is support for this device on Linux through the :doc:`socketcan` interface and for Windows using this
``usb2can`` interface.
Support though windows is achieved through a DLL very similar to the way the PCAN functions. The API is called CANAL
(CAN Abstraction Layer) which is a separate project designed to be used with VSCP which is a socket like messaging system
that is not only cross platform but also supports other types of devices.
Installation
------------
1. To install on Windows download the USB2CAN Windows driver. It is compatible with XP, Vista, Win7, Win8/8.1. (Written against driver version v1.0.2.1)
2. Install the appropriate version of `pywin32 <https://sourceforge.net/projects/pywin32/>`_ (win32com)
3. Download the USB2CAN CANAL DLL from the USB2CAN website.
Place this in either the same directory you are running usb2can.py from or your DLL folder in your python install.
Note that only a 32-bit version is currently available, so this only works in a 32-bit Python environment.
Internals
---------
This interface originally written against CANAL DLL version ``v1.0.6``.
Interface Layout
~~~~~~~~~~~~~~~~
- ``usb2canabstractionlayer.py``
This file is only a wrapper for the CANAL API that the interface expects. There are also a couple of constants here to try and make dealing with the
bitwise operations for flag setting a little easier. Other than that this is only the CANAL API. If a programmer wanted to work with the API directly this is
the file that allows you to do this. The CANAL project does not provide this wrapper and normally must be accessed with C.
- ``usb2canInterface.py``
This file provides the translation to and from the python-can library to the CANAL API. This is where all the logic is and setup code is. Most issues if they are found
will be either found here or within the DLL that is provided
- ``serial_selector.py``
See the section below for the reason for adding this as it is a little odd. What program does is if a serial number is not provided to the usb2canInterface file this
program does WMI (Windows Management Instrumentation) calls to try and figure out what device to connect to. It then returns the serial number of the device.
Currently it is not really smart enough to figure out what to do if there are multiple devices. This needs to be changed if people are using more than one interface.
Interface Specific Items
------------------------
There are a few things that are kinda strange about this device and are not overly obvious about the code or things that
are not done being implemented in the DLL.
1. You need the Serial Number to connect to the device under Windows. This is part of the "setup string" that configures the device. There are a few options for how to get this.
1. Use ``usb2canWin.py`` to find the serial number.
2. Look on the device and enter it either through a prompt/barcode scanner/hardcode it. (Not recommended)
3. Reprogram the device serial number to something and do that for all the devices you own. (Really Not Recommended, can no longer use multiple devices on one computer)
2. In ``usb2canabstractionlayer.py`` there is a structure called ``CanalMsg`` which has a unsigned byte array of size 8.
In the ``usb2canInterface`` file it passes in an unsigned byte array of size 8 also which if you pass less than 8
bytes in it stuffs it with extra zeros. So if the data ``"01020304"`` is sent the message would look like
``"0102030400000000"``.
There is also a part of this structure called ``sizeData`` which is the actual length of the data that was sent not
the stuffed message (in this case would be 4). What then happens is although a message of size 8 is sent to the device
only the first 4 bytes of information would be sent. This is done because the DLL expects a length of 8 and nothing
else. So to make it compatible that has to be sent through the wrapper. If ``usb2canInterface`` sent an
array of length 4 with sizeData of 4 as well the array would throw an incompatible data type error.
3. The masking features have not been implemented currently in the CANAL interface in the version currently on the USB2CAN website.
.. warning::
Currently message filtering is not implemented. Contributions are most welcome!
Bus
---
.. autoclass:: can.interfaces.usb2can.Usb2canBus
Exceptions
----------
.. autoexception:: can.interfaces.usb2can.usb2canabstractionlayer.CanalError
Miscellaneous
-------------
.. autoclass:: can.interfaces.usb2can.Usb2CanAbstractionLayer
:members:
:undoc-members:
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