Send and Receive¶
Bus On / Bus Off¶
When the CAN controller is on bus, it is receiving messages and is sending acknowledge bits in response to all correctly received messages. A controller that is off bus is not taking part in the bus communication at all.
When you have a canlib.canlib.Channel object, use
busOn to go on bus and busOff
to go off bus.
If you have multiple Channel objects to the same controller,
the controller will go off bus when the last of the Channel
objects go off bus (i.e. all Channel objects must be off bus
for the controller to be off bus). You can use
readStatus and watch the flag
BUS_OFF to see if the controller has gone off
bus.
You can set a channel to silent mode by using the SILENT mode
if you want it to be on-bus without interfering with the traffic in any way,
see CAN Driver Modes.
This example opens a channel, takes it on-bus, then takes it off-bus and closes it:
>>> from canlib import canlib
... with canlib.openChannel(channel=1) as ch:
... ch.busOn()
... ...
... ch.busOff()
Reading Messages¶
Incoming messages are placed in a queue in the driver. In most cases the
hardware does message buffering as well. You can read the first message in the
queue by calling read, which will raise the exception
canlib.canlib.CanNoMsg if there was no message available.
The flags attribute of the Frame returned by
read contains a combination of the
MessageFlag flags, including FDF,
BRS, and ESI if the
CAN FD protocol is enabled, and error flags such as
OVERRUN which provides you with more information
about the message; for example, a frame with a 29-bit identifier will have the
EXT bit set, and a remote frame will have the
RTR bit set. Note that the flag argument is a
combination of the MessageFlag, so more than one flag might be
set.
See CAN Frames for more information.
Sometimes it is desirable to have a peek into the more remote parts of the queue. Is there, for example, any message waiting that has a certain identifier?
If you want to read just a message with a specified identifier, and throw all others away, you can call
readSpecificSkip. This routine will return the first message with the specified identifier, discarding any other message in front of the desired one.If you want to wait until a message arrives (or a timeout occurs) and then read it, call
readwith a timeout.If you want to wait until there is at least one message in the queue with a certain identifier, but you don’t want to read it, call
readSyncSpecific.
The following code fragment reads the next available CAN message, (using default bitrate 500 kbit/s):
>>> from canlib import canlib
... with canlib.openChannel(channel=0) as ch:
... ch.busOn()
... frame = ch.read(timeout=1000)
... ch.busOff()
>>> frame
Frame(id=709, data=bytearray(b'\xb5R'), dlc=2, flags=<MessageFlag.STD: 2>, timestamp=3)
Acceptance Filters¶
You can set filters to reduce the number of received messages. CANlib supports
setting of the hardware filters on the CAN interface board. This is done with
the canlib.canlib.Channel.canAccept function.
You set an acceptance code and an acceptance mask which together determine which CAN identifiers are accepted or rejected.
If you want to remove an acceptance filter, call
canlib.canlib.Channel.canAccept with the mask set to
NULL_MASK.
To set the mask to 0xF0 and the code to 0x60:
>>> from canlib import canlib
>>> ch = canlib.openChannel(channel=0)
>>> ch.canAccept(0x0f0, canlib.AcceptFilterFlag.SET_MASK_STD)
>>> ch.canAccept(0x060, canlib.AcceptFilterFlag.SET_CODE_STD)
>>> ...
>>> ch.close()
This code snippet will cause all messages having a standard (11-bit) identifier with bit 7 - bit 4 in the identifier equal to 0110 (binary) will pass through. Other messages with standard identifiers will be rejected.
How acceptance filters can be used in a smaller project:
>>> from canlib import canlib
>>> ch = canlib.openChannel(channel=0)
>>> # The acceptance filter only have to be called once for each ch object
>>> ch.canAccept(0x0f0, canlib.AcceptFilterFlag.SET_MASK_STD)
>>> ch.canAccept(0x060, canlib.AcceptFilterFlag.SET_CODE_STD)
>>> ...
>>> # We can now run the rest of the program and the acceptance filter
>>> # will reject unwanted CAN messages.
>>> while(True):
>>> frame = ch.read()
>>> ...
>>> ...
Code and Mask Format¶
Explanation of the code and mask format used by the canlib.canlib.Channel.canAccept function:
A binary 1 in a mask means “the corresponding bit in the code is relevant” A binary 0 in a mask means “the corresponding bit in the code is not relevant” A relevant binary 1 in a code means “the corresponding bit in the identifier must be 1” A relevant binary 0 in a code means “the corresponding bit in the identifier must be 0”
In other words, the message is accepted if ((code XOR id) AND mask) == 0.
Sending Messages¶
You transmit messages by calling canlib.canlib.Channel.write. Outgoing CAN messages are buffered
in a transmit queue and sent on a First-In First-Out basis. You can use
canlib.canlib.Channel.writeSync to wait until the messages in the queue have been sent.
Sending a CAN message:
>>> from canlib import canlib, Frame
... with canlib.openChannel(channel=0) as ch:
... ch.busOn()
... frame = Frame(id_=234, data=[1,2])
... ch.write(frame)
... ch.busOff()
Using Extended CAN (CAN 2.0B)¶
“Standard” CAN has 11-bit identifiers in the range 0 - 2047. “Extended” CAN,
also called CAN 2.0B, has 29-bit identifiers. You specify which kind of
identifiers you want to use in your call to canWrite(): if you set the
EXT flag in the flag argument, the message will be
transmitted with a 29-bit identifier. Conversely, received 29-bit-identifier
messages have the EXT flag set.
The following code fragment sends a CAN message on an already open channel. The CAN message will have identifier 1234 (extended) and DLC = 8. The contents of the data bytes will be whatever the data array happens to contain:
>>> frame = Frame(id_=1234, data=[1,2,3,4,5,6,7,8], flags=canlib.MessageFlag.EXT)
>>> frame
Frame(id=1234, data=bytearray(b'\x01\x02\x03\x04\x05\x06\x07\x08'), dlc=8, flags=<MessageFlag.EXT: 4>, timestamp=None)
>>> ch.write(frame)
Object Buffers¶
Object buffers are currently not supported in the Python wrapper.