I have several ECAN within the PIC18 and PIC24 (on OpenCan) with Can Transceiver attached to the CAN Bus network. In event one module send a message and received by other modules (within ECAN), will all ECAN do CRC check and if passed, make dominate bit or just one one of many make this response?. In other words, does PIC ECAN make ACK response even the message is not assigned for that module?
CAN controllers generate dominant ACK bits if they receive the frame without any errors. ID filtering takes place after that. So yes, the CAN controller generates ACK even for the frames it's not interested in.
If a transmitter detects dominant ACK bit, it concludes that at least one node in the bus has received the frame correctly. However, it's not possible to determine if this receiver was the intended one.
As far as I understand, ACK bit makes it possible for a transmitter to self-check. A transmitter can think "If no one hears my message, then I should be the one having problems." if it samples recessive ACK bits. The reception of the message by the intended node should be checked by higher layer protocols, like CANopen.
Transmitter node transmits CAN MSG and monitors the bus for a dominant bit in the ack. slot. Receiver if receives the message correctly, will overwrite the ack. bit and make it dominant. If it does not receive the message correctly, it will not overwrite the ack. slot. Then the transmitter knows that one node has not received the message correctly because it will detect a dominant bit written by the other nodes and assume that all the nodes have received the correct message. Even if one node does not receive the data correctly the message is retransmitted by the transmitter.
Check if you can successfully transmit CAN messages. The problem you could have is in receiving messages. When you send a message to PIC, the message is not received. The message received flag is never set. You have to check that a message is being sent with the scope, check if your PIC stores it. Check which mode is it in, I assume 0, and if it is configured to receive all messages, even with errors.
Check on the scope if the PIC sends and receives the Ack response. When a message is then transmitted back to the pic, check if it sends an Ack response or receives the message!
CAN is a broadcast network so a node does not really know how many other nodes share the bus with it.
With that manner, all the nodes shall do the CRC check and ACK whether the messages are "assigned" (supposed to be received in application layer) by the listened node or not.
There are no conflict, since if there a error with CRC or ACK, all listened nodes shall send (active or passive) Error Frame which are same form from every nodes.
I recommend you to refer this excellent article:
http://www.copperhilltechnologies.com/can-bus-guide-error-flag/
Related
The first time I skimmed the zeromq docs, I assumed that the sender high watermark was there to ensure that the sender did not get too far ahead of the receiver. Now that I'm looking at it more carefully, it seems that this can't possibly be true, since the wire protocol doesn't have any concept of ACKs so the sender can't know whether the receiver is keeping up or is way behind. After staring at jeromq code in the debugger for way too long, it seems that the watermark is actually a purely "within-same-process" mechanism to ensure that the application thread that's writing to the ZMQ socket does not get too far ahead of the background thread that's responsible for taking messages off the ZMQ socket and writing bytes into the OS's TCP socket.
It seems like a rather fringe thing to worry about, relative to how much attention it's given in the docs. It doesn't even seem like a great way to control memory usage, because if you have a high water mark of 10, then 15 messages of 2kb each is not allowed, but 5 messages of 100 megs each is allowed, so things are still pretty un-predictable.
Am I understanding all this correctly or am I hopelessly confused.
I think that another thing that says it's not to prevent a sender getting too far ahead of the receiver is that if one set the HWM to 0, that's taken as infinity not actually zero. For 0 to mean zero, it'd have to have some too-ing and fro-ing with the receiver to know whether the socket was actually empty throughout the whole connection.
I wish that 0 did mean zero, because then ZeroMQ could implement both Actor Model and Communicating Sequential Processes architectures. But it doesn't, so it can't.
Possible Uses
None the less, a potential useful aspect is related to the fact that ZeroMQ is Actor Model. Suppose one were sending messages, and it kind of mattered whether or not those messages got through. In the situation where the link has collapsed (something that ZeroMQ's heartbeat can tell you, pretty quickly), messages already sent are potentially lost forever. However, if the HWM is being used to throttle the rate of messages being sent by the application, then the number of lost messages when the link breaks is minimised.
Obviously with CSP - the perfect architecture so far as I'm concerned! - you lose no messages (because the acts of sending and receiving are an execution rendezvous; the send won't complete until the receive has also completed).
What I have done in the past is to queue up messages for transmission in the sending application, sending them as and when the socket / connection can ingest them. Having the outbound message queue in the sending application's control (instead of in ZeroMQ's control) means that sender state can potentially get ahead of the transfer of messages, but still recover easily from a network connection fault.
I have written systems where a sender has a choice of two pathways to send messages through - prime and spare - and if the link to prime has collapsed the sender continues to send to spare instead. Having queued the messages inside the application and not in the socket allows the sender's state can get ahead of the actual transfer of messages, knowing that if a link goes down it's still got all the unsent outboud messages that have been generated in the meantime. These can then be directed at spare instead, without having to rewind the sender's internal state (which could be really tricky) to the last known successful transfer.
Something like that, anyway.
"Why not send to both prime and spare anyway?" is a valid question. Well, sometimes things can be complicated...
Using Omnet++ and INET framework.
I want to see the data after the transmission process with errors, noise and all channel impairments.
I found out that I can insert it in the .msg file, but it won't be affected by channel impairments.
My question is: Where can I insert the data (packet content) to be transmitted such that I can see the effect of channel impairments on it?
When a packet (for example an IPv4 packet) is transmitted between two nodes via channel with some impairments, in the destination node only the following events may occur:
The packet is received without errors, then link layer delivers it to the upper layers.
The packet is received without errors but with extended delay, then link layer delivers it to the upper layers.
The packet is received with error, then link layer discards the whole packet.
So there is no possibility that upper layer will receive the packet with errors.
I have a computer (that use as server) and several board with Atmega microcontroller Something like:
The computer connect to board on UART & RS485 (with a USB to RS485 converter)(I have limitation that lead to I could not use ModBus). I want to broadcast a message from server over bus and fetch the ID from of each board (Board ID is 4 digit).
When the boards receive broadcast message and try to send their own ID and the server receive some fake ID and I think it related to Collisions problem when all boards want to send data in one time.
After I search about this problem found a way that put a constant in each board that save a special delay for send data and when board receive broadcast message send ID with that delay...in this way it work fine and I dont see the Collisions but have some problem:
May be the delay number of 2 board be same.
Good way for small count of board.
Extra process when want to install board on bus.
Anybody know with this problem and could help me how to solve this problem with better solution?
You are mentioning Modbus in your question although some of your other stated facts seem to deviate from there (like 4-digit device numbers, and Modbus only has 1-255). Also, Modbus does not support responses to broadcast messages. I thus doubt a bit you are actually using Modbus.
A scheme you could use (and that is classically used in MA networks) would be:
Once a broadcast is received, have the clients scan the bus for responses for a time frame based on its station ID. If your client can see one, have it wait a minimum bus time (the time a module needs to answer your broadcast message based on current bus timing + the round trip for the master acknowledging the broadcast answer) plus an additional time based on its module ID, then go back to (1)
If a client sees the bus unoccupied for the specified time, send back a broadcast answer.
Have the master acknowledge the broadcast response from this client with the shortest possible message.
If a client that has sent a broadcast response does not receive a proper ack, go back to (1)
This is not 100% secure and absolutely not according to the Modbus specification, but could work.
* is a transmission, - is a "wait"
**** (Bus master broadcast)
--------- station 100 waits 100ms
------------------ station 200 waits 200ms
**** Station 100 sends broadcast response
------------------ station 200 sees bus active and waits another 200ms
*** master acknowledges broadcast response of 100
------------------ station 200 sees bus active again and waits 200ms from last seen activity
**** Station 200 has seen bus quiet for 200ms and sends broadcast response
*** master acks brc response of 200
This can take quite a bit of time and needs the waiting times finely adjusted against the transmission time of broadcast responses and response acks, but can work, and actually is implemented that way in a lot of CSMA/CD networks.
It will probably take longer, but here is another way to do it. First, design your protocol so that each command contains (or an can contain) an ID, and boards only respond to commands for their ID. Then, on your host, you would iterate through each of the possible IDs and send a simple command to each of them. If you get a response, you know there is a board with that ID. If you don't get a response after some period of time, you know there is no board there.
I'm working with the Veins framework over the OMNeT++ simulator and I'm facing a weird situation where certain few nodes lose all received packets.
To put everybody in context, I'm simulating 100 nodes (4 flows of 25 nodes), all under coverage (apparently), and sending 10 packets per second each. Depending on the moment the nodes enter the network (i.e: are created by SUMO), some of them (usually just 1 but can be 2, 3, 4...) enter in a mode where all packets are marked as lost (SNIRLostPackets) as they receive a packet while another packet is already being received (according to the Decider the NIC is already synced to another frame).
That doesn't suppose to happen in 802.11 unless there are hidden nodes and the senders don't see each other at the moment of sending their respective frames (both see the the channel idle) right?
So, this behavior is not expected at all and destroys the final lost packets statistics. I tuned the transmission powers of transmission and interference range but nothing changes.
It happens too often to ignore it and I would like to know if anybody has experienced this behavior and how it was solved.
Thank you
(OK, apparently the issue comes in an special case where a packet is received (started to being received) OK but at the end of the reception, the node has changed to a TX state.
Then, the packet is marked as "received while sending" but the node has already marked this frame as the next correctly reception. So it discards all receiving ones with no end.
It seems a bug and the possible workaround is adding these lines
if (!frame->getWasTransmitting()){
curSyncFrame = 0;
}
in the processSignalEnd function (Decider80211p file), inside the "(frame->getWasTransmitting() || phy11p->getRadioState() == Radio::TX)" case.
I'm not quite sure if this is a case that whether should happen or not, as a node should not send a packet while receiving.
Hope it helps.
I have a question about the high water mark (HWM) for a ZeroMQ PUB/SUB connection. Effectively I want to set the HWM value to zero. i.e.: if a message can't be delivered, just drop it.
Unfortunately it seems that the HWM value can only be set as low as 1.
"0" means infinity according to the API docs and my testing.
In my opinion, using "0" to mean "infinite" in the API was a mistake
:/ It's probably unlikely to change.Is there a workaround that
doesn't require recompilation of ZeroMQ?
The problem I'm encountering is that with a non-zero HWM, when a connection fails, at least one message sits in the queue and is sent when the connection is re-established. By that time the message is no longer valid and shouldn't be trusted.
I've thought about discarding messages on the receiving side by including a time stamp generated at the sending side. Unfortunately the systems clocks are not connected to the internet and drift significantly. Syncing the clocks with an additional REQ/REP socket will introduce other complicated startup states to the sender and seems like an unnecessary workaround.
Yes:
. . . may try to hunt the same deer from the opposite forest
Based on the need,
declared in the initial problem motivation,
the solution may be derived from setting not the ZMQ_???HWM,
but another parameter:
ZMQ_CONFLATE: Keep only last message
Default value 0 ( false )
Applicable socket types ZMQ_PULL, ZMQ_PUSH, ZMQ_SUB, ZMQ_PUB, ZMQ_DEALERIf set, a socket shall keep only one message in its inbound / outbound queue, this message being the last message received/the last message to be sent. Ignores ZMQ_RCVHWM and ZMQ_SNDHWM options.Does not support multi-part messages, in particular, only one part of it is kept in the socket internal queue.
and some help may come from a "preventive-measure" hidden in:
ZMQ_IMMEDIATE: Queue messages only to completed connections
Applicable socket types all, only for connection-oriented transports.
By default queues will fill on outgoing connections even if the connection has not completed. This can lead to "lost" messages on sockets with round-robin routing (REQ, PUSH, DEALER).
If this option is set to 1, messages shall be queued only to completed connections. This will cause the socket to block if there are no other connections, but will prevent queues from filling on pipes awaiting connection.