ArCanSend Data Length

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1

Hello Community,

I have to send 11 bytes with the ArCanSend function block.

I see that the ArCanFrameType has: Data (array of 64 bytes) and Data Length.

The question is: how can I send 11 bytes if the ArCanSend FB accepts a DataLength <= 8? With values greater than 8 I have arCAN_ERR_INVALID_DATALENGTH.

This is the code I’m using:

memcpy(ADR(ArCanSenderAT2HI2.ArCanSend_0.Frame.Data), ADR(PGN_AT2HI2_Data_Byte), SIZEOF(PGN_AT2HI2_Data_Byte));
ArCanSenderAT2HI2.ArCanSend_0.Frame.DataLength := dataLength;
ArCanSenderAT2HI2.ArCanSend_0.Frame.ID := ArCanSenderAT2HI2.ID;			
ArCanSenderAT2HI2.ArCanSend_0.Frame.Format := arCAN_29BIT;						
ArCanSenderAT2HI2.ArCanSend_0.SendFrame := TRUE;

ArCanSenderAT2HI2.ArCanSend_0();

P.S.

PGN_AT2HI2_Data_Byte is an array of 11 bytes

Thank you

2

Hi @Andrea_Machina ,
Thanks for your post, now I know that CAN_lib is obsolete and should no longer be used, in new prj is recommended to use the ArCAN (time to study new library…)

The question is: how can I send 11 bytes if the ArCanSend FB accepts a DataLength <= 8? With values greater than 8 I have arCAN_ERR_INVALID_DATALENGTH.

Weird…maybe there is another issue and we are focus on wrong direction

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Have you checked our examples in the help?

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You can add them from logical view:

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I have reviewed with @marcusbnr your issue and, thanks Marcus!, the limitation is on CAN Protocol itself where is limited to a maximum payload of 8 bytes. You will not be able to send a payload of greater than 8 bytes in one frame while working with CAN Protocol.

I asked some ideas to AI: “How Can I send 11 bytes with B&R Automation library ArCAN?”

To send 11 bytes using the B&R ArCAN library, you must account for the 8-byte payload limit of Classical CAN (CAN 2.0). Because ArCAN frames are fixed-length structures, you cannot simply increase the data size beyond 8 bytes.

1. The Multi-Frame Method (Segmentation)

Since a single frame is too small, you must split your 11 bytes into two frames.

  • Frame 1: Send the first 8 bytes.

  • Frame 2: Send the remaining 3 bytes.

Implementation Steps:

  1. Define a variable of type ArCanFrameType for each frame.

  2. Use brsmemcpy to copy the first 8 bytes of your data into the .Data array of the first frame.

  3. Copy the remaining 3 bytes into the .Data array of the second frame.

  4. Call the ArCanWrite() function block twice—once for each frame.

2. Example Code (Structured Text)

Here is a simplified logic for splitting the data manually using ArCAN:

(* Frame 1: First 8 bytes *)
canWrite1.Frame.ID := 0x123;
canWrite1.Frame.DataLength := 8;
brsmemcpy(ADR(canWrite1.Frame.Data[0]), ADR(MyData[0]), 8);
canWrite1.Execute := TRUE;

(* Frame 2: Remaining 3 bytes *)
canWrite2.Frame.ID := 0x124; // Use a different ID or a sequence counter
canWrite2.Frame.DataLength := 3;
brsmemcpy(ADR(canWrite2.Frame.Data[0]), ADR(MyData[8]), 3);
canWrite2.Execute := TRUE;

Additional input from Marcus (AI didn’t tell me that…): “You have to make sure those frame IDs aren’t already used by other devices on the network“

Back to AS Samples, the third one: “Receiving and transmitting multiple CAN frames“ should be a good reference for your case.
Thanks
Ciao
Valerio

3

You mentioned PGN_AT2HI2_Data_Byte is an array of 11 bytes.
A PGN is used in CAN J1939 communication, which also uses still only 8 bytes in the can message.
You should take a look at mapp J1939: SAE J1939 network protocol

As you can see in the structures in this section, they seem also bigger than 8 bytes.
So depending on the values some J1939 conversion is done to be able to put this data in an 8 byte can message. I have no access to the J1939 documentation but for instance a REAL values is not communicated as 4 bytes, but with less bytes.

4

a reason might be a preparation for CAN FD.

‘For CAN FD, a frame is labeled with a 29-bit identifier and carries a 64-byte message payload’

1 Like
5

Similar to what Christoph mentioned, I saw that in the Error description of the error -1070584150: Invalid data length, it states that only “CAN-FD” (CAN Flexible Data-Rate) supports up to 64 bytes per frames, while standard CAN only supports up to 8 bytes per frame.

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This would make me believe that the ArCan library only supports standard CAN. Can anyone confirm this? I could not find any information in the Help documenting this, other than that it only mentions CAN 2.0A and CAN 2.0B - both limited to 8 bytes per frame.

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This may need to be investigated in a support ticket to get a final answer.

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6

Hello Everyone and thank you for all the information you sent.

@corne.geerts got the point. We are communicating with an engine that is using the J1939 protocol. All the other data sent or received works fine because it’s equal or smaller than 8 bytes.

As per what you’re saying probably the split of the frame will not work in my case.

Do you know if there’s a way to use that protocol with the standard ArCAN library instead of using the mapp component that is under license?

Thank you

7

The mapp J1939 is available in the mapp Services “Starter” package and is free of charge.
Licensing

1 Like
8

Hello, sorry to bother you again..

I’m using the MpJ1939Transmit and Receive services and everything was going pretty good.. as far as the engine asked for a broadcast communication on our side.

It looks like the broadcast address should be 255 but, in the MpJ1939Transmit configuration, we can enter values just from 0 to 253..

Any idea?

Thanks