can camunication using MCP2515,pca82c250 AND AT89S8252

[debabrta kar]

hi , i have a doubt regrading can communication.....during IFS(interframe space) if an illigal bit is detected then what frame will will be generated an OVERLOAD FRAME or an ERROR FRAME (for form error).....because a as per the can document IFS is of fix format so form error must generated ...so please guide me ..thank you .....

 

[embpic]

if during interframe space any illegal bits detected then then all can nodes will neglect this bits.
bcoz if any frame is completed and interframe space is filled with recessive and any kinda bits detected they will not obey the protocol.
And they will not match ID with any of node's ID.

 

[debabrta kar]

if during interfame space detected illigal bits are neglected then how the frame error occuers due to IFS...because in document of can it is written that if any illegal bit is being detected during interfame space then the fixed format destroys and frame error occures....so plz explain how it happens thank you.......

 

[embpic]

i think there is no such error frame regarding in CAN bus and if u have document where this error is mentioned post this document.

- - - Updated - - -

yes but in interframe space is normally recessive

 

[FvM]

MCP2515 datasheet specifies:

6.3 Form Error
If a node detects a dominant bit in one of the four
segments (including end-of-frame, interframe space,
acknowledge delimiter or CRC delimiter), a form error
has occurred and an error frame is generated. The
message is repeated.

 

[debabrta kar]

yes sir i was also saying the same as u have mentioned ("in 6.3 form error")...but i have the confusion that as per can document overload frame also starts after detecting a dominant bit during intermission.....so i want to know that when a dominant bit gets detected in intermission then "form error" or "overload frame " which frame will arise ....

 

[embpic]

Ok if dominant bit is detected in intermission then first overload frame is generated. It will get generated for maximum two times. And if receiver still not ready to receive frame then ERROR FRAME will get generated.

 

[debabrta kar]

ok thank you sir .....

 

[debabrta kar]

actually i read again the document can and mcp2515 can controller again today and got a point that except the CRC error if any type of error like frame error is arised then error frame get generated just after the next bit ...so now m confused again as u said that error frame will be generated after overload frame so please suggest something thank you sir.....

 

[embpic]

yes after yesterday i also read the CAN document again i found that sentence like


i was comment previous from reading one pdf which was re edited by some one so i download the document from bosch site and read again.

 

[Emb4U]

Dear Embpic Sir, Overload frame is detected when the last bit of the EOF is dominant bit and the 1st & 2nd bit of the intermission field detected as a dominant bit .CAN frame neglact that bits. There is no Form error occurs.

As per you no error frame will generate after the overload frame.
And the document which you attached here means if last bit of the intermission field will detected as a dominant, it wii interpret as a SOF but not act as a SOF.
means there is a overload flag is generated after that dominant bit which is detected in last bit of intermission field .Generally overload flag consists of 6 dominant bit
which is caused as a Stuff error.

 

[embpic]

yes your right that's why they produce superposion of overload flag that is it extend up to 12 bits of overload flag.

 

[Emb4U]

Dear Embpic sir, can u help me abut CRC ?

my question is if I am not using CRC field then which type of problem occurs ?
please tell me the effect.

 

[FvM]

How do you plan "not using CRC field"? If you send CAN messages by a CAN controller, a CRC field will be generated automatically, when receiving messages, it's automatically checked.

 

[Emb4U]

Dear FvM sir ,please help me on another one question

If two node trying to send message ,one node will won the arbitration. If the another node which has lost arbitration can causes an error or not. If there is an error then which error occurs if not then tell me why ?
thank you very much.

 

[embpic]

Yes crc field get auto filled by controller and and also others like sof , eof, etc.
The node which lost in arbitration will send the message again after node which won the arbitration.