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TCAN1042G-Q1: The CANL voltage difference between the first half frame and the second half frame is relatively large。

ian wu
Intellectual 985 points

Part Number: TCAN1042G-Q1

hi 

In a project of our company, CAN chip TCAN1042GDRQ1 of TI company is used for CAN bus communication. When testing CAN communication, CAN communication frame waveform is found as follows. Please kindly answer:
There are voltage jump spike pulses at the head and end of frame 1
2 The CANL voltage difference between the first half frame and the second half frame is relatively large

CAN 通信调试疑问咨询-0321.pdf

could you tell me what causes the receiving and transmitting voltage pressure difference to be so large？

  • 0
    TI__Guru 53535 points

    Hi Ian,

    This scope shot appears to show two CAN frames transmitted by different ECUs. The last bit of every CAN frame is the ACK bit which is transmitted by any receiving node once the frame is fully received. In this case, the different voltage levels show us which node is transmitting. Labeling the higher voltage node 1 and the lower voltage node 2, we see node 1 transmit fist with a lower voltage ACK bit from node 2. Node 2 then transmits a frame with as met with a higher voltage ACK by node 1. 

    Seeing different Vod from different nodes is normal when monitoring a CAN network. The most common reason we will see this is due to the voltage drop across the resistive elements of the network cabling. This will cause the probed node (where the oscilloscope is connected) to appear to have a larger Vod than any remote nodes. Other reasons may include different supply voltages at each node, different drive strength of different transceiver models, or atypical series resistance on one of the ECUs. 

    In this case, both Vod measurements show that the voltage is well within the 1.5V minimum specification for CAN drivers, so I don't have any concern with this impacting the reliability of communication.  

    Let me know if you have any more questions. 

    Regards, 
    Eric Schott