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SN65HVD251: Regarding restrictions when 120 nodes are connected

Masanori Tachibana
Genius 5350 points

Part Number: SN65HVD251

Hello support team,

Could you teach me the conditions under which the CAN communication operates well, in the case 120 SN65HVD251s are connected to the bus line?
Are there restrictions on data rate and cable length?

Best regards,
M. Tachibana

  • 0
    TI__Intellectual 2090 points

    Hi Masanori,

    There are multiple factors that may constrain the maximum functional data rate and/or the maximum cable length in your system. 

    First, total bus impedance may play a role.  Taking typical values from the SN65HVD251 data sheet for bus input resistance (70Kohms) and bus input capacitance (10pF), along with a 60-ohm bus termination resistance, the equivalent bus impedance with 120 nodes could significantly increase the Dominant-to-Recessive edge, which will affect the total loop delay of the network.  Adding any impedance from the cabling will also contribute to slowing down the loop delay and/or introducing DC variations at separate points in the network.

    The other, perhaps more significant, factor is the physical configuration of the network.  Any stubs in the network could contribute reflections that may affect communication.  In this case, cable length and speed really become dependent on the individual network's structure.  Any information that you could provide regarding the arrangement of the nodes and the lengths of their cable connections will help us to provide a more tailored answer to your inquiry.  

    Best Regards,

    Max Megee

  • 0 in reply to Max Megee
    Genius 5350 points
    Hello Max-san,

    Thank you for teaching in detail.

    Our customer asked us how to check pseudo operation in case of connecting 120 pieces of SN65HVD251.
    So I proposed to constitute point-to-point communication using a pair of SN65HVD251 and to add 118 of differential input capacities (10 pF x 118 pcs) and 118 of differential input resistance (40 kΩ / 118 pcs) between them.
    In addition, I advised this dummy circuit wasn't considering that capacitance and resistance of the cable.

    Sincerely,
    M. Tachibana