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SN65HVD72: Differential bus termination resistor in slave device

Rishav kumar verma
Prodigy 110 points
Part Number: SN65HVD72

Hi team,

We have designed an RS485 device that will act as a slave device, as per the attached schematic,

Please suggest your inputs on the bus termination resistor R20 and R26, and their overall impact on the network when ~32 devices are connected to the network, should we mount that resistor or not?

 

Is our Termination resistor topology creating this stability issue? What will be the appropriate Termination Resistor Placement method as per RS-485 Guidelines in such type of networks?

What is the most appropriate value for the termination resistors on the network?

  • +1
    TI__Mastermind 40136 points

    Hi Rishav,

    This receiver device has a fail-safe high feature that allows the receiver output to go HIGH in the case of any bus-idle, bus-short, or bus-open circuitry- this means that the external fail-safe resistors are not required, but still can be used to increase noise immunity. Its important to note that if external fail-safe resistors are to be used, they are only at one node.

    Since this device can support up to 200 nodes, we can calculate the estimated impedance to be 75Kohms- 200*375. If we use 820Ohms for the external fail safe resistors, we are still able to satisfy the requirement of >=375Ohms minimum impedance (1(32/75k+1/820)). For more information on this implementation, please refer to this app note: 

    https://www.ti.com/lit/an/slyt324/slyt324.pdf

     

    Regards,

    Jack