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RS-485 impedance and termination requirements

Other Parts Discussed in Thread: SN65HVD72, SN65HVD75

Hello Team,

One of my customer is using RS-485 bus to communicate with multiple slaves.

Total length of bus is less than ~6ft or 1.8m. Within this distance they have 10 nodes connected through daisy chaining. Stub length at each node is less than 20cm.

Baud rate is 9.6kbps for their application.

However, they have used 5pin ribbon cables for daisy chaining. (2 for RS485, 1 for VCC, one for GND and one as chip select logic).

There is no impedance control or termination resistor or fail safe biasing on the bus.

I request your feedback on how critical are these parameters for such a short range communication, considering that there are 10 nodes connected along the path.

I have suggested them to use SN65HVD72 or SN65HVD75 for the application, let me know your recommendation as well.

Thanks and regards,

Mahendra Patel

 

  • Hi Mehendra,

    Termination and impedance control are always a good idea even in short networks especially with multiple nodes. With an unterminated bus and multiple nodes signal integrity could be a major concern. In a point to point topology that is relatively short termination may not be used but even here it is still a good idea.

    Has your customer actually builtt any boards yet or are they still in the discovery/design phase? If they have built up their system already are they having issues with sending and receiving data correctly between the nodes?

    The SN65HVD72 and SN65HVD75 are a good choice due to the extended IEC protection built into the IC. Is ESD a concern for them in their end application?

  • Hi Michael,

    Thanks for your inputs.

    Earlier they had considered I2C bus with repeaters instead of RS-485 bus & built the board.

    With I2C they faced latch up issue, which couldn't be resolved. Hence they have used small PCBs for connecting 485 transceiver with MCU UART in the same board with ribbon cables for testing RS-485 communication.

    In this configuration, there are some error occurring on the bus after which they need to RESET the UART peripheral of the MCU to start the communication again.

    They will definitely change the layout once they are sure about the performance using RS-485.

    There is not a specific requirement for ESD, but it would be good to have the protection because all the nodes are connected in daisy chain through connector & not via direct PCB traces.

    Thanks and Regards,

    Mahendra Patel