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SN65HVD06: Resistor termination

HS FAE Germany
Mastermind 31170 points
Part Number: SN65HVD06

Hello,
customer wants to connect several device with RS485 in a row. Is there any recommendation for termination? Is termination needed for the first and the last device in the row? For the customer it would be an advantage if no termination at first and last device is needed and termination can be done on each device. This makes it easier and more flexible in different network configurations.

Regards, Holger

  • 0
    TI__Guru 50795 points

    Hi,

    We get a lot questions regarding termination schemes for RS-485. The common recommended configuration for a system with many nodes is to daisy-chain each node along a single cable length keeping the stub lengths as short as possible. The network should then be terminated at the two nodes farthest from each other – or the two ends of the full cable length. Though this setup is ideal to reduce reflections and maintain signal integrity, it is not required to design a successful network.

    In a case where termination is applied at each node, the total loading (total resistance) should aim to be 60-ohms (two 120-ohm in parallel) between the differential lines. For example, in a system with 16 nodes, each node could have a termination value of 960-ohms. Though this loading is equivalent to end-node termination, this approach will have sub-optimal dampening of signal reflections. Additionally, if another node was to be added, the needed termination value would change for all nodes to maintain a 60-ohm equivalent load. If this layout is desired loading can be chosen to be 60-ohm equivalent for the maximum node-count case. The system would operate with a lower loading (higher resistance) in cases where less nodes are used.

    Termination schemes may vary system by system depending on design requirements including signal rate, cable length, node count, environmental conditions, cost, etc. It is the responsibility of the designer to weigh these factors so the network fits the end use case. If the system in question can tolerate the reflections that will be introduced by sub-optimal termination, this may be a way to simplify the design and reduce cost. Care should be taken to ensure the system can tolerate these conditions.

    For more information on RS-485 termination networks, check out this app note.

    For more information on RS-485 node count calculations, check out this app note.

    Let me know if you have any more questions or have particular concerns regarding your customer’s configuration.

    Regards,

    Eric