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RS485 Transceiver: Problem to use at lower data rate? (e.g. SN65HVD75)

Graf Daniel
Prodigy 10 points
Other Parts Discussed in Thread: SN65HVD01, SN65HVD75

For the use of MODBUS RTU protocol on RS485, I evaluated the RS485 transceiver SN65HVD75. I want to use the IF at a data rate of  921.6 kbps. Will the SN65HVD75 (20Mbps) also work at much lower rates? In other words: Does a transceiver capable of higher data rates constrain a use of it at lower speeds? The reason for my question is IC SN65HVD01, that has two selectable data transmission speeds (one higher speed would have worked, too?).

Thanks in advance

  • 0
    TI__Guru 62920 points
    Hi Graf,

    Yes, an RS-485 transceiver will work at rates much lower than its rated maximum speed (even down to DC). The maximum rated speed of a transceiver is usually a function of the transition times of the differential output, and there are a couple of advantages to using slower transition times.

    One is for reduction of EMI - i.e., slower edges mean less high-frequency content in the bus waveforms that could potentially radiate or couple onto other conductors. Another is for flexibility in bus topology. When multiple nodes are connected to a common bus, their connections typically form short transmission line "stubs" as they branch off from the main bus. Since these stubs are not terminated, signals that propagate down them tend to reflect back at the the main bus.

    For a slow-changing signal, the total round-trip delay down this signal path may only be a fraction of the transition time. When this is the case, the stub does not have much of an impact on signal integrity. For a fast-changing signal, though, the reflected signal may be substantially different in phase/amplitude when it returns to the main bus. The interference caused by this reflection can cause signal integrity issues (e.g., overshoots/undershoots, non-monotonic edges, etc.).

    So, while there are no limitations at the device level which limit the minimum rate of a "high speed" transceiver, it would be good to consider from an overall system perspective as well.

    Regards,
    Max