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SN65HVD1176: Device spacing on RS-485 buses

Dimitri Ten Lohuis
Prodigy 10 points
Part Number: SN65HVD1176

We are looking for a bus solution for up to 300 nodes were the nodes are mounted relatively close to each other (spacing 5cm).

While reading the 'Device spacing on RS-485 buses' Analog Applications Journal (http://www.ti.com/lit/an/slyt241/slyt241.pdf I noticed that there is a minimum node spacing when using a RS-485 driver.
The document is referring to the SN65HVD1176, which is a half-duplex driver.

I would like to know if this minimum node spacing is only applicable for half-duplex drivers or also for full-duplex RS-485 drivers?

  • 0
    TI__Guru 62920 points
    Hi Dimitri,

    The same concepts and equations apply equally for half-duplex and full-duplex applications. The paper is basically pointing out that closely-spaced nodes will function electrically as lumped capacitive loads, which could form a reflection-generating impedance mismatch when installed on an electrically long network. The equations in this paper are general guidelines, though, and these reflections may or may not be an issue depending on the rate of data transfer, the transition times of the differential driver, and the lengths of any cable segments used to connect to nodes outside of this cluster.

    Max