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RS-422 & RS-423 multidrop configuration

Ananya Mishra
Prodigy 10 points


Hi,

I have a requirement of using RS-422 and RS-423 interface in multidrop configuration with 4 loads.

For RS-422 interface, I am using AM26LV31INS(3.3V supply) and AM26C32IPW(5V supply).I am using 480 ohm termination resistors at each receiver.

For RS-423 interface, I am DS3691 and DS26LS32.

Is there any issue in the design?

Is there any RS-423 driver IC available with only positive supply?

Ananya

  • 0
    TI__Expert 5400 points

    Hi Ananya,

    It is always best for evaluation purposes if you can provide a schematic or system diagram, as well as additional information such as data rate, distance, cable type/impedance, etc.  Please provide additional information for review. 

    A good starting point for RS-422 applications can be found here.

    RS-423 is a bipolar signaling scheme, and thus requires 2 supplies.

  • 0 in reply to Bart Stiller
    Prodigy 20 points

    Hi,

    Thanks for the response. I am attaching a sample schematic for your reference. 

    Data rate < 1 Kbps

    Distance < 5 meters

    Cable type is twisted pair.

    Hope this information is sufficient.

    I need your support for RS-423 also. Because of some constraints,we can't use RS-422 interface for few signals.

    Hence, we are using RS-423 interface in multi-drop configuration. Please suggest the driver and receiver part no. and please provide a sample circuit for multidrop configuration with RS-423 driver and receiver.

    Ananya

    1185.RS422_TX_RX.pdf

    RS422_TX_RX.pdf
  • 0 in reply to Ananya Mishra1
    TI__Expert 5400 points

    At 1kbps and 5 meters, your RS-422 application should not present any issues.  I am curious why you have chosen 480ohms for termination at multiple nodes?  For only 1kbps, you could likely do without termination at all, but if you choose to include them, ~100ohms is a more typical value, to match the characteristic impedance of the cable, and is only placed at the most distance point on the bus from the driver.

    For your RS-423 application, I would point you here, to an application note describing RS-422 and RS-423 operation with the DS1691A.

  • 0 in reply to Bart Stiller
    Prodigy 20 points

    Hi Bart,

    Thanks for the response. In our system, the RS-422 driver is nearly at the center and all the RS-422 receivers are approximately at the same distance from the driver.In this case, how should we place the termination resistor and what should be its value?
  • 0 in reply to Ananya Mishra1
    TI__Expert 5400 points

    Hi Ananya,

    At 5 meters, the round trip propagation delay of your signal will be approximately 45ns1, or 1/22500 of your total bit time at 1kbps, meaning that any reflections in the line will have ample time to settle.  This is why I mentioned earlier that you likely don't need any termination at all.

    I have seen other designs which are constrained to a star network place some termination at the end of each stub, and it is not unreasonable to believe that this reduces reflections relative to the unterminated case.  But strictly speaking, you no longer have a transmission line, and thus the notion of termination itself is less applicable.  Again, however, this is not an issue, given your specifications.

    1 10m / [ ( 3 x 108m/s ) * ( 0.75) ]