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RS422 using ISO3086 - circuit questions

MSP
Prodigy 20 points
Other Parts Discussed in Thread: ISO3086, TIDA-00058, SN6501

Hi Community,

I'm relatively new to serial communication, RS485 in particular and I hope you can help me understanding the ISO3086.

The scenario:

Two devices are talking to each other over a long (but variable) distance. Since it is a rough environment and the cable length inbetween the two devices can vary from 50m up to 500m, I decided to use RS422 communication and also decided to use an isolated interface.

First question:

The datasheet of the 3086 says, it has an internal fail-safe bias network.

Does that mean, I do not have to add bias resistors to pull the A/B lines to a defined state when the cable is disconnected?

If so, I still need to add the 120 Ohm resistor inbetween A and B, right?

Second question:

The cable that is being used does only have two wires. Thus, I cannot have a separate GND wire inbetween the two transceivers.

So what do I do with the VCC2 and GND2 on the isolated side of tranceivers? Can I put a DC/DC converter to have an isolated power supply and connect that to VCC2/GND2? Or do I just connect VCC2 to the isolated Power Supply and leave GND2 open?

I have attached the schematics of how I would put the 120 Ohm resistors... is that correct?

Any help or hint is highly appreciated as I have never really used this component before nor have a used rs422 in the past.

Thanks a lot in advance,

Mike

  • 0
    TI__Guru 53715 points

    Hi Mike,

    First Question:
    Yes you need not have to add bias resistors for ISO3086 as the device has internal biasing when cable is disconnected. Yes 120Ω resistor is needed on both ends of the cable for termination per RS-485 requirement and impedance matching. Please refer the below image for typical RS-485 network, please not that intermediate taps on the cable do not termination and is required only at both ends of the cable.

    Second Question:
    You will have to generate isolated power supply and connect it VCC2 & GND2. You can SN6501 to achieve this as given in section 11 of ISO3086 Datasheet. You can refer to TIDA-00058 for more details on www.ti.com.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 20 points
    Hi Koteshwar,

    thanks a million for your much appreciated reply.
    Your information is really useful to me and the reference design section on ti.com is just great! I didn't see that before.

    Have a nice day,

    Mike