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THVD9491-SEP: Single Master Multi Slave Communication

Uday Kumar Urimi
Prodigy 90 points
Part Number: THVD9491-SEP

Tool/software:

Hi,

I need to control around multiple slave, say 20 from a PC. I also need to get data from slaves upon request. I require data rates similar to UART and max distance from slave is around 15meters.

Can I use RS485? or any other options available?

Thanks,
Uday

  • 0
    TI__Guru 73991 points

    Hi Uday,

    From what you provided RS485 looks like it would be a possible solution.

    What software protocol are you intending to use? (Mainly want to know if it's bidirectional where the dataline is shared between the master and slave).

    RS485 can communicate across 1.2km and is mainly used at UART speeds. It was technically designed around 32 nodes but with modern day input biasing, it can technically support 256 with the correct cable network set up. (daisy chained and slower data rate). 

    If you need to share the data line, your software will need to have a way to tell the slave/masters when they can communicate to avoid data collision from occurring. RS485 doesn't support arbitration. 

    You could potentially also look at CAN (the physical layer) or use both CAN (physical and software/firmware). CAN is kind of similar to RS485 but allows for arbitration. 

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 90 points

    Hi Bobby,

    Thanks for the response.

    I would like to use UART protocol.

    I would be needing the data from slaves only upon request from PC. 

    Here is my Plan :

    1. Assign unique byte in UART protocol to represent each of the slave

    2. Based on this, only one of the slaves Tx will be enabled

    3. Once Slave finishes data transfer, Tx will be disabled.

    4. Thus no chance of data collosion and arbitrator also not require.

    Let me know if I miss anything.

    Thanks,
    Uday

  • 0 in reply to Uday Kumar Urimi
    TI__Guru 73991 points

    Hi Uday,

    That should definitely be doable. If you choose the THVD9491, here are some tips:

    If you want the cable to be bi-directional and share the same communication lane, you can short the Y pin to the A pin and B to Z pin on the THVD9491 to turn it from full duplex to a half duplex bus. For your application, it would be better to choose the 20Mbps mode on SLR. Faster isn't always better, in this case choosing a slower slew rate would be more than sufficient for UART speeds and lower the chance of reflections on the bus. 

    When you get to the design stage, feel free to post a snippet of the THVD9491 (or whatever TI RS485 transceiver you choose) for us to review. 

    -Bobby