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DAC8742H: How is Power Drawn from the Fieldbus

Cyrus Kelly
Prodigy 240 points
Part Number: DAC8742H
Other Parts Discussed in Thread: PROFIBUS, TIDA-01504

The information on using the DAC874xH for Foundation Fieldbus and Profibus PA seems very limited.  Is there somewhere I am not looking?

The question I have at the moment is about the ability for the DAC874xH support for bus-powered Fieldbus devices.  Fieldbus devices are typically powered from the same bus they communicate on.   The device will draw a fixed amount of current to power itself then modulate the current to generate comms signals.

Does the DAC874xH support configuring the current draw from the Fieldbus as necessary to power the Fieldbus device?  If so, how is that done?  If not, can you suggest how to do it external to the DAC874xH?

Thanks for your help.

  • 0
    TI__Mastermind 46290 points
    Cyrus,

    We are working on a reference design dedicated to Foundation Fieldbus, the main source of difficulty / delay with respect to when these devices released has been regarding the FF stack itself, as we did not previously have a FF stack. Right now we're pretty close to having a working / complete stack, and are making some adjustments to the hardware to enable continued progress on that reference design. We're hopeful to have something early next year.

    The DAC874xH is essentially only providing the encoder and decoder functionality required for the Manchester Encoded waveforms used in Foundation Fieldbus. The rest of the physical layer for controlling the loop current etc. are assumed to be external components.

    In a lot of ways, Foundation Fieldbus transmitters look very similar to 4-20mA / HART transmitters. We produced a reference design using discrete components for a HART enabled 4-20mA transmitter called TIDA-01504 which you may be able to leverage as a starting point.

    In Foundation Fieldbus of course there is no need to have a dynamic DC current value, so you could conceivably replace this piece with a static bias voltage. Alternatively, perhaps having the programmability is valuable to leverage for different sensors which may require different quiescent current values on the loop.

    Also when comparing versus TIDA-01504 the filter component values and gain setting resistors would need some modification in order to fit into the FF requirements, but for the most part I believe HART versus FF/PA can be facilitated through a BOM change. We will also publish a reference design on that topic pending the FF certification completion.
  • 0 in reply to Kevin Duke
    Prodigy 240 points

    Kevin,

    Thanks for the detailed reply.  The reference design is helpful.  I see what you mean about using a static bias voltage.  I'm going to get an eval board and see what I can do with it.  It looks like it will take a bit of external circuitry.

    Are you developing your own stack or just customizing one from someone else?  If you are making your own, is there a plan in place to make it available to FF/PA device developers?  If not, may I ask whose stack you are using?  The closer I am to using what TI does, the easier it is to use the application notes and other support methods.

    I will anxiously watch for the FF reference design.

    Thanks again,

    Cyrus

    PS:  Are you a bike rider?  I noticed your user icon.  There are several of us here that try to get out regularly when the weather allows.

  • 0 in reply to Cyrus Kelly
    TI__Mastermind 46290 points
    Hi Cyrus,

    My team and I are more analog-focused folks, so we do not internally have the expertise to develop a full Foundation Fieldbus stack. As such, we decided to engage a third party on a stack development. If you'd like to discuss the stack further, I can reach out to you at the email address you used in creating your myTI.com account.

    And yes, I try to put as many miles in on the weekends as I can, though the time change / weather make that less enjoyable this time of year.