• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 35 subscribers
  • Views 2354 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DMX512 Communication on the AC LED Lighting and Communications Developer’s Kit

Andy Yan
Prodigy 65 points
Other Parts Discussed in Thread: SN65HVD3082E, SN65HVD12

Hi Everybody,

Now I use DMX512 to communication with the PWM Control Module(Piccolo + LED Driver). I find a Application Report writtend by Brett Larimore from TI. The hardware block function showed below:

My question is:

1) RS-485 XCVR SN65HVD3082ED device is optimized for signaling rates up to 200 kbps, but the DMX512 specificaiton is 250Kbps, will using the XCVR be OK?

2) SN65HVD3082ED Output VOH is 4.6V (Typ.) and the Piccolo IO Voltage is powered by 3.3V, is there any problems if SN65HVD3082ED R/D pins connecting to Piccolo SDI pins derectly?

Do me a favor, Thanks.

  • 0
    TI__Expert 5400 points

    Hi Andy,

    It is a valid question, and has come up before.  As you may know, a commonly stated rule of thumb is that the rise/fall time of an RS-485 transceiver should be no more than 1/3 of the total bit time.

    For the SN65HVD3082E, the max rise time is 1500ns, implying that it should be used for applications requiring a data rate of 222kbps or less [ 1 / ( 3 * 1500ns ) ].  The typical rise time of 900ns implies a more generous maximum data rate of 370kbps, so it may possible for the SN65HVD3082E to support 250kbps in some applications.

    So you are correct that the SN65HVD3082E is a marginal choice for this application.  A transceiver with a maximum rise/fall time of ~800ns or less would be a more conservative choice.

    Regarding (2), I am not familiar enough with the Piccolo devices to say whether 5V would cause damage to its I/O pins, or how it would function in such a case.  In any case, if you ultimately end up using a 5V transceiver, I would recommend level-shifting the output of the transceiver in accordance with this post.  Or you could address both of your questions by using a higher speed, 3.3V transceiver, such as the SN65HVD12.

  • 0 in reply to Bart Stiller
    Prodigy 65 points

    Dear Bart,

    Thanks for your kindly help. I think SN65HVD12 is a good choice.

    If SN65HVD12 used, whether the clamp voltage with BAT54S should be 3.3V or still 5V?

    Best Regards

    Andy

  • 0 in reply to Andy Yan
    TI__Mastermind 21769 points

    Hi Andy,

    Sorry this wasn't clarified sooner. The absolute max stand-off voltage of the bus pins are -9 to +14 volts. Therefore, any clamping voltage that guarantees the bus pins are not subjected to higher voltage will do.

    The BAT45S is not a TVS diode it is simply a Schottky diode. Is there a reason you are choosing this device for bus protection? I would recommend something like the SM712 (datasheet link below). This TVS device is specifically designed for 485 applications and even has the asymmetric breakdown voltages of +12 to -7 volts.

    www.semtech.com/images/datasheet/sm712.pdf

    Thanks,

    John

  • 0 in reply to Andy Yan
    TI__Mastermind 28265 points

    Andy,

    DMX512 uses isolated bus nodes. in this case common-mode is removed and you don't need an SM712. The issue with Schottky diodes or steering diode is, when they switch they cause quite some EMI. So as long as you don't operate your circuit in EMI sensitive environment you will be okay.

    Thomas

  • 0 in reply to Andy Yan
    TI__Mastermind 28265 points

    Andy,

    DMX512 uses isolated bus nodes. in this case common-mode is removed and you don't need an SM712. The issue with Schottky diodes or steering diode is, when they switch they cause quite some EMI. So as long as you don't operate your circuit in EMI sensitive environment you will be okay.

    Thomas

  • 0 in reply to Thomas Kugelstadt
    TI__Mastermind 28265 points

    Andy,

    sorry for the double response, I don't know how this happened. I have attached a DMX512 article. You might want to add at least a high-voltage capacitor (4700pF/2kV from NOVACAP) between GND and ISO-GND.

    regards, Thomas

    DMX512 Application.pdf