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TB5D2H: PECL Driver and RS-485 Receiver

Darren (FAE)
Expert 5483 points
Part Number: TB5D2H
Other Parts Discussed in Thread: SN65LBC173A, TB5D1M, SN65LBC172A

I found this app note:
https://www.ti.com/lit/an/snla041a/snla041a.pdf

It mentions:

Differential ECL or even Pseudo ECL (PECL) will typically directly inter-operate with a RS-422 receiver.

Is it possible to use TB5D2H as Driver, and a device like SN65LBC173A as Receiver?

I have a use-case where there are two boards directly connected together (no cabling) and the trace-length is <0.5m maximum.

Receiver-side has 100Ω termination resistor between differential signal lines. There is no pullup or pulldown.

I am a little worried about using the TB5D2H or TB5D1M driver as-is, because the output stage, as far as I can tell, is just a high-side transistor.

The receiver has good common-mode capability, but I'm not sure how the device would work...

Figure 15 ~ Figure 17 in the Datasheet show some examples. Figure 15 would be closest to customer use-case (100Ω termination resistor on Receiver-side), but they can't add pulldown resistors to the design.

Are the pull-down resistors necessary? I think they are used to "pull the voltage low" when that transistor is off.
This way, if one transistor is conducting, then that line is "high"
And if the other transistor is "off", then that line is pulled "low" and this creates the necessary voltage differential.

Is there anyway to use the device without these Rs resistors? What kind of behavior would we see?

  • 0
    TI__Guru**** 157476 points

    Darren

    I would think THVD family device will be better fit for RS-485 application, and move your ticket to the INT TRX team to get their support.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    TI__Guru 69962 points

    David,

    Thanks for directing to the correct forum.

    Darren,

    An engineer from the TRX team has been notified of your question and will respond by tomorrow.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Guru 69962 points

    Darren,

    After taking a closer look at this, it looks like you're asking about the loading capabilities of the TB5D2H. I can redirect you to the HSDC forum, but I want to understand if you're looking for RS-485 help, or more about if the TB5D2H will work in an RS-485 bus.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Expert 5483 points

    Hi Eric, maybe should have posted internal...this is related to replacement of SN65LBC172A due to delivery schedules.

    I need to know about this device specifically in this application...

    Will TB5Dxx work without pull down resistors...?

    I understand PECL has only a high side transistor. So without pull down resistors, I don’t know if it can maintain >250mV differential voltage between On and Off outputs.

    If it can maintain above 250mV without those resistors when one transistor is high and the other low, I think the customer will want to evaluate these parts...

    sorry, but this is really urgent...

  • 0 in reply to Darren (FAE)
    TI__Guru 69962 points

    Darren,

    TRX doesn't own that device so I don't want to speak out of turn for the TB5Dxx devices. But I would like to repeat David's question, is a THVD RS-485 transceiver possible here? I know supply is tight right now everywhere, but just wanted to make sure you looked at other RS485 transceivers before going to the TB5Dxx devices.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Guru**** 157476 points

    Darren

    The pulldown resistors provides a DC current paths for the emitter follower output transistors. There will be no output until pull-down resistors are connected to the open emitter.

    Thanks

    David