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SN65MLVD201: 65MLVD201 Proper Rt value on Multi Point Connection

Part Number: SN65MLVD201

Hello, I am using the 65MLVD201 MLVDS transceiver in two applications

Application 1)

I have 4 backplane connected cards in which one card has an always enabled 65MLVD201 driver driving a MLVDS pair and 3 cards have 65MLVD201 recievers connected to that same pair. The 3 backplane connected cards always have their receivers enabled and do not assert this line. What should be the total equivalent Rt on this line for the assuming the furthest receiver has the Rt ?

What signal levels should I see with this value of Rt (term resistor) using this  65MLVD201 arrangement (e.g. using the Receiver voltage diagram below from 65MLVD201 datasheet)

Application 2)

same 4 backplane connected cards in which one card has an always enabled 65MLVD201 receiver receiving a different MLVDS pair and 3 cards have 65MLVD201 drivers connected to that same pair. The 3 backplane connected cards have their drivers enabled such that their is no (never) contention on the MLVDS bus.

What should be the total equivalent Rt on this line for the assuming the furthest receiver has the Rt ?

What signal levels should I see with this value of Rt (term resistor) using this  65MLVD201 arrangement (e.g. using the Receiver voltage diagram below from 65MLVD201 datasheet)

I have made measurements with Rt=100 Ohms (application 1 and 2) and have found that the 65MLVD201 pins do not diverge (e.g. they track) when driving 100 ohms but when Rt=50 Ohm I see what appears to be correct MLVDS levels. Can TI explain this ? 

Thanks, Tom

  • Hello Tom,

    For the application 1, the Rt must be 100 Ohms, because the receiver has a high input impedance when compared with the terminator resistor. It would take a large number or receivers (over 100) before the effects would interfere with the performance of an overall system.

    Regards,
    Gerardo
  • MLVDS_backplaneconn.pdfHere are some questions

    1) The 65MLVD201 data sheet says that "Low-Voltage Differential 30-Ω to 55-Ω Line Low-Voltage Differential 30-Ω to 55-Ω Line · Low-Power High-Speed Short-Reach Drivers and Receivers for Signaling Rates"
    Q: Can you explain how the Drive current source is configured for 100 Ohms or 30 to 55 Ohms given MLVDS signalling ?

    2) I have found multiple references on MLVDS which state that we should use 50 Ohms and some references stating that we should use 100 ohms. For the 65MLVD201 it appears that it is nominally set to yields MLVDS Levels at 50 ohms based upon Figure 2 in the datasheet
    Q:Can you explain figure 2 if we are using this driver to produce MLVDS levels with Rt=100 ohms ?

    3) Is it true that MLVDS is most frequently characterized as (M)ultiDrop or (M)ultipoint LVDS meaning double terminated (e.g. 2x // Rt = 100 ohms or Rt equiv = 50 ohms ) ?

    4) Also, I have noticed that some of the TI documents imply lower MLVDS Rt values : ( Ref SLLD009 the LVDS app handbook)  (the RL for 65MLVD201 seems to be 30 ohms )

  • Hello Thomas,

    I'm working on this post and I will answer your questions with all available info that I could offer you.

    Thanks,

    Gerardo

  • Hello Thomas,

    Sorry for the mistake. You can use a double termination resistor of 100 Ohms (to get a 50Ohms bus). The figure that you've mentioned is a test circuit that simulate a 50 ohms line impedance.

    For further details you can check the application report of the link below

    www.ti.com/.../snla113b.pdf

    Regards,

    Gerardo

  • Hello Gerardo,

    I think my only question has not been addressed yet. In the configurations defined above (the diagram details it clearly) what can explain the waveform differences at Rt=50 Ohm and Rt=100 Ohms. Recall, at Rt=100 Ohms I see the voltage signals not diverging. That is, the pos/neg lines rise to potential but do not diverge.

    From above:
    "I have made measurements with Rt=100 Ohms (application 1 and 2) and have found that the 65MLVD201 pins do not diverge (e.g. they track) when driving 100 ohms but when Rt=50 Ohm I see what appears to be correct MLVDS levels. Can TI explain this ? "

    -Tom
  • Hi Tom,

    Could you give more details about your measurements? Like data speed, bus length, etc. 

    If you have some test waveform figures, it would be helpful.

    Regards,

    Nic