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I am trying to wrap my head around the difference between differential signaling (mainly LVDS and LvPECL) and RS-485.

All seem like differential signaling for serial communications.

  • From the TI Page, it seems like the difference between LVDS and LvPECL is just the common mode voltage (2V vs 1.2V), differential voltage (800mV vs 400mV), and termination?

  • From reading about RS485, it seems to be based of RS-422 which is also a type of differential signaling, but with a sensitivity of 200 mV? Is the only difference here that the RS485 spec doesn't reference a common mode voltage?

  • How does 485 differ from LVSD and LvPECL? 485 seems to be referenced but never grouped with differential signals but I don't see why. I'm sure I'm missing something simple?

If I have an LvPECL signal I want to ingest and my receivers (LVDS or RS485) can handle input voltages up to 5V as well as detect differential voltages as low as 200 mV and all are 100 Ohm terminated, will either receiver work?

Any guidance or clarification would be appreciated

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    \$\begingroup\$ Different design objectives. One is for distance and ruggedness and one is for speed. RS-485 is not a logic family like the others. \$\endgroup\$
    – DKNguyen
    Commented Aug 26, 2021 at 15:32
  • \$\begingroup\$ @DKNguyen so if my signal is low speed and not far (<15m), is it up to me? Can you clarify how its a different logic family? Isn't it if A-B <-200mV = 1 and if A-B > 200mV =0? Isn't that essentially the same for LVDS and LvPECL? \$\endgroup\$
    – bchang32
    Commented Aug 26, 2021 at 16:25
  • \$\begingroup\$ There may be some cross-compatibility but none of it is designed or specified of it. I don't now the details. edn.com/interfacing-lvds-with-other-differential-i-o-types \$\endgroup\$
    – DKNguyen
    Commented Aug 26, 2021 at 16:32

2 Answers 2

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RS-485 uses differential signaling, like LVDS and (Lv)PECL.

RS-485 is designed for industrial environments and can tolerate much higher voltages than the other standards. (The actual common-mode voltage of a transmitter is near VCC/2.)

If you have LVPECL signals, then RS-485 receivers will be able to handle them. (This also works in the other direction as long as the actual voltages do not exceed what the weakest component can tolerate.)

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  • \$\begingroup\$ Isnt LVDS and LvPECL also differential signaling? I'm trying to understand the difference and it seems like they're essentially the same except for minimum differential voltage thresholds and that 485 doesn't have a specified common mode voltage? I keep trying to see/look it up and it seems the common answer is that "theyre different," and then proceeds to just list what 485 is but it never goes into how theyre different. I do appreciate you providing more info than the "they just are," or "theyre not the same," that I've been seeing online. \$\endgroup\$
    – bchang32
    Commented Aug 30, 2021 at 12:24
  • \$\begingroup\$ It isn't differential signallin vs RS485. It is 485 vs LVDS vs LvPECL. RS485 has a specified common mode voltage (nothing can operate at infinite common mode volts) but different transeivers can vary from the standard (-7V to 12V) but I've seen as high as +/-25V. RS422 and RS485 steps down the voltage internally (divider) to achieve this since it actually runs off 5V or less. \$\endgroup\$
    – DKNguyen
    Commented Aug 30, 2021 at 13:32
  • \$\begingroup\$ Ok so sounds like theyre all differential signaling (which is what I thought but the internet seemed to show otherwise). Sounds even more like they're just different sensitivities and Vcms. As long as I ensure that the Rx can handle the common mode and sensitivity, I should be good. The main thing I need to be cautious of is the data rate? Thanks! \$\endgroup\$
    – bchang32
    Commented Aug 31, 2021 at 12:27
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I'll answer your question another question which might help as well.

Can you integrate 2 seperate systems when one has an RS485/422 and the other has an LVDS. Let's say the RS485 is the driver here.

On the input side things look possible, but not without pitfalls. LVDS guarantees an output swing of +/-350mV. LVDS input buffers look for +/- 100mV levels. This gives 250mV of noise margin. While this is plenty for a set of traces of a backplane it is very small for an external interface exposed to radiated fields. RS422 receivers look for +/- 200mV of signal, so they would only be left with 150mV of noise margin.

The output side is more problematic. LVDS receivers are typically only built to withstand +/- 450mV at their inputs. The driving system is going to output +/- 3.3V or +/- 5.0V depending on how the exact interface is setup. Depending on the bus architecture you might be able to get around this by turning the termination resistor on the recipient system into a voltage divider. The RS485 drivers will also have either 1.625V or 2.5V of common mode on this signal. That will fall within the acceptable range of many, but not all LVDS inputs, and this cannot be as easily divided off the signal.

The final potential pitfall is with timing. Most LVDS applications today are running at speeds in the 100MHz – 2GHz range, all of which is faster than RS-422 can go. Even if your potential use case is slower, you would also need to verify that the additional jitter from the RS422 doesn’t cause other issues.

Hope this helps.

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  • \$\begingroup\$ I think so? It seems like as long as I ensure my receiver can handle the data rate, common mode voltage, and sensitive enough to detect the differential voltage, I should be fine? Then back to my original question, it seems like they're all essentially the same; just different value parameters (Vcm, differential voltage, and data rate, but they're all differential and such). My internet searches made it seem like 485/422 was entirely a different signaling/category (but I couldnt tell how). \$\endgroup\$
    – bchang32
    Commented Aug 31, 2021 at 12:30

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