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Thread: Ground loops

  1. #27
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    Quote Originally Posted by numbawunfela View Post
    The shield CAN be used as a reference, but SHOULD it be used is another story. If there is a noise free environment it works fine, but in noisy envirenments your poor transciever is trying to put 3v against a 'reference' that is doing the cucaracha with spikes and whatnot. Just like a flooded river crests a day or 3 after the rain, the noise does drain, but not before showing up as induced voltage. You should google images of oscilloscope captures of noisy trunks, you will see what I mean.
    I've never understood why ALC specifically specifies the shield for their 3 wire stuff.
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  2. #28
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    Thanks all. Makes more sense for sure!
    I also ended up looking around at documentation and saw how KMC mention how they suggest powering up their devices half wave, and that if you make them full wave by grounding the common of 24vac you can cause communication issues. That’d be because they are using the common as a reference for their mstp then.



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  3. #29
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    Quote Originally Posted by bolt950rider View Post
    Thank you for the extra detail Sam.

    Makes sense about the confusing JCI devices, as some are actually ABB drives with the name slapped on I think , or weren’t the TECs vykonics? Makes sense.

    So then about the difference of with reference an without, does the below make sense..
    Like the KMC mstp troubleshooting video has you going + or - to an actual ground as they do not have a reference terminal. You’re saying these types are using that controllers local ground as a reference? Even though they do not have a physical ground wire to it, so would it be using the common of the 24vac?
    Then when trying the KMC troubleshooting on JCI controllers it seems out of whack because your comparing the voltages to an actual ground instead of the reference. When you do measure the +,-to the reference the values all make sense and are in the correct ranges. All the controllers on that trunk are referencing the same wire and should be at the same resistance/voltage?

    Why not just use the shield as a reference then, although it’s meant to drain out noise, but if it’s drained out then they all should be referencing nothing/the same thing then?


    Thanks for helping me understand this more.

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    Good day bolt950rider,

    You are most welcome!


    So then about the difference of with reference an without, does the below make sense..
    Like the KMC mstp troubleshooting video has you going + or - to an actual ground as they do not have a reference terminal. You’re saying these types are using that controllers local ground as a reference?
    This is correct. Thing of it this way and ignore the whole RS485 thing. You have a electronic component that is powered. this device needs to be powered (+ and ground). Now you have an electrical signal being fed into this device (RS485 "A" and "B"). This component has a limitations as to what maximum voltages it can be fed before it is damaged and this/these specs are referenced to the component's "ground" pin. So... if the RS85's "A" and "B" are not somehow referenced to the component's local ground, then the voltage's of this signals could exceed the components limits and thus problems can occur (frying the component or misinterpretation of the voltage levels). The third wire (RS485 "reference") is used to provide a common reference point for all of these signal voltages. Now, interconnecting a bunch of devices sprinkled all over the place can create a differential ground level (referenced to say Earth ground) resulting in some nasty currents (which could be actually lethal) and so some level of impedance is used between the electronic component (RS485 transceiver)'s ground and the RS485's reference. This impedance is usually a higher wattage resistor of about 100 ohms. In cases where full isolation is needed... then specialty circuitry (opto, magnetic, capacitive isolation) is designed to electrically isolated any current paths.

    Even though they do not have a physical ground wire to it, so would it be using the common of the 24vac?
    I cannot say, as each manufacturer design's their controllers and RS485 interface circuits differently. Adding to this mess is that some manufacturers actually tie their 24VAC common to Earth ground which can create a ton of other problems and in some cases could violate electrical codes. Sadly, it is a bit of a "wild west" when it comes to product design.

    Then when trying the KMC troubleshooting on JCI controllers it seems out of whack because your comparing the voltages to an actual ground instead of the reference.
    This is a major problem when interconnecting different manufacturers on the same RS485 bus... and especially so with manufacturers that do not have a RS485 reference and/or the reference is not connected to its RS485's transceiver. One must remember that in order to have an accurate meter reading that the meter must be connected to to signal locations that have some electrical connection between them.

    Now... one must also be aware of the RS485's spec for a valid voltage level. If you recall I stated that RS485 is a differential signaling technology and so the voltage difference between the "A" and 'B" wires denote the digital data. The spec is that the resulting differential voltage must be either greater than 200mV or less than 200mV in order to be a valid signal level. Any differential voltage that is -200mV <= diff voltage <= +200mV is considered indeterminate and thus could incorrectly extract the correct digital value (logic 1 or 0). The 200mV levels may seem low and they are, but in reality it is not an issue if proper cable is used (proper cable ensure that any common mode electrical noise will cancel out), proper wiring practices are implemented, and the RS485 bus is not overloaded (which affects the 200mV levels).

    Why not just use the shield as a reference then, although it’s meant to drain out noise, but if it’s drained out then they all should be referencing nothing/the same thing then?
    You could and some people do. Will it work? It can and in most cases it will. However, there can be problems if this is done. I am simply amazed at how many projects and sites I have been to where improper cable, etc is used. Proper cable is not that expensive and especially so if you have to reinstall proper cable in the future in order to sort out a nasty RS485 bus.
    I always say to people "Is it not better to use the proper cable that you know will work as opposed to using a cable that might work?".

    Cheers,

    Sam

  4. #30
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    Quote Originally Posted by bolt950rider View Post
    Thanks all. Makes more sense for sure!
    I also ended up looking around at documentation and saw how KMC mention how they suggest powering up their devices half wave, and that if you make them full wave by grounding the common of 24vac you can cause communication issues. That’d be because they are using the common as a reference for their mstp then.



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    Good day bolt950rider,

    This is a major kludge in my opinion...Clearly, the manufacturer has no real understanding of RS485, etc.

    Secondly, one has to be clear and careful in using the term "grounding the common".... To some a ground could me Earth ground and this could be down right dangerous, as an Earth ground creates a current path for the high voltages. As an example, let's say that someone earth grounded the 24VAC common, the RS485 reference is connected to this same point... that means that any point along the RS485 bus and all the controllers could be pathways for possible high voltage currents. These high voltages could be someone touching an incorrectly grounded/powered piece of equipment... or could be the result of a close proximity Lightning Strike... the results could be lethal and/or at a minimum damage to all of the connected controllers.

    Anyway, stuff to be aware of.

    Cheers,

    Sam

  5. #31
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    The control hasn't lock down since I added the filter and ferrites cores,I'm keeping my fingers crossed!
    Have a question about VFD generated interference, the Honeywell VFD I suspect is causing the interference has the switching frequency set at 5KHZ and could be increase all the way to 16KHZ or decrease to 4KHZ, my understanding is that increasing the switching frequency will decrease harmonics and increase EMI, should I leave it right where it is at or maybe bring it down to 4KHZ?
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  6. #32
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    Quote Originally Posted by valdelocc View Post
    The control hasn't lock down since I added the filter and ferrites cores,I'm keeping my fingers crossed!
    Have a question about VFD generated interference, the Honeywell VFD I suspect is causing the interference has the switching frequency set at 5KHZ and could be increase all the way to 16KHZ or decrease to 4KHZ, my understanding is that increasing the switching frequency will decrease harmonics and increase EMI, should I leave it right where it is at or maybe bring it down to 4KHZ?
    Good day valdelocc,

    That is great that all is working... and if all is fine then I would simply leave well enough alone.

    Also with regards to switching frequency and harmonics... Harmonics are always present with any non sinusoidal waveforms (e.g. square waves like those used with modern power supplies and VFDs, communication protocols like RS485, RS232, etc). The reason is because square waves are actually made from a number of sinusoidal waveforms and these waveforms tend to be multiples (or harmonics) of the base or primary frequency (fyi...a perfect square wave is made from an infinite number of sinusoidal waveforms). Thus changing the VFD frequency up or down will not decrease the harmonics, but only change the various harmomic frequencies.

    As for EMI (Electromagnetic Interference)... EMI is always present with a non DC waveform. Whether or not the EMI generated has any affect on things is determined by the magnitude and frequency of the waveform (and its harmonics) and what governmental standard (FCC Class A, B, etc) the device is designed towards.

    Now... for your situation...I would be more concerned about conducted emissions (electrical noise coming back on the input power and control wires) affecting your controls. Failing all else and if I wished to try a "Hail Mary" I would select a frequency that is not an integer multiple of your communication rate.

    There is a lot more to what I mentioned above and I simply abbreviated as best as I could without writing a ton about the physics of electromagnetic and transmission line theory.

    Cheers,

    Sam

  7. #33
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    Quote Originally Posted by MaxBurn View Post
    I've never understood why ALC specifically specifies the shield for their 3 wire stuff.
    Same thing with Continuum b3’s. Although I have a few networks of those where the shield is connected to each device & comm’s is pretty solid. The real issue with them seems to be the auto baud rate.
    I’ve also run foul of it with Titan window controllers. That pulled the whole network down.

    I do wish manufacturers could be clear in their instructions.


    On the subject of VSD’s, one of our senior guys was telling us that they’re different again.
    The shielding around the cable must not be twisted up into a tail & clamped to the earth as commonly happens, because in this case it’s all about the whole circumference being earthed, and keeping the nasty stuff inside the shield. (With RS485 the idea is to let the nasty drain out).
    So what’s supposed to be done is a ring shaped earth clamp goes all the way around the shield at each end, and they are connected to the earth point(s).

  8. #34
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    Just stumbled on this. It has some examples of what not to do and some scope examples of what that looks like.

    https://www.maximintegrated.com/en/a...dex.mvp/id/763
    Scott Jalbert
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  9. #35
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  10. #36
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    Quote Originally Posted by MaxBurn View Post
    Just stumbled on this. It has some examples of what not to do and some scope examples of what that looks like.

    https://www.maximintegrated.com/en/a...dex.mvp/id/763
    Also look at the Niagara summit talk I posted to the troubleshooting post. The biggest downfall is they didn't show how they connected things so the traces leave alot to be interpreted. The overall effects and results are still relevant.
    Propagating the formula. http://www.noagendashow.com/

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