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  1. #1
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    RS-485 Wiring Question

    Will this work if the Bare drain wire is wired as per the Diagram below?
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  2. #2
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    Assuming everything else is correct, I don't know why it wouldn't.

  3. #3
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    Good day TechServices,

    The RS-485 Reference is used to maintain a common or "Reference" RS-485 transceiver ground between the various interconnected RS-485 devices and is not actively part of the RS-485 differential signalling system. Consequently, using your drain-wire, etc will work fine. The caveat here is that ensure you do not Earth ground this drain wire, as this can create all sorts of other issues... ground line differential currents, potential shock/safety hazards, coupling of 60Hz AC noise onto the bus, etc. That is not to say that the "reference" is not important... in fact it is very important and without it one runs the risks of reducing the overall network reliability and potentially places the connected RS-485 devices at risk of damage during various electrical events (i.e. Lightning storms, power line disturbances, etc). If you are interested in more details simply do a search on here you will find a number of posts discussing this and RS-485 in general.

    Cheers,

    Sam

  4. #4
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    Who is the manufacturer of the controller if you don't mind me asking.
    If you can't fix it with JB Weld, Duct Tape, and Ty Wire it has to be replaced.
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  5. #5
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    Do not Earth Ground the Drain wire?
    Interesting,that may explain a few problems with other sites.
    I have seen this wire touching bare metal conduits and junction boxes
    These are Johnson FX controllers

  6. #6
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    Quote Originally Posted by techservices View Post
    Do not Earth Ground the Drain wire?
    Interesting,that may explain a few problems with other sites.
    I have seen this wire touching bare metal conduits and junction boxes
    These are Johnson FX controllers
    You ground the shield wire at ONE and ONLY ONE END. Make sure other exposed shield it tape up neatly and not to have it touch any ground. Some controller gives you a third terminal to connect your shield wires together. But this terminal goes NO WHERE and touch nothing. Sometimes I don't even use this 3rd terminal since it is hard to cut the wires short enough not to touch anything then have to connect them to this terminal. I just twisted them shield wires together and tape them.

    I typically ground it at the begin. This way I don't have "wonder if I ground it or not"

  7. #7
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    Quote Originally Posted by techservices View Post
    Do not Earth Ground the Drain wire?
    Interesting,that may explain a few problems with other sites.
    I have seen this wire touching bare metal conduits and junction boxes
    These are Johnson FX controllers
    Good day TechServices,

    That is correct... you typically should not connect the Reference lead to Earth Ground. There are a multitude of variables that affect this decision, but typically avoiding Earth ground for communication lines is a good idea. Connecting the reference to Earth can create a very nice antenna for 60Hz and other AC noise to couple onto your network. If the equipment has a well designed RS-485 interface and if you use the correct RS-485 cable, then the RS-485 network has a tremendous amount of noise immunity from electrical noise... however a lot of manufacturers have poorly designed RS-485 interfaces and a number of people continue to use incorrect cable... the result... problems.

    Another reason for avoiding Earth Ground is to minimize shock/safety hazards... Depending upon how the RS-485 equipment is designed and powered this reference Earth Ground would serve as a nice connection for any line voltage issues within the equipment...this is bad for both the equipment, but especially for the installer who is potentially touching this wire.

    As I said, ideally you wish to avoid Earth grounding the reference.

    Cheers,

    Sam

  8. #8
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    Quote Originally Posted by just_opinion View Post
    You ground the shield wire at ONE and ONLY ONE END. Make sure other exposed shield it tape up neatly and not to have it touch any ground. Some controller gives you a third terminal to connect your shield wires together. But this terminal goes NO WHERE and touch nothing. Sometimes I don't even use this 3rd terminal since it is hard to cut the wires short enough not to touch anything then have to connect them to this terminal. I just twisted them shield wires together and tape them.

    I typically ground it at the begin. This way I don't have "wonder if I ground it or not"
    Good day just_opinion,

    I just posted my response to doing this in my previous post.

    Cheers,

    Sam

  9. #9
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    Good day All,

    To those that do not agree with my post about avoiding Earth grounding the RS-485's Reference lead... I have to ask them what purpose do they think Earth Grounding achieves on a RS-485 network? If they think it is because of electrical noise immunity, then think again... Remember the RS-485 bus is a differential signalling bus and so any common mode noise that is present on the RS-485 signalling wires (A, B, or +, - or to the JCI people N2+, N2-) is cancelled out by the RS-485 transceiver when it does its differential calculation. The caveat here, of course, is that that one uses correctly spec'd RS-485 communication wire (i.e. twisted-pair, etc) and the that bus is not overloaded (max 32 Unit loads).

    Cheers,

    Sam

  10. #10
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    Quote Originally Posted by s2sam View Post
    Good day All,

    To those that do not agree with my post about avoiding Earth grounding the RS-485's Reference lead... I have to ask them what purpose do they think Earth Grounding achieves on a RS-485 network? If they think it is because of electrical noise immunity, then think again... Remember the RS-485 bus is a differential signalling bus and so any common mode noise that is present on the RS-485 signalling wires (A, B, or +, - or to the JCI people N2+, N2-) is cancelled out by the RS-485 transceiver when it does its differential calculation. The caveat here, of course, is that that one uses correctly spec'd RS-485 communication wire (i.e. twisted-pair, etc) and the that bus is not overloaded (max 32 Unit loads).

    Cheers,

    Sam
    Maybe sometimes the screen/drain is the antenna for reasons unknown...install practices unseen etc. We have pretty good success in many mstp and Lon networks taking shield to ground but via an RC network to provide a path to ground for HF ...especially switching junk coming from VFD and switchmodes.
    1 + 1 = 3 ( *** for very large values of 1)

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  11. #11
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    Quote Originally Posted by MatrixTransform View Post
    Maybe sometimes the screen/drain is the antenna for reasons unknown...install practices unseen etc. We have pretty good success in many mstp and Lon networks taking shield to ground but via an RC network to provide a path to ground for HF ...especially switching junk coming from VFD and switchmodes.
    Good day Matrix,

    Indeed, there are cases where one terminating the drain in other ways (like you mentioned) is beneficial to those networks that are exceptionally cranky... and even some cases where it is a necessity. My previous post's point was was simply Earth Grounding the Drain/reference as a standard practice not such a good idea... One has to understand why and when to do something, as opposed to simply doing it a certain way because "we have always done it this way".

    Cheers,

    Sam

  12. #12
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    Quote Originally Posted by MatrixTransform View Post
    Maybe sometimes the screen/drain is the antenna for reasons unknown...install practices unseen etc. We have pretty good success in many mstp and Lon networks taking shield to ground but via an RC network to provide a path to ground for HF ...especially switching junk coming from VFD and switchmodes.
    Just curious, what are you using for an RC Network? Are you making them up yourself or buying the OTC?
    If you can't fix it with JB Weld, Duct Tape, and Ty Wire it has to be replaced.
    No good deed goes unpunished.
    If you want to take off friday to go fishing then make sure you train your helper right.

  13. #13
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    I think the confusion here is because the original post is a little vague. If the connection to the DDC controller is a shield connection, the wiring shown should work. The green conductor that is apparently a bare conductor drain for the cable shield should be grounded in a single location for the whole length of the drain. On the other hand if the DDC terminal is a "ref", the wiring shown should still work. The same wire that is likely the shield drain should not be grounded anywhere. The electric potential of this wire should come from, and equalize, all the communication boards on the segment.

    Ideal RS485 wire has a twisted pair for plus and minus and a reference conductor for balancing the potential of the communication. Then an additional shield drain cable would be present, if shielded wire is used. IMO shielded cable is not necessary unless there are special circumstances. The physics of inducing potentials with common voltages and frequencies where your com wire is installed is better handled by the twist of the conductor than by a shield. I don't remember where I read this info and I am not qualified to determine this. I just read it and remember it in a general way.

    So, in this example it looks like; A twisted pair is used for com. There is a shield drain that is connected to a terminal on the DDC controller. Without more info this connection is 99% of the time going to be OK and work just fine. There are details not indicated that could make this not work or be a bad installation, but that information is not provided.

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