BACNet wire

[VAEngineer]

I have been reviewing submittals lately on a couple projects and I consistently notice that even really big companies like JCI recommend wire that does not meet the requirements of the RS-485 specifications. For instance for JCI they recommend a Belden part #B5501FE (The recommendation is in this document https://cgproducts.johnsoncontrols.c...F/12011034.pdf). When I go to the Belden catalog for that wire this is what I find https://catalog.belden.com/techdata/...E_techdata.pdf it is actually wire intended for "security and sound" applications with an impedance of only 16.4 Ohms. The RS-485 specification specifically calls for an impedance of 120 ohms. I've also seen similar recommendations from companies like Delta controls and Schneider Electric on their MODBUS systems. I know when I look up the datsheet for the IC's that are the RS-485 transceivers the applications notes always are very specific about termination resistors and cable impedance. It's not like Belden and other don't make a cable specifically for RS-485 with the correct impedance, so why not just use the correct cable?

Is it because true 120 ohm cables cost much more than the "speaker" wire that they don't recommend it?

I wonder what effect using cable with the incorrect impedance would have on the reliability of the BACNet/MODBUS network? Is anyone aware of any studies?

Here is what one manufacturer of the driver IC's for RS-485 has as far as application notes https://www.maximintegrated.com/en/a...dex.mvp/id/763

[VAEngineer]

BTW - I do realize that these systems are forgiving enough that they will most likely work using any 22/24ga twisted pair and shielded cable, but why take the chance and not use the correct wire? I would hate to troubleshoot something like the actual wire being the cause of an issue, no one would believe you if you said that was the issue.

BTW - Schneider Electric also says it's OK to use a T-tap on their MODBUS wiring as long the T-tap is under 20m!!! I was always told you keep it to the absolute minimum you can get away with.

[orion242]

Pretty sure Sam will comment on this and has many times in the past.

JCI doesn't define the 485 standard.

[VAEngineer]

JCI doesn't define the 485 standard.

Not sure what you mean by this? Are you saying go with their recommendations or go by the TIA/EIA working group that does define the RS-485 standard?

[orion242]

The 485 standard is well established. Every piece of mass produced silicon that works with 485 is designed for and will be present in 99.995% of 485 devices. The impedance and capacitance is spelled out in the specification.

Maybe JCI has some custom non-standard transceivers, or they just use any crap cable because they can't be bothered to document things correctly. Might just recommend networks be limited to a handful of devices to sell more equipment and deal with coat hangers for comms cable. Who knows.

The spec is clear. Its been time tested and what the ecosystem is designed around. If JCI says coat hanger is fine, that's certainly in their world only.

[MaxBurn]

Seems to me this is one reason people are pushing for Ethernet, they can go to the supply house and look for a box that says catX and be good. None of the boxes will mention RS485 comm and you make people think about the spec and that's where things go really wrong.

[incontrol]

BTW - I do realize that these systems are forgiving enough that they will most likely work using any 22/24ga twisted pair and shielded cable

I don't know about that.

[zzip7]

I have been reviewing submittals lately on a couple projects and I consistently notice that even really big companies like JCI recommend wire that does not meet the requirements of the RS-485 specifications. For instance for JCI they recommend a Belden part #B5501FE (The recommendation is in this document https://cgproducts.johnsoncontrols.c...F/12011034.pdf). When I go to the Belden catalog for that wire this is what I find https://catalog.belden.com/techdata/...E_techdata.pdf it is actually wire intended for "security and sound" applications with an impedance of only 16.4 Ohms. The RS-485 specification specifically calls for an impedance of 120 ohms. I've also seen similar recommendations from companies like Delta controls and Schneider Electric on their MODBUS systems. I know when I look up the datsheet for the IC's that are the RS-485 transceivers the applications notes always are very specific about termination resistors and cable impedance. It's not like Belden and other don't make a cable specifically for RS-485 with the correct impedance, so why not just use the correct cable?

Is it because true 120 ohm cables cost much more than the "speaker" wire that they don't recommend it?

I wonder what effect using cable with the incorrect impedance would have on the reliability of the BACNet/MODBUS network? Is anyone aware of any studies?

Here is what one manufacturer of the driver IC's for RS-485 has as far as application notes https://www.maximintegrated.com/en/a...dex.mvp/id/763

I'm not a JCI guy, but I know JCI recommends a maximum of 32 devices per segment on their facility explorer MS/TP products which is quite a bit lower than other vendors. This may be due to the transceivers they use, but I suspect they are being careful to advise smaller networks to let installers use more varieties of wire and not be as strict about wiring practices.

In that JCI guide you referenced, it also allows the use of 18AWG and various other cable types, some unshielded for shorter runs. I'm guessing they're designing the guidelines for cost and flexibility. Given that they approve it, I would hope that means it's well tested.

[desert guy]

Are you getting your information from this?
https://cgproducts.johnsoncontrols.c...f/12011670.pdf

[s2sam]

I have been reviewing submittals lately on a couple projects and I consistently notice that even really big companies like JCI recommend wire that does not meet the requirements of the RS-485 specifications. For instance for JCI they recommend a Belden part #B5501FE (The recommendation is in this document https://cgproducts.johnsoncontrols.c...F/12011034.pdf). When I go to the Belden catalog for that wire this is what I find https://catalog.belden.com/techdata/...E_techdata.pdf it is actually wire intended for "security and sound" applications with an impedance of only 16.4 Ohms. The RS-485 specification specifically calls for an impedance of 120 ohms. I've also seen similar recommendations from companies like Delta controls and Schneider Electric on their MODBUS systems. I know when I look up the datsheet for the IC's that are the RS-485 transceivers the applications notes always are very specific about termination resistors and cable impedance. It's not like Belden and other don't make a cable specifically for RS-485 with the correct impedance, so why not just use the correct cable?

Is it because true 120 ohm cables cost much more than the "speaker" wire that they don't recommend it?

I wonder what effect using cable with the incorrect impedance would have on the reliability of the BACNet/MODBUS network? Is anyone aware of any studies?

Here is what one manufacturer of the driver IC's for RS-485 has as far as application notes https://www.maximintegrated.com/en/a...dex.mvp/id/763

Good day VAEngineer,

In regards to your questions

I have been reviewing submittals lately on a couple projects and I consistently notice that even really big companies like JCI recommend wire that does not meet the requirements of the RS-485 specifications. For instance for JCI they recommend a Belden part #B5501FE <snip>. When I go to the Belden catalog for that wire this is what I find https://catalog.belden.com/techdata/...E_techdata.pdf it is actually wire intended for "security and sound" applications with an impedance of only 16.4 Ohms. The RS-485 specification specifically calls for an impedance of 120 ohms.

Firstly, be aware that a wire's specs are and can be different depending upon how the wire is to be used. For example, for low frequency or DC use the wire's resistance or resistance per foot is usually the dominating spec. However, for higher frequencies or if the wire is to be used for a communication bus then there are wire specs that take precedence... for example capacitance, inductance, or the wire's characteristic impedance. For MS/TP (or JCI N2, or any RS485 use) the DC parameters (resistance per foot, etc) are not a concern as the primary use of the wire is for communications and so capacitance (and indirectly inductance) and
characteristic impedance are the ones that are important. Why are they important you ask? Well, these high frequency parameters affect how the communication signal travels along the wire and so improper wire specs will result in all sorts of communication problems like offlines, packet loss, etc... generally a flakey communication bus. With that said the 120 ohm spec for RS485 is not the resistance per foot that most people think off, but is the characteristic impedance (usually denoted by Zo) of the cable. Matching Zo will make the communication cable highly noise immune and will ensure a very high signal integrity of the data signals resulting in a properly operating RS485 bus. Basically if you are planning to use a cable as a communication bus then ensure that cable has a Zo spec... if the cable does not have a Zo spec then it will be a gamble if the wire will work or not... or if it will work for different baud rates (frequencies), weather conditions (changes in humidity will affect the signals, etc).

Why did JCI spec such a crappy cable? That is the question, as they and pretty much all larger manufacturers should know better. It is true that low data rate (low frequency) applications (e.g. JCI N2 bus) are far more forgiving with regards to improper cable. However, even in these cases the crappy cable can bit you in some locations and conditions (weather, long distances, etc). Sadly, if JCI would have spec'd the proper cable even in the N2 days the same cable could have been using for higher data rate applications (e.g. MS/TP) without issue. Now, these N2 customers will need to install new cable in order to support higher MS/TP data rates...

The bottom line is to use the proper cable and wiring practices and not have to worry that any unexpected issues are not cable related.

Cheers,

Sam

[s2sam]

BTW - I do realize that these systems are forgiving enough that they will most likely work using any 22/24ga twisted pair and shielded cable, but why take the chance and not use the correct wire? I would hate to troubleshoot something like the actual wire being the cause of an issue, no one would believe you if you said that was the issue.

BTW - Schneider Electric also says it's OK to use a T-tap on their MODBUS wiring as long the T-tap is under 20m!!! I was always told you keep it to the absolute minimum you can get away with.

Good day VAEngineer,

I do realize that these systems are forgiving enough that they will most likely work using any 22/24ga twisted pair and shielded cable

That is a big gamble and I beg to differ. Your statement would be highly dependent upon the data rate (frequency), cable length, number of devices attached the cable, communication protocol, background electrical noise, weather conditions, etc. For a thorough explanation as to "why" I would suggest you review books on transmission line theory.

As for T-taps... You can use these if the t-tap length is really short compared to the wavelength of the signals that are propagating. Again, this is transmission line theory and it is based upon the frequency and high frequency characteristics of the cable. Will large length t-taps work? "Maybe" .... the question is are you willing to take the risk and then troubleshoot and potentially reinstall the cable properly later? Sadly, vendors are not directly in front of the customer and so they are not the ones that have to solve these types of problems...

Cheers,

Sam

[s2sam]

The 485 standard is well established. Every piece of mass produced silicon that works with 485 is designed for and will be present in 99.995% of 485 devices. The impedance and capacitance is spelled out in the specification.

Maybe JCI has some custom non-standard transceivers, or they just use any crap cable because they can't be bothered to document things correctly. Might just recommend networks be limited to a handful of devices to sell more equipment and deal with coat hangers for comms cable. Who knows.

The spec is clear. Its been time tested and what the ecosystem is designed around. If JCI says coat hanger is fine, that's certainly in their world only.

Good day Orion242,

Well said and indeed JCI's RS485 interfaces use standard RS85 transceivers with an excellent design to address electrical transients. However, it seems to me that their electrical design group are not the ones who generate the wiring suggestion documents...

Cheers,

Sam

[s2sam]

Seems to me this is one reason people are pushing for Ethernet, they can go to the supply house and look for a box that says catX and be good. None of the boxes will mention RS485 comm and you make people think about the spec and that's where things go really wrong.

Good day MaxBurn,

I agree. Ethernet use has exploded over the past several years and so the availability of proper Ethernet cable is everywhere and its practices are reasonably well understood. However, back in the day Ethernet had its growing pains and supporting it in hardware was not nearly as easy as RS485. The software requirement for Ethernet is much higher and so it was use only in higher end processors with sufficient horsepower and memory. Nowadays processors, etc have a lot more performance, etc and so Ethernet is much easier and cheaper to deploy. RS485 had the advantage of being multidropped and this has now changed with Ethernet, as now one can get Ethernet physical layers (PHYs) that have multi ports and so technically it can also be multidropped.

Cheers,

Sam

[s2sam]

I'm not a JCI guy, but I know JCI recommends a maximum of 32 devices per segment on their facility explorer MS/TP products which is quite a bit lower than other vendors. This may be due to the transceivers they use, but I suspect they are being careful to advise smaller networks to let installers use more varieties of wire and not be as strict about wiring practices.

In that JCI guide you referenced, it also allows the use of 18AWG and various other cable types, some unshielded for shorter runs. I'm guessing they're designing the guidelines for cost and flexibility. Given that they approve it, I would hope that means it's well tested.

Good day zzip7,

I suspect the 32 device load has more to do with the controller's ability to support this number of devices than the cable. These days one can use RS485 transceivers that can have 64, 128, or 256 devices on a bus. The caveat here is that the wire has to be properly spec'd and if one can deal with the bandwidth limitations of having so many devices attached.

Given that they approve it, I would hope that means it's well tested.

I would not bank on this. Having said that the latest JCI MS/TP cable specs are much better than the old security/intercom wire they originally spec'd. In fact I have a spool of this new spec'd wire and it tests OK for lower baud rates. I have yet to test it at higher baud rates, distances, etc, but I will at some point to see how it compares to a properly spec'd RS485 wire. Once I do I will post up the results.

Cheers,

Sam

[s2sam]

Good day All,

Although not directly related to "wire" per se... Here is a past post that has some links to some RS485 videos that some may find interesting.

https://hvac-talk.com/vbb/showthread...ideo-Tutorials

Cheers,

Sam

[orion242]

Seems to me this is one reason people are pushing for Ethernet, they can go to the supply house and look for a box that says catX and be good. None of the boxes will mention RS485 comm and you make people think about the spec and that's where things go really wrong.

Good day MaxBurn,

I agree. Ethernet use has exploded over the past several years and so the availability of proper Ethernet cable is everywhere and its practices are reasonably well understood. However, back in the day Ethernet had its growing pains and supporting it in hardware was not nearly as easy as RS485. The software requirement for Ethernet is much higher and so it was use only in higher end processors with sufficient horsepower and memory. Nowadays processors, etc have a lot more performance, etc and so Ethernet is much easier and cheaper to deploy. RS485 had the advantage of being multidropped and this has now changed with Ethernet, as now one can get Ethernet physical layers (PHYs) that have multi ports and so technically it can also be multidropped.

Cheers,

Sam

Not really buying into this.

What installing controls contractor isn't already buying cable by the pallet directly from wire distributors? I have a full menu of cables to select most of which is stocked / short lead time. One hassle with cheap cable is going to piss 10x the cost in labor vs just getting the correct cable. Now the fact JCI calls out non-standard cable seems to be a nightmare. So what does support say if you use 485 standard cable, not JCIs blessed cable, and need comms support? Do they just blow you off if you haven't used the blessed coat hanger? Seems like a total fiasco between installing contractors and manufactures if this is what's going on.

Ethernet is just trading one set of problems with another IMO. The install needs a bit more attention to detail, networking knowledge and all the added security issues. How many hours have you pissed away with customer IT trying to get things working? Pick your poison.

[s2sam]

Not really buying into this.

What installing controls contractor isn't already buying cable by the pallet directly from wire distributors? I have a full menu of stocked cables to select most of which is stocked / short lead time. One hassle with cheap cable is going to piss 10x the cost in labor vs just getting the correct cable. Now the fact JCI calls out non-standard cable seems to be a nightmare. So what does support say if you use 485 standard cable, not JCIs blessed cable, and need comms support? Do they just blow you off if you haven't used the blessed coat hanger? Seems like a total fiasco between installing contractors and manufactures if this is what's going on.

Ethernet is just trading one set of problems with another IMO. The install needs a bit more attention to detail, networking knowledge and all the added security issues. How many hours have you pissed away with customer IT trying to get things working? Pick your poison.

Good day Orion242,

Understood and like you said... pick your poison.

One benefit with Ethernet is that the cable is pretty much everywhere and so there is no real need to stock large quantities of it. Secondly, most people understand that you "must" use Ethernet cable (Cat 5, 5x, 6) in order to have an Ethernet network work properly. For some reason the same people that complain that you have to use the correct cable for MS/TP (or any RS485 network) are perfectly content to get and use the proper Cat cable for Ethernet... Given how "common" Ethernet Cable is I am thinking it is becoming a commodity and so eventually the differentiating factor will be price...When this happens there will be manufacturers/vendors that will cut manufacturing costs to a point were the Ethernet cable spec's are compromised and how will the customer know? Only when it is installed and tested will one know if the cable is up to snuff.

Also, an Ethernet network can be cranky as well simply because of its high data rate. This means one has to be aware of the bend radius, or the amount of "pull" on the cable, otherwise problems will arise. I wonder how many Cat cable installers have the right test equipment to quantify how well the Cat cable is working? On many sites I see installers simply pull cable and terminate without running diagnostics and reports on the cable's signal integrity. I feel sorry for the networking installer, as now they have to figure out if it is their equipment or the cable.

You are quite right that one has to have a solid understanding of Ethernet topology and networking... and it is pretty much a guarantee that one will be working with the site's IT department. Another factor is that once you are gone the IT department or others may use the same Ethernet network for other unintended uses (user's desktops, etc). RS485 has the advantage that you are the master of this domain and there is a very slim chance that the RS485 cable/network will be used for anything else but the BMS. In the end I use and design with both technologies, as I see distinctive use cases where one is the clear winner over the other and so I a happy to use both.

As for JCI's cable recommendations... all the doc's I have seen these are "recommendations" and not "must use" cables. Will JCI not support you if you use better cable? If they did then a lot of JCI Branches past installs would also be punted for support, as they use any kind of wire that they could get their hands on... including the coat hanger style you mentioned On the positive side, using proper cable and wiring practices will pretty much focus any issues on JCI's field devices.


Cheers,

Sam

[orion242]

One benefit with Ethernet is that the cable is pretty much everywhere and so there is no real need to stock large quantities of it.

Not a teledata contractor. I need lots of different cables for I/O, so having to carry good comms cable is a nothing burger. We deal with 485, lon and Etherent yet and stock appropriate levels based on what we do. If we need wire, I think its less than 3 days order to dock.

Secondly, most people understand that you "must" use Ethernet cable (Cat 5, 5x, 6) in order to have an Ethernet network work properly. For some reason the same people that complain that you have to use the correct cable for MS/TP (or any RS485 network) are perfectly content to get and use the proper Cat cable for Ethernet...

Where the heck where these same jokers when Lon was big? Every try use screwball cable with that?

Given how "common" Ethernet Cable is I am thinking it is becoming a commodity and so eventually the differentiating factor will be price...

We only work with plenum rated cables just to avoid the sting of some installer screwing up and using non-plenum rated when its required. Is CATx plenum as common...I think not.

I wonder how many Cat cable installers have the right test equipment to quantify how well the Cat cable is working?

Almost none, which leads to nightmares. Have a site now that works fine normally. If I try to pull 200GB server backup image over the network, things go wonky and its not the switches...its a $hit cable or termination somewhere that only presents itself under heavy transfer. Almost all installers I have seen, including our sub-contacted work using nothing more than sub $100 test gear that simply looks for opens/short/swapped wires. That might link up fine, but under load / pushing the distances to the limit...be a real PITA. Let's compare the cost of a cheap tablet 70mhz scope to CAT6+ cable analyzer.

You are quite right that one has to have a solid understanding of Ethernet topology and networking... and it is pretty much a guarantee that one will be working with the site's IT department. Another factor is that once you are gone the IT department or others may use the same Ethernet network for other unintended uses (user's desktops, etc). RS485 has the advantage that you are the master of this domain and there is a very slim chance that the RS485 cable/network will be used for anything else but the BMS. In the end I use and design with both technologies, as I see distinctive use cases where one is the clear winner over the other and so I a happy to use both.

I have no issue with either. We have the option of Ethernet based controllers since ~2003 and used tons. To say one is better than the other just because of the cable required is silly.

As for JCI's cable recommendations... all the doc's I have seen these are "recommendations" and not "must use" cables. Will JCI not support you if you use better cable? If they did then a lot of JCI Branches past installs would also be punted for support, as they use any kind of wire that they could get their hands on... including the coat hanger style you mentioned On the positive side, using proper cable and wiring practices will pretty much focus any issues on JCI's field devices.

No idea. I limit my contact/use of them as much as possible. Just the fact they would "recommend" non-standard cable is a support nightmare IMO. Why the hell do that??

[s2sam]

Not a teledata contractor. I need lots of different cables for I/O, so having to carry good comms cable is a nothing burger. We deal with 485, lon and Etherent yet and stock appropriate levels based on what we do. If we need wire, I think its less than 3 days order to dock.




Where the heck where these same jokers when Lon was big? Every try use screwball cable with that?



We only work with plenum rated cables just to avoid the sting of some installer screwing up and using non-plenum rated when its required. Is CATx plenum as common...I think not.




Almost none, which leads to nightmares. Have a site now that works fine normally. If I try to pull 200GB server backup image over the network, things go wonky and its not the switches...its a $hit cable or termination somewhere that only presents itself under heavy transfer. Almost all installers I have seen, including our sub-contacted work using nothing more than sub $100 test gear that simply looks for opens/short/swapped wires. That might link up fine, but under load / pushing the distances to the limit...be a real PITA. Let's compare the cost of a cheap tablet 70mhz scope to CAT6+ cable analyzer.



I have no issue with either. We have the option of Ethernet based controllers since ~2003 and used tons. To say one is better than the other just because of the cable required is silly.



No idea. I limit my contact/use of them as much as possible. Just the fact they would "recommend" non-standard cable is a support nightmare IMO. Why the hell do that??

Good day Orion242,

Yup, no arguments on my side... I especially like your comment:

To say one is better than the other just because of the cable required is silly.

Cheers,

Sam

[orion242]

Ran into this looking in my inbox to remind me what we use for comms cables.

Had a project that the equipment required a specific cable for comms. Just comms between some air compressors and dryers, need 150’ max. OEM docs were pretty useless in real info I wanted (modbus integration, nothing for registers…each setup is custom yea). Only thing I got from the docs was ample notes on that specific cable, its CAN and they have these special terminations that look like wildly expensive DB9s. Whatever, we are on the hook for the inter unit comms cable and know this up front. Ask WCW for a quote on a box. Not like one can walk into the home center and get boutique cable by the foot.

Belden 3106A

Looking at this again, I see Belden markets this as 485 cable. Pricing was leaving earth orbit and assume that's about all its used for. Long done project now, critical and never a comms issue. The fact this has been the case, the cable price was insignificant compared to issues this could have caused using anything other than the recommended cable. That said I can’t imagine any reason this OEM has for this cable. They have no high-speed requirements, sure they run into nasty environments, long distances. Nothing different that what we deal with. Wrap coat hanger around 1K HP VFD...yep have to account for stupidity. If I had to guess as to why they went to this extreme, they got burned with coat hanger install once and said never again. This isn’t your typical HVAC air compressor / dryer setup. Still, absolutely nothing they do requires this cable far as I can see.

This cable request was apparently unique enough that the WCW wire rep reached out to see what the hell I was using it for. After talking with them, apparently Big A$$ Fans also spec that cable for some of their installs. Only baffles me more why something as simple as their gear would require this stuff unless its just a knee jerk reaction too getting screwed once. “Just give me the best cable and require it. Fail, its unsupported and piss off.”

OEMs recommending, suggesting or failing to specify 485 standard cabling are just trashing the ecosystem that’s otherwise solid and proven. Be it $hit cable or gold-plated moon dust insulated assembled by unicorns, its a real industry problem with some OEMs. The same jokers will be writing the specs on any other medium they move to and many new to the field will blindly follow these recommendations.