RS-485 cable VS. Cat-6

[mrdom]

Hi everyone: We're in the process of installing an Interlogix TruPortal door access control system on our campus, and we're seeing if it would be feasible to connect the systems together so we can have one user database and event log. We have three buildings on our campus, and each one will be upgraded to automated door access control.

In order to connect everything together, The TruPortal documentation refers to the SNAPP bus. The bus is located at the system board, and can be used to connect devices "downstream". The modules get their own power, so it doesn't seem we're running power, only comm to connect the modules together. The documentation calls for RS-485 22AWG wire. Right now, the buildings have extra Cat6 wire running between them, and it would be super simple to utilize one of the extra wires to provide the comms to the remote door controllers. The Cat6 cable we have is either 23 or 24 AWG, so I'm not sure if we could get it to work. I've read several posts in which users have used Cat6 in place of RS-485.

Would this be a possibility, or do we need to bite the bullet and pull some RS-485 cable through to the buildings to connect the remote controllers up to the system board? Total run from the farthest building to the mother ship is no more than 700 feet.

Thanks for the help everyone!

 

[budbyrd]

RS-485 is a spec for a higher voltage, long haul version of RS-232 serial connection. The cable it's self isn't that important. Cat6 would be fine.

Dermis and feline can be divorced by manifold methods.*
*(Disclaimer for all advise given)--'Version Dependent'

 

[Wanebo]

I would stick with the manufacturers specs on this one simply because manufacturers get picky about supporting configurations that do not adhere to their specs when it comes to something not working correctly.

22AWG is a bit more robust than anything you will find in a CAT6 cable.

 

[mforrence]

Consider using RS485 to Ethernet gateways instead of running cables end-to-end. I'm actually a bit surprised that your access controllers don't have native Ethernet connectivity. Not to dis RS485, as it's a solid solution that has been around for 30 years - i't simply that a single-technology infrastructure is often easier to maintain. Good luck

Mike Forrence

http://www.goteamvox.com

 

[libellis]

Keep in mind that native RS-485 distance will depend upon the speed it's running at over the particular cable type, and the speed wasn't identified. RS-485 can be delivered over significant distances on Category rated cables, but speed, the quality of installation, and the field conditions the cable is exposed to matter. For example, assuming a good quality link, at 1Mbps, 700' is probably achievable over Cat 6. (Per the original post, delivering power is not a factor here.)

Ethernet at the 700' that was mentioned would violate the total 328' channel length specified in the standards. That isn't to say it might not work at 10Mbps over Cat 6, but that solution introduces a similar sort of uncertainty to the solution, and 700' really seems to be pushing the envelope.

 

[mrdom]

Thanks for all of the helpful posts - lots to think about. The access control system interfaces into the network through TCP/IP. At the system card, I can log in, assign users, schedules, etc. That portion is easily configured through our network. When it comes to connecting other add-on modules though, they apparently don't connect through TCP/IP. They connect using this SNAPP bus system that's connected with RS-485 cable.

For our network installation, we start with a switch in building one, and a cable from there goes through the building, then underground through conduit into building two, and plugs into a switch there. Both ends have lightning protection. From building two, another cable leaves the switch and goes through an in-building pipe into building #3 and gets plugged into a switch there. Each building has its own network switch and everything is connected together that way.

I'm not sure what the COMM speed is for the TruPortal system. I tried looking at a data sheet and can't find any documentation on the speed of the SNAPP bus.

I've never used an RS-485 to Ethernet gateway. Is the gateway physically on our network, or does the Cat6 cable connect to the ethernet port on one end and feed to another device at the other end and back into the SNAPP bus? Not sure how that would that work in our situation?

With regard to the SNAPP bus, this was everything I could find on it. Perhaps this information will spur more comments:

From the Interlogix web site:

*There are four independent RS-485 buses. You can use any or all of the four buses. (31 2-door add-ons maximum; 8 I/O add-ons maximum; 24 modules per bus maximum)

•Using multiple buses limits the failure scope in cases of individual bus outages

•Maximum distance for each bus is 4000 feet (1200 meters).

Not sure if this helps or not. Thanks for your expertise!

 

[libellis]

4000' maximum distance implies that the speed is relatively low, so 700' on Cat 6 would seem possible. However, note that there are other considerations regarding the maximum number of RS-485 interfaces that can be attached to the bus and that maximum would be affected by wire gauge. Although probably not a factor here, out of curiosity what is the maximum number of RS-485 devices you expect on a bus section?

Per my earlier comment, Ethernet at 700' - even at 10Mbps over Cat 6 - should really not be considered a solution.

 

[mrdom]

I did some checking today, and it appears that my estimate on cable run length was sorely wrong. I'm a terrible estimator! We're talking 400 feet max. It's 200 feet between buildings, and not even 200 feet to the next location.

Although probably not a factor here, out of curiosity what is the maximum number of RS-485 devices you expect on a bus section?

We would be attaching 16 devices in building 1 and 13 devices in building 2.

 

[libellis]

At 400' speed/distance likely disappears as an issue.

As long as you keep the number and type of devices/modules on a single bus within the limits given by the TruPortal specs, seems like you should be okay. (Assumes no odd issues with noise or other problems in the shared pathways - RS485 is reasonably robust since it uses balanced signaling and the Cat 6 grade cable also helps to reduce the likelihood of problems.)

 

[mrdom]

Soooooooo ... I found out today that the wire needs to provide both comm (1 pair) and voltage (6-12 volts). Would Cat6 still be a contender, or is that going to force me to RS-485 now that we have a voltage requirement?

 

[libellis]

That will depend on how much current is drawn, and consequently what worst case voltage drop the equipment can tolerate due to the current and cable resistance. Note that the current must be within the ampacity rating of the wire gauge in the Cat 6 cable, and the cable pair and overall jacketing must be able to safely tolerate the resultant heat rise. These could be complete non-issues, or they could be important - all will depend on current and resistance.

Assuming 12 volts is both the source voltage and the maximum voltage the equipment can tolerate, then the voltage drop through the cable at the furthest equipment location must be less than 6 volts to keep the minimum voltage above 6 volts. Knowing the cable length and wire gauge (from which resistance can be determined), it's of course simple to calculate what current would produce a maximum 6 volt drop. (If the assumption about 12 volts being the source voltage is not correct then other conditions will apply, but the key is to know the current and resistance.)

 

[libellis]

I think you should go for it - no reason to make this project painful since you are not anywhere near the maximum allowable run length (4000'), and the resistance differences from that of #22AWG at the real distances involved (400') shouldn't matter. Note that:

#24 AWG copper wire has a resistance of 25.67 ohms/1000' at 68 degrees F
#23 AWG copper wire has a resistance of 25.36 ohms/1000' at 68 degrees F
#22 AWG copper wire recommended by the documentation has a resistance of 16.14 ohms/1000' at 68 degrees F


Also, just for the record - doesn't change what was said above - I want to modify something I did say earlier:
Your situation might involve devices running from this voltage source that are attached to the bus at various distances along the cable run.. In that case the maximum current flows only the distance to the first device and then current decreases along the cable as each device siphons some off, which is a better situation from a voltage drop perspective, but more tedious to calculate.

 

[mrdom]

Thanks for all of the help. libellis - I really appreciate your expertise and knowledge in helping us figure this out. Once we have it up and working, I'll report back with how things went!

 

[mrdom]

Well ... the saga continues. The installers checked with Interlogix, and they flat out refused to allow us to use our existing Cat6 runs to connect the remote controllers ... they wouldn't honor the warranty if there was a problem. Argh!

They said we had to use an "RS-485 compatible" cable. I've got a note into Interlogix to ask them to clarify, but this is all becoming way too confusing for me I think. In the TruPortal documentation, they recommend 24 AWG wire, and give Belden 9842-spec wire as an example. The installers feel as though the wire is too thin for our run of approximately 400 feet. They want to run 18/4 shielded wire for both data and power (data on one pair and low voltage on the other pair). I'm guessing the 18/4 wire they're referring to is the standard security wire they use, but this stuff is insulated. If we can use 18/4 instead of the Belden style cable, it would be much better for the budget because the Belden cable is super expensive.

I think I'm going to have to wait for Interlogix to give me more information, but could a 18/4 wire work for RS-485 data comm and voltage?

Thanks again for the continued help.

 

[libellis]

I wouldn’t know if 18/4 cable in general, or the specific variant of that cable the installer is proposing to use, is suitable for RS-485 in your application (or at all). However, I would recommend that you ask the installer if they have ever used that 18/4 in an RS-485 installation, and if so, if the circumstances were similar (or even more stringent) to yours. If the answer is “no”, I would be wary.

I do know that when a non-RS-485 specific cable is used in an RS-485 application, it’s best that the cable have twisted pairs and that the cable impedance be a reasonable match for the RS-485 bus (120 ohms). Category (5e, 6) cables - which are commonly used for this application - of course have twisted pairs, and their nominal impedance is 100 ohms – not a perfect match. For 18/4 cables I’ve seen impedances in the 40 ohm range (don’t know about the particular cable your installer proposes), but I simply do not know how much any of this would matter in your case.

One other set of factors is important here. Since your installation is in a campus environment with conduit running between buildings, I assume the conduit runs outdoors (i.e. buildings not physically connected) – you did say the Cat 6 cables were lightning protected at each end. If that’s the case, were “outside plant” type cables, or dual indoor/outdoor fire rated cables. used in the conduit runs (cable could include internal water blocking)? If the answer is “yes”, then any new cable should meet similar requirements, including those imposed by Codes (e.g. Electrical Code), and any shield would need to be grounded. After the cable exits its point of entry in the building it must have a fire rating suited to the environment it will be running in as per Electrical Code, or make a transition to a suitable cable. If the cable will be running exposed (i.e. not in metallic conduit) between floors then it must have a riser fire rating. If it will be running exposed in plenum spaces (e.g. a ceiling space that is also used for return air), it must have a plenum fire rating. How does the proposed 18/4 fit into any of these requirements? The Belden cable was rated “CM” which is neither riser nor plenum and can only be used in general spaces, but other than having a polyethylene outer jacket – normally associated with outside plant cables – it wasn’t clear from its specification if that is an intended use, and no water blocking was mentioned.

 

[robertjo24]

Your installers do not know the difference between communications cabling and control cabling. The 22 AWG cable may be too small for the controls signal (voltage drop, current rating…), but it’s much better for the communications signal. Google cable capacitance and line speed.

Cat-6 works better for RS-485 than 18/2 or 18/4 ever will.

 

[mrdom]

Thanks everyone for the continued help. Interlogix has approved the use of 18/4 wire for the comm needs. Interestingly enough, the cable is NOT carrying voltage ... only COMM. I would agree with Roberts' comment above ... I can't understand why Cat6 wouldn't be acceptable here. Interlogix claims that Ca5/6 used on their SNAPP bus can have "undesired results" and cause intermittent communication failures between the remote readers and the main panel. Who's to say ... I guess we'll use the 18/4 as they have advised.

For the first run, the cable will be going underground and into the next building. For the second run, the cable passes through a pipe that's installed in the walls between both buildings, so it's not really leaving the outdoors on that run. Interlogix advised that I not use shielded cable for the runs.

Thanks everyone,
M

 

[libellis]

For COMM, Cat 6 is for sure superior to 18/4, that wasn't in question, it was if 18/4 was really usable. Interlogix's response really doesn't make sense. Anyway, let us know how you make out. Good luck.