RS485 implementations

[mike_au]

I'm in the early stages of designing a home automation system. I'm thinking of using RS485 for most of the communication, but I'm not quite sure how I want to wire it.

The building is only about 30x10M and I am planning to use either Cat5e or Cat6 cable (I realise Cat6 is excessive but I have quite a bit spare).

The three options I have come up with are:

A single run that hops from once device to the next - This minimises the length of cable and is the normal approach but seems like it might be harder to maintain and troubleshoot.

Point to point runs from the controller to the devices - Obviously RS485 is a linear bus topology, but I have 4 pairs available per cable so I could easily run out to the device on one pair and back on another. Cable length is much longer but should be OK at low speed, and it seems like it would be easier to isolate a device by repatching at the controller if a device or cable fails.

A combination of the two - I could run a number of cables out from the controller to different areas of the house, in each area the cable comes out, runs in a loop to the devices in that area and returns, the return pair is then jumpered at the controller onto the cable running to the next area. Obviously this is a compromise between the other two options. One advantage of this approach is that the main run to each area could be terminated with a standard RJ45 cable and wouldn't be specific to the HA system, it could be used for ethernet security cameras, wireless APs, etc. so it is easier to justify over building it a bit giving better flexibility in the future.

I've never designed any wired communications over more than a couple of meters so I am interested in other people's experience in setting up and maintaining an RS485 or other long range (well...building sized) communications systems. Is a physical star topology significantly easier to work with?  Is it likely to cause more problems than it solves due to the extra length that would be introduced? Is there a better alternative to RS485 (bandwidth in the low 100's of Bps, half duplex would be sufficient). Are there any alternatives that use a logical star topology (all the major ones seem to be a linear bus)? Any other issues I should be thinking about?

I've read plenty of technical documents but I am curious to hear about people's real world experiences.

[koky]

rs 485 is very solid, i have make many system and your ideea can work without problem

i have tested rs485 (9600 bps) long more than 600 mt make with normal ac cable 2x2.5mmq not twisted, working without problem by more the 10 year

[Catcatcat]

You can apply any twisted pair. At a distance of 30-60 meters at low speed (9600) will work even on regular cable.

[Ichan]

30x10 meter is not large, better run the cable as linear bus - as you said multiple pair cables will help the hassle on installation of some separated unit.

On larger building let say a factory, consider to use repeater / multiplexer - it will then form a mixed star + bus topology.

-ichan

[Vineyards]

Twisted pair cable is pretty much the standard. If it is shielded it needs to be grounded on one end only to avoid ground loops.

[mike_au]

Sounds promising... I've got several boxes of Cat6 and a couple of Cat5e, all UTP so once I've finished cabling for ethernet I'll use whatever is left for the HA cabling.

Koky: I hadn't considered running over mains cable. I won't try it for the new sections, but longer term I was thinking of replacing light switches with touch screen interfaces and being able to reuse existing mains cable might make that a simpler project.

Ichan: As I understand it the repeaters/multiplexers are used to convert to a physical *and* logical star topology, I was thinking of using a physical star but logical bus. So the devices would still be connected one after another, but the cable effectively runs back to the central controller (over one of the spare pairs) before running back out to the next device.

Maybe I'm over estimating the amount of maintenance that would be required, I have good quality cable and as long as the devices are well designed I'm hoping I won't have to touch it much once it is in place, so perhaps I shouldn't stress too much about how easy it is to isolate areas/devices.

So no other competing protocols I should be considering?

[zab]

In implementing RS485 normally twisted pair cable is used.You have to add protection devices across it to make it long lasting.
For distance Just reduce the baud rate and increase the distance and at the end use a repeater to double it.I have used rs485 with 40 controllers spread over more 5 Km.
One more thing to take care of is that the controller's default mode should be listening(receiving)so if some one just hangs may not create problem for the other devices on line.

[myheadhurts]

National used to have an application note called "10 ways to bulletproof rs485 interfaces"

Its been updated by TI and can be found here..

Code:

http://www.ti.com/lit/an/snla049b/snla049b.pdf

Well worth reading.

Rob

[max]

Hi,
see the following link, single wire is used to carry the power and data signals.

http://www.romanblack.com/blacknet/blacknet.htm

Holtek also make an ic to do the same job, dc power line data transceiver.
see the following link and app notes.

http://www.holtek.com/english/docum/uc/71d0x.htm

 

[mike_au]

Max: Thanks, those look interesting.

I quite like RB's set up. I believe it would be more susceptible to noise since it is single-ended rather than differential but I suspect that at low speed, over a relatively short and not too noisy run it should be OK. I will be running some ethernet cabling around the place so I can jumper a few runs together and see how it goes before committing.

Much appreciated

[Gallymimu]

I've had very good luck in VERY noisy environments (100KW PECVD plasmas with heavy arcing) using Cat5 twisted pair, and RS485 over 100' at 10mbit.

Siemens commonly uses profibus which is RS485 at serveral mbit.

Use terminator resistors though!

[dipchip]

Hello Mike,

I have faced the same problem, how to wire a star topology into a linear cable.  My project required that I supply Vcc and ground from the cable also.  Using Cat5 I wired it like this.

org/org wht -> 12v
brn/brn wht -> gnd

these two do NOT need to be wired as a ring.  you can tie all of them together and run them to a power supply as needed.

grn/grn wht -> A/B send
blu/blu wht -> A/B return

the return pair gets jumpered to the send pair of the next cable and so on.
you can terminate the last pair right at the wiring hub also.

At low speeds and in as small of an area as your talking about, this will work.  I hope its not to late to make use of this idea.
The last post is almost 2 months old

--Chip

[f22kma]

Have you thought about making a bridge to the X10 or UPB home automation protocol? You can then use a number of off-the-shelf devices and devices where power is available but running data cables is difficult.