Modbus Max. Length

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Thread Starter

Franco Kew

Hi guys,

Currently I am designing modbus routing from control room to a field consists of 15 tanks. I plan to have a junction box place in the tank area.

So, here my question. Since I am using RS485, 1 1/2 pair modbus cable running from control room to junction box. Then, from junction box, I will have 15 single modbus cable going to each of the tank. How the modbus maximum length (1.2km) calculate from the design?

For exm: control room to junction box = 400m
junction box to each of the tanks = 20m

So my total length would be 400m + (20m x 15)?
or my total length would be 400m + 20m?

Please help me... Thanks for any feedback.
 
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Curt Wuollet

That doesn't sound like a workable configuration. 485 doesn't like star layouts. You can use special transceivers to make it work. Wire is cheap, do a daisy chain and all will be well. I get 1km that way but your signal integrity will be much better. Distance isn't the problem with a star, it's all the unterminated stubs. Or, if you terminate them all, the impossibly low impedance coupled to a 120 ohm transmission line. National Semi has a really good application note "How to bulletproof your RS485 network" or something like that. You would do well to read it. Here's a link:

http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snla049

Apparently TI owns the asset now

Regards
cww
 
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You're not supposed to run RS485 in a star topology. Not sure if short branches are officially permitted by the spec or not, but I've done 3ft or so branches off the trunk many times with no issues. 20m branches, I'll bet you have problems.

You should run in a daisy-chain topology. Now you can still run the conduit in a star if you want. Just run 20m down the conduit to your node and back, then up the next conduit. You'll burn 600m of cable that way though. Better if you can arrange to get more directly from slave to slave without coming all the way back to the J-box.
 
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Lynn August Linse

This is an RS-485 issue, not Modbus question.

Max length is more an issue of the cable quality and any 'surge protection' loading your signal beyond the expected RS-485 norms. You can never assume 1.2km is possible for any Modbus. I think the ISO equivalent of EIA485 allows 5-m 'drops', whereas EIA485 assume zero drops (aka: not possible to even have PCB traces ...)

If you really want 20 'home runs', you should buy 10 units of RS-485 repeater. Then make each pair of tanks a single bus (boomerang shaped) with the repeater at the center/inner edge. Then the 10 repeaters share a single internal RS-485.

That said, you will find that by LOWERING the baud rate, you can make most any badly cabled RS-485 work ... if you are willing to talk 600 baud instead of 9600 baud :). For example, I once showed up at a jetty/ship loading site in Malaysia to find simple twisted pair wire designed for 4-20mA on a 600-m run to a batch computer at the jetty end ... main contractor had too much of 'that' cable and none of true RS-485 cable. I knew complaining was pointless, so I just set the baud rate low & poll rate low, insisting that was the only reliable speed this cable allowed, and the end user finally decided they wanted better data rates and forced main contractor to replace the cable! Haha!
 
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Hadn't thought of segmenting the network with repeaters to fix the topology - slick!

Franco, if you do that, IIRC the 485 spec requires minimum of 3ft of cable between nodes or something like that. You probably don't need to follow that religiously, but don't just do the two-inch jumpers the junction box would invite.
 
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Dear CWW,

Maybe I use the wrong wording in my post, but I am not using the star topology. I try to explain the configuration having in my design as following:

"I have using one triad cable running from control room to my junction box (e.g. one shielded twisted pair for signals and third wire for 0V).

From junction box, I use terminal block for each of the wire (L+ TB, N- TB, and E TB). Then, I have 15x triad cable to each of my tanks."

Isn't this is a bus topology? I haven't try out this (to be truth).

Daisy chain topology is a tested and worked-well with modbus (also called multi-drop). But if one of the cable failure will disable the whole network. This is why I use the design above.

Please do advise me if my design isn't really a good idea. I would welcome your reason on top of that.

Many Thanks,
 
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Curt Wuollet

No that's a star and it's a bad idea because 20 M of unterminated cable will give you reflections which will be get worse with each stub added and the succeeding reflections from the long cable. And if you terminate all the stubs, the impedance where they join will be 120/15 which is impossibly low for most transceivers. Adding to this you have a long cable which is now terminated in what amounts to a short circuit which will cause reflection on that cable whose length and propagation delay will almost insure that you completely corrupt your data at any reasonable speed. This is a balanced transmission line and behaves like one, it's not like DC or low frequency AC power. If, as Lynn said, you make each line it's own network with a repeater or similar and you drive the repeaters properly with the long cable it would work fine. But that would be spendy enough where you could just pull spare wire and handle a break a lot cheaper. You can have a solid reliable network for essentially the price of the wire if you actually adhere to the RS485 spec. Or you can spend the cash to work around that with electronics. But I wouldn't bet coffee on the star configuration driven by a long line. I _have_ seen some pretty awful ideas work by some weird anomaly. But if you want long distance and reasonable speed, you want to go by the book on networks. Living with a bad network far outweighs any perceived benefit or cost savings. This is essentially what I said in the first post, so I'll simplify, It won't work. But, here's what you can do. Pull two cables to each boiler. It's only 20 meters of cable. Then you can try it the first way and when it doesn't work, you do the daisy chain and no rework needed. Or if the star works, you can blame me for the extra cable or call it a spare.

Regards
cww who groks transmission lines.
 
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