RS485 with many devices

What about using ethernet, and buying cheap switches for the nodes where things fan out? You can get small processors with built-in stacks these days.

Tim Wescott
Wescott Design Services
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Tim Wescott
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I need a bus solution with the following requirements:

- Differential (current or voltage mode).

- 2 wires (so, half-duplex).

- Minimum speed: around 10 kbps. So, relatively low.

- Nodes will be interconnected forming a network with branches. Maximum branch length may be around 100 m, and worst-case point to point distance could be 300 m, roughly. Can't tell exactly the shape of the network, yet.

(So far, RS485 would do the job, but...)

- I need around 4000 devices connected to that network. The highest number of RS485 transceivers that I have found that can share a common bus is 256, using some MAXIM IC's (I'm sure there are also other brands).

Does anyone know of any solution that allows so many devices?

Also, any recommendation on cheap repeaters?

Thanks in advance.

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The limit on RS485 node numbers is usually quoted because of the number of bus termination resistors. At low speeds, you can probably get away with much higher termination resistance, or even only one resistor per cluster, if many nodes are near each other on a branch. You will have to do the calculations, but it seems to me that you should be able to have many more than 256 nodes under your conditions.

Another option of course is to subdivide your net into several sub-nets,

256 or less nodes per subnet, and then use another net for the master interconnect, but it adds another level of complexity.

Adrian Jansen           adrianjansen at internode dot on dot net
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Adrian Jansen

Easier to maintain too, if you ask me. 4000 devices on a single partyline sounds like a nightmare, if you have a problem.

Thanks, Frank.
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Frank Bemelman

Adrian Jansen wrote in news:4314e192

Another thing to consider is addressing, no? I don't know what protocol he has in mind but I think Modbus is limited to 247 devices or something. Are there other protocols that support more devices?

Unless he only intends on broadcasting.

Joel Moore

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Joel Moore



Any protocol could be extended to include any number of address bytes.


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Meindert Sprang

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