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Author Topic: RS-485 for physical layer  (Read 5992 times)
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Graynomad
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« on: January 29, 2011, 11:40:22 PM »

I've decided to use RS-485 for the physical layer, in preference to LIN.

Why? Mostly because the network was originally designed for vehicles and now I also see it as being used in light-weight process and building control. And RS-485 is pretty much the standard in that field.

This means that the cable has to be 4-core.
« Last Edit: February 10, 2011, 04:29:22 PM by Graynomad » Logged

Rob, aka the GRAYnomad
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« Reply #1 on: February 11, 2011, 04:42:38 AM »

You had earlier said the BUSnet would be somewhat independent of physical layer, is that still true? Can be both and more, as long as it meets all other requirements?
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« Reply #2 on: February 11, 2011, 10:10:02 AM »

It should still be true, however...

One problem with allowing any physical layer is compatability, my temp sensor won't work with your MCU etc.

One reason for defining a "standard" is to allow connectivity between gadgets made by different people, and while flexibility is nice it doesn't help that goal.

So I think we have to say BUSnet uses 485 end of story, if someone wants to make a Node that uses a different format for their own network that's OK but it won't be compatable and therefore won't inherit any previous work (at the hardware level anyway).
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« Reply #3 on: February 11, 2011, 04:56:40 PM »

OK. It allows others to use "adapters" as needed, like the USB to serial adapter (or in fact the 232 to 485 adapters I have seen). Do what you want to the signal as long as you put it right later, eh? BTW, giving up the LIN loses some features, if I remember right and might therefore raise the minimum parts count a bit.
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Graynomad
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« Reply #4 on: February 14, 2011, 02:53:04 AM »

Quote
BTW, giving up the LIN loses some features, if I remember right and might therefore raise the minimum parts count a bit.
The only thing I can think of is the 50mA on-board voltage reg that many LIN transceivers have. There is however at least one 485 transceiver with this feature (only 20 or 30mA though) so I think we're still OK with the parts count.

Worst case it to add a tiny 5v reg I suppose, but I think even 20mA should be enough for most simple Nodes.

RS-485 also needs a few resistors to terminate the line, however I think at the data rates I'm talking about these won't be required. (maybe with a really long line, will have to do some maths there)
« Last Edit: February 14, 2011, 02:54:57 AM by Graynomad » Logged

Rob, aka the GRAYnomad
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« Reply #5 on: March 18, 2011, 12:58:43 PM »

Since I am fairly unfamiliar with RS-485 I was doing a bit of reading on the subject and noticed that the official spec only caters for 33 devices.

Quote
Maximum Devices: 33 - Including one detatchable terminal (32 drivers and 32 receivers)
  - source: wikipedia
http://en.wikipedia.org/wiki/EIA-485

Is additional hardware required to increase this number? Words like impedance and inductance come to mind but I have yet to find out if they apply here..  Smiley

Cheers,
marcel
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Graynomad
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« Reply #6 on: March 18, 2011, 01:17:02 PM »

Hi Marcel,

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the official spec only caters for 33 devices
That's true.

Actually it's 33 "unit loads" or ULs, which when the spec was designed was the same thing as a transceiver chip, however for some time now chips have been better and they are rated at 1/4 or even 1/8th UL.

So worst case these days would I think be 32 x 4 nodes, but even 32 x 8 would be possible.

BUSnet has an 8-bit address field but only allows for 64 physical addresses (58 for user Nodes) so we should be right with this. The remaining addresses are logical which don't count in this context.
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Rob, aka the GRAYnomad
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