Author Topic: 24IO pins vs 24 nodes on CAN line  (Read 595 times)

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Offline mack_30Topic starter

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24IO pins vs 24 nodes on CAN line
« on: May 11, 2020, 08:31:39 am »
I am working on a home automation project, and I have around 24 inputs and 8 relay outputs.
Its simple 1 or 0 input consisting of various reed sensors and switches to monitor door, window status or glass shattering.

In my current implementation, I have simply used the IO's but I realised there is an enormous amount of wiring involved plus it gets very bulky when all the wires converge to the central monitoring panel.
When I compare it with market products, most of the products they tend to follow the same logic (single IO's for the single sensor) and I feel I can simply put a cheap controller on each sensor and communicate everything on single CAN/LIN or RS485 channel.

Now my question is more of system analysis rather than a technical one. Is there any added advantage of using a single IO's for single sensor rather than making all of them on a single communication channel.
Only drawback I can think of in single communication channel is since everything is dependent on a single communication line, so 1 fault will cause everything to be useless. Is there anything I am missing here?
 

Offline Niklas

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Re: 24IO pins vs 24 nodes on CAN line
« Reply #1 on: May 11, 2020, 12:20:27 pm »
Cost! For a home project this is not an issue, but for commercial products it could be a deal breaker. CAN transceiver, more complex microcontroller with integrated CAN controller, external crystal instead of internal RC, connectors with more pins, cables with more wires...
 

Offline Siwastaja

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Re: 24IO pins vs 24 nodes on CAN line
« Reply #2 on: May 11, 2020, 01:46:08 pm »
It isn't either or, it can be a combination.

This would be a classical cost minimization problem which you could try to do in Excel, for example.

Assign a cost for each meter of wire. Assign cost for the labor of routing those wires.

Assign a cost for each module - note it's likely much more than the MCU and CAN transceivers; it needs assembly, programming, testing, casing, etc. It's very hard to produce any "box" for less than maybe $10-$20, no matter how stupidly simple that "box" is, and however small the electronic component BOM cost is.

Then group the IOs in different ways, and find the minimum total cost.

My take is that design a module which has preferably excess IO, than too little. Say, a CAN IO box with 1 IO connector might cost $20 to build (electronics, casing, assembly, testing), but with 10 IO connectors, it's not $200, but maybe just $25.

Now, with excess IO in place but unit cost still as low as it gets, you can decide at install time whether you just leave most IO unused, but if you happen to have a lot of devices there close-by, you do have the IO available without needing to build a horrible spaghetti nest of chained single-IO modules.
 
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