Peter Todd wrote:

>>> If your communication is downstreams only a current-loop might be a
>>> nice idea, although at 100 nodes the source voltage might be a bit
>>> high. 
>> 
>> Main consideration here is probably whether the public has access to
>> the wires. The nice thing is that you don't have to deal with large
>> currents.
> 
> The public very much has access to the wires, squirrels too. :)

Ouch... that kills this one probably. Unless you can use a star
configuration and keep the number of nodes on one end low enough for the
voltage not to get dangerous.


>>> There is no law that the lower voltage on the bus should be 0V: I can
>>> imagine a but with for instance signalling levels 9V and 12V. 
>> 
>> Works for downstream only. You have some power to get rid of at the
>> sender, though.
> 
> Hmm... such a system would nessesitate some sort of voltage regulator at
> the slaves, unless the voltages were 5V and, say, 3V 
> 
> What would the advantage be of such a system?

Not sure what Wouter thought of, but for one you want to keep the voltage
high enough so that the lower voltage plus any possible variations is high
enough. Additionally, you may be able to use the higher voltage and PWM the
LEDs with a reduced on time, and with this reduce the average current. In
your setup, it seems the processor currents are negligible and the major
problem are the LED currents. So it may make sense to make the voltage as
high as possible for the LEDs. The processor supply can be had easily and
cheaply with a resistor and a zener (a these days often forgotten "sort of
voltage regulator" :).


>>> Alternatively signalling can be in HF, which can easily be added/removed
>>> from the power line. 
>> 
>> Yes, RF or even audio (DTMF or modem type modulation).
> 
> So what would the node need to detect a HF signal? High-pass filter w/ a
> capacitor?

Exactly. If the regulations permit this, this could be quite simple. You
don't have to do anything fancy on the receiver side, I guess. Just use an
encoding that makes it very difficult to get a false positive.


>>> 100 nodes might be too large for some setups, maybe split in separate
>>> strings, or add repeaters.
> 
> Note that for installation reasons, the nodes really should be in a star
> configuration, by strings do you mean strings of nodes?

Yes, I think that's what he meant. I guess it would help if you could give
a notion of how many nodes on a "string" :)

>> Good ideas. Add the possibility to do the communication in short
>> intervals where you don't have power on the bus. 
> 
> What do you mean by the above?

You don't necessarily have to have power on the bus all the time. You may
be able to shut it off for short intervals. During that time, you send the
information. The nodes survive with big enough capacitors. If there's a
time where the LEDs are off, this would help. Otherwise, it may or may not
be an option.

Gerhard

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