Signal over long wire-Active high or Low?

[rocky79]

Hello,

I am designing two circuits that will communicate using discreet signals over ~ 7feet of 24 AWG wire.
My goal is to make this communication as reliable and as low power as possible
I have two possible scenarios ( shown below) I am leaning towards the active high design. Please comment on which one you would think will be better. I appreciate your response.

 

[betwixt]

Over 7 feet it would be quite difficult to make it UNrelaible with either method. This afternoon I've been laying 86 metres (~280 ft) of signal cable through a forest and expect to get at least 10Mb/s reliable throughput.

If you use the active low method you probably want to drop the pull-up resistor at the receiving end to something much lower. With that value the rise time when the FET turns off wil be quite long so your data rate will have to be kept fairly low. For 7' you probably want a few K Ohms, for my link I'm using 120 Ohms! For best results use dedicated line driver ICs which have controlled slew rates, short circuit protection and internal clamping to reduce signal reflections.

Brian.

 

[rocky79]

Great, thanks Brian. Would a low VF shotcky diode work for spikes or ESD? or should I use TVS instead?

 

[dick_freebird]

It depends a lot on what remote resources you expect or can
insist upon.

If you want minimal wiring and no opportunity for supply
integrity problems, you might make the signals more like
open/short to the return/ground; this would be active low
open drain / open collector.

If you want to push a positive voltage or current then
you start to care about things like whether this will
power the remote unit via input protection networks,
perhaps into a non-valid or marginal state. But if the
load is dumb and has no power, then you would be
looking to supply energy, not just a little-as-possible
bit of state information.

And speed matters; schemes like open collector do
depend on the pullup resistance for one of the transitions,
long wires can be capacitive and slow way down when
you get past inches of run and use pullup impedances
consistent with logic drive strengths. If you're sub-MHz,
OK; above this, at that length, you want to consider
transmission line effects and styles (the phone company
does, for audio at neighborhood scale; power company
does, at 60Hz at county/state/continent scale).

In my experience this choice often comes down to the
available remote and head-end resources, fault modes
and such. You mention signal reliability but nothing about
the "threat set" (against which it would be deemed reliable).
Give that some thought. You might like to look at app
notes for industrial long wire interfaces, even if you don't
intend to adopt any of them.

 

[betwixt]

There would be little difference electrically but low Vf Schottky diodes will probably have lower capacitance than a TVS so they would load the signal less. They would probably be cheaper too.

Brian

 

[rocky79]

Hi Dick,

Thanks for your response. The data is only a status signal that sends a high signal( in the active high mode) once the user pushes a button. No high speed communication data or any of that sort. I am mostly worried about transients on the wires. Any additional thoughts?

 

[mtwieg]

Neither circuit would be especially effective in the presence of large EMI. What exactly are you trying to achieve? What sort of baud rate/bandwidth do you need? You may want to use differential signalling, or optoisolators to avoid ground loops.

 

[rocky79]

Neither circuit would be especially effective in the presence of large EMI. What exactly are you trying to achieve? What sort of baud rate/bandwidth do you need? You may want to use differential signalling, or optoisolators to avoid ground loops.

Thanks for the reply. This is just a status/signal line that only goes momentary high or low.
A user presses a button then a momentary signal change gets transmitted through the 7 feet wire and into a microcontroller input. Very simple.
My questions was would an active low or high makes sense for this application?

Thanks

 

[BradtheRad]

Consider making HI the normal state. It will let you verify continuity when you hook up connections.

If it doesn't go high, then you have a bad connection somewhere.

 

[rocky79]

Consider making HI the normal state. It will let you verify continuity when you hook up connections.

If it doesn't go high, then you have a bad connection somewhere.

Great idea Brad, thanks!

 

[permute]

The design is also affected by any extra needs for detection of the switch. For example, if you have a pullup resistor you can also place a resistor across the other end terminals. This would give a 3 level circuit, which would let you know if you have 1, 0, or invalid.

Active high/low could also be chosen based on the consequences of the incorrect action. With a pull down, an open or a short would result in '0'. If it is always safe to declare '0', then this would make sense to do.