Hi Eric,
I would think that simply copying the PLC input circuit on your own
PCB would be the simplest approach if you're making a PCB.
This would consist of an OPTO Isolator with an inline resistor and
possibly some filter components (normally a simple RC filter).
I just did a Google search and found:
http://www.eod.gvsu.edu/~jackh/books/plcs/chapters/plc_wiring.pdf
Page 6 has the input circuit (you would pull up to 3.3V rather than 5V
though), and the first resistor diode is probably a visual indicator.
If you search the web you can find information on the actual values of
the resistors and capacitors, but these will depend on your input
voltage and it's current sourcing capabilities.
Another option would be the TI sn65hvs882:
http://focus.ti.com/lit/ds/symlink/sn65hvs882.pdf
This chip turns eight high voltage (<34V) inputs into a serial stream
accessible via SPI.
Hope this helps.
PJE
--- In "eric.robishaw" <> wrote:
>
> What kind of opto-isolation / voltage suppression circuitry or
devices
> would you recommend for a 7800 DIO?
>
> I'll be reading outdoor field devices highly susceptible to voltage
> spikes potentially significantly > than the 5V toleration.
>
> I know I can spend $250 for a device from Phoenix Contact, but I
need
> to go cheap (I know, you get what you pay for, but it doesn't make
> sense to pay the same amount for protection as I do for the board
itself).
>
> I'm looking for either a cheap DIN-rail device (~$50 for 4 inputs),
or
> some simple circuitry I can put on my own PCB.
>
> Ideas?
>
> Thanks
> Eric
>
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