Thanks for all the feedback, very useful!

What I'm currently looking at is having two circuits:

Circuit 1:
Remote Site
Standard 400mA 4.5V wallwart -> 470 ohm resistor -> twisted pair with ground
and 5V
--> CABLE --> Central Site
HCPL-2731 Opto-isolator -> PIC input pin.

This circuit will simply signify binary 1 for power on (i.e. the PIC input
pin will be held at +5V), else power fail.


Circuit two:

Remote site:
5V output from above wallwart -> PIC -> MAX232 -> Twisted Pair for RS232
---> CABLE ---> Central site
--> MAX232 --> PIC.

The MAX232 will be there to give a higher line drive across the cable and to
protect the PIC pins.
I could also add HCPL-2731 opto-isolators between the MAX232 and the PIC at
the RX ends to isolate it completely?

How does that sound?

On 25/10/2007, Richard Prosser <rhprosser@gmail.com> wrote:
>
> Woops - what happened there? - just sent a blank email while the
> screen jumped  around on me! Sorry about that.
>
> Re long length RS232. I've seen problems, even over short lrngths 9but
> higher bit rates) with crosstalk between the RX & TX lines. This
> created problems since the RS232 line was feeding an RS232-RS485
> converter & the whole thing sort of locked up.
>
> But at 300baud you may be OK as the edges cause the problem & you
> could slow them down with some series R.
>
> But I'd strongly advise against connecting directly to PIC pins for
> the reasons Jinx listed
>
> A major advantage of the constant current method is the fault
> detection capability. 20mA = "High",  4mA = low and 0mA = fault.
>
> RP
>
> On 25/10/2007, Jinx <joecolquitt@clear.net.nz> wrote:
> > > connect each end of the pair to the TX and RX pins on two PICs,
> > > or would using say a MAX232 as well help ?
> >
> > If it was me, I'd want *something* between the PIC and the wire,
> > even if it was just a couple of transistors, (a) for protection and
> > (b) for drive. 1/2 mile is quite a length, and will pick up noise like
> > RF, EMI, nearby lightning etc if it's in the open
> >
> > Since this is a one-off, you don't strictly need to follow any "rules",
> > just make it work reliably, so any way that gets the job done is OK
> >
> > http://www.lammertbies.nl/comm/info/RS-232_specs.html
> >
> > Cable length is one of the most discussed items in RS232 world. The
> > standard has a clear answer, the maximum cable length is 50 feet, or
> > the cable length equal to a capacitance of 2500 pF. The latter rule
> > is often forgotten. This means that using a cable with low capacitance
> > allows you to span longer distances without going beyond the limitations
> > of the standard. If for example UTP CAT-5 cable is used with a
> > typical capacitance of 17 pF/ft, the maximum allowed cable length is
> > 147 feet.
> >
> > The cable length mentioned in the standard allows maximum
> > communication speed to occur. If speed is reduced by a factor 2 or 4,
> > the maximum length increases dramatically. Texas Instruments has
> > done some practical experiments years ago at different baud rates to
> > test the maximum allowed cable lengths. Keep in mind, that the
> > RS232 standard was originally developed for 20 kbps. By halving
> > the maximum communication speed, the allowed cable length increases
> > a factor ten!
> >
> > RS232 cable length according to Texas Instruments
> >
> > Baud rate Maximum cable length (ft)
> >
> > 19200 50
> > 9600 500
> > 4800 1000
> > 2400 3000
> >
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> >
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