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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