One wire would not work with "TTL" either -- you need a common
ground for return signal current. Either way you need two-wire,
unless you use earth ground as a common ground. That's generally
a bad idea over any length of cable as the earth ground can be
quite different at both ends.

Speaking of current...

I've done some longer cable run transmission before
using a current loop. You can do this quite easily with a
transistor current source and sense resistor on the other end.
You can also have a fully isolated current loop using
optocouplers and gain the advantages of 1500V+ of isolation. A
web search should turn up lots of current loop circuits.

Matt Pobursky
Maximum Performance Systems

On Wed, 3 Jul 2002 10:24:13 -0400, Josh Koffman wrote:
>Ok, I'm not an EE, but won't this not work? I thought the
requirement
>was one wire only, and RS485 is balanced. Then again, that might
be
>taken care of in that resistor comment at the end that I didn't
>understand.
>
>Enlighten me please :)
>
>Josh

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