For serial data, the signal idles high (5V) at the UART. It's
active low.

For RS-232 serial data, the valid levels are -12V and +12V, with
a deadband of +/-3V in the middle around zero. Many people can
violate the rules reliably, because virtually everyone these days
uses something like a MAX232 or equivalent, and those all have
their levels set to consider zero volts a valid state. So, for
RS-232 level-shifted data, anywhere from -12 to 0 is idle, and
anything from about 2.5 up to +12 is 'active'. An inverter in both
the transmitter and receiver gets you back to 'idle high' at
the UART terminals.

If you need to send serial data only a few hundred feet, you
can use a simple totem-pole driver on each end of the wire (the
ACT logic family works nicely) and a series-resistor-limited
CMOS input for detection. Put a clamp diode inside the resistor
to soak up any zaps.

For longer runs out to 1000 feet or so, or for multidrop use,
you might consider optocouplers. Ground potential differences
can be a problem over long distances, and that can get onto
your cable if you don't isolate carefully. We're able to pass
19.2K full-duplex data around over 4-wire cables, with multiple
slave receivers listening to a single master. I can't discuss
the details of it publicly, but it's not complex and if you've
got specific questions I'll do my best to point you in the
right direction.

Mark G. Forbes, R & D Engineer  |  Acres Gaming, Inc.    (541) 766-2515
KC7LZD                          |  815 NW 9th Street     (541) 753-7524 fax
forbesm@peak.org                |  Corvallis, OR 97330
http://www.peak.org/~forbesm
mforbes@hq.acresgaming.com

"There has been an alarming increase in the number of things I know nothing
about."
---Anomalous