In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Beau Schwabe wrote:

<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">[url=http://forums.parallax.com/forums/profile.aspx?f=7&amp;m=258293&amp;p=9822]mwkohler[/url],<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>

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<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">38400 baud = 38.4kHz over the wire.<o:p></o:p>

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<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">A 24AWG copper wire has about 19.1 feet per Ohm or (5.24 Ohms per 100ft).  Also a standard shielded 24AWG wire has about 31.4pF per foot of wire.<o:p></o:p>

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<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">What forms in a wire is basically a low-pass R-C filter with increased distance you have increased resistance and increased capacitance.<o:p></o:p>

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<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">At 38400 baud with the conditions above, you start to see a significant roll off at about 300 feet   R = 31.44 C = 18,840pF (~.018uF)<o:p></o:p>

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<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">Note:  Remember that you need to provide a signal path there and back, so R and C values are doubled from what you might expect.

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<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">         This does not take into account for any external interference that might enter the wire.  A "safe" rule of thumb might be to take half of the

<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">         distance "ball parked" above down to 150 feet or even 100 feet.

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