Serial Communications

mwkohler

I am communicating serially between several sx48 proto boards.· Does anyone know what the practical distance for the wiring is before I need to boost the signal with rs-232?

Bill Chennault

mwkohler--

I am trying to learn about the SX. However, I imagine folks will want to know what baud you wish to use. (I would like to know·the "maximum"·baud two 50 MHz SX MCU processors can communicate at when the distance between them is measured in fractions of an inch.)

Thanks!

--Bill

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You are what you write.

pjv

Hi Bill;

About 10 Mega bits per second bursts.

Cheers,

Peter (pjv)

mwkohler

I would like to send at 38400 baud.

Beau Schwabe

mwkohler,
·
38400 baud = 38.4kHz over the wire.
·
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.
·
What forms in a wire is basically a low-pass R-C filter with increased distance you have increased resistance and increased capacitance.
·
At 38400 baud with the conditions above, you start to see a significant roll off at about·300 feet·· R = 31.44 Ohms·C = 18,840pF (~.018uF)
·
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.

········ 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
········ distance·"ball parked" above down to 150 feet or even 100 feet.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.