David Woolley wrote:
> Mike S wrote:
>> unruh wrote:
>>>
>>> Make sure you properly terminate the cable with 100 ohms.
>>> Otherwise you will get reflections back and forth along
>>> the cable, and the cable will slowly fill with charge,
>>> broadening the pulse by a huge amount.
>>> It may also be that the signal is decreasing due to
>>> resistance in the line.
>>
>> The proper termination for EIA-232 is 3000-7000 ohms.
>
> EIA-232 deliberately mis-terminates in order to make the line
> capacitive, and slow down the rise times.
> It also has a 50 foot limit, to limit the capacitance and rise time.
The RS-232 standard (currently TIA/EIA-232-F)
{normalized with ITU-T V.24, V.28, and ISO/IEC 2110}
<BlockQuote> Electrical Specification:
The maximum cable length originally was defined in RS-232-C
as 15 meters; However, this has been revised in EIA-232-D
and TIA/EIA-232-E and is now specified more correctly as a
maximum capacitive load of 2500 pF. This equates to about
15 to 20 meters of line length, depending on cable capacitance..
</BlockQuote>
I'd have to dig up the specs, but Cat5e is usually around
mid 50's pf per meter (@ some frequency).
With +/- 12VDC RS232 drivers, I've done 800' and 1000' runs
of Cat5e @ 19.2Kbps without issues.
{On the other hand, on cables designed for RS232 decades before Cat5e,
I've had issues at one tenth that far, (~100pf to 200pf / meter)
at 19.2Kbps.
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