[ntp:questions] Very large jitter and offsets on GPS ref clock after upgrade to "p5"

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Mon Jan 9 19:29:49 UTC 2012


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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