[ntp:questions] Explaining Technical Detail to Technically illiterate journalist

William Unruh unruh at invalid.ca
Wed May 21 16:03:28 UTC 2014


On 2014-05-21, Martin Burnicki <martin.burnicki at meinberg.de> wrote:
> E-Mail Sent to this address will be added to the BlackLists wrote:
>> Martin Burnicki wrote:
>>> I've recently started to write a whitepaper about computer time
>>> synchronization. It is not yet complete but already contains a lot of
>>> basic information which may be helpful for folks who are not very
>>> familiar with that stuff.
>>>
>>> A preliminary copy is available here:
>>> http://www.meinbergglobal.com/english/info/#whitepaper
>>
>> Re: <http://www.meinbergglobal.com/english/info/cablelength.htm>
>>   RS-232 signals (COM) via shielded data cable
>>      dependant on cable quality
>>      dependant on the baud rate of the signal (e.g. 15m at 19200 Baud)
>>
>>   Yes, _very_ dependent on cable quality.
>>
>>      However a Shielded Cat5e cable will work great
>>       to 300m at 19.2k bps;
>>       {I do it all the time, with "real" (to spec) RS232 drivers}.
>>
>>       Higher baud rates have shorter cable limits.
>>
>>        The capacitance of the cable
>>         (even almost all normal "RS232 serial cables"),
>>         turns the square waves into something the receivers
>>          can't recover correctly.
>>
>>         {Take a look at the signals at the far end of the
>>           cable with a oscilloscope!}

Cables have an internal impedence. If the cable is not terminated by an
impedance equal to that, the signal will reflect back up the cable, and
back and forth. Now, one advantage to that is if the reflection is the
same phase (eg terminating impedance greater than cable impedance) , then
the signal will slowly (travel time up and back) build
up in amplitude, perhaps making it large enough for the electronics to
detect the transition, But it will produce a very rounded signal.
Otherwise make sure that the cable is terminated with the appropriate
resistance. (50 ohm I believe for Cat 5). Note that source had better be
able to drive that impedance or the signal will be very small. 

>
> Personally I'm familiar with requirements and potential problems due to 
> cable types, etc.
>
> However, I'm not the author of that web page and just saw that there are 
> also misspelled words on it, so I'm forwarding this to the guy who has 
> created that web page and hopefully will fix it.
>
>
> Thanks for the hints!
>
> Martin



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