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

David Woolley david at ex.djwhome.demon.invalid
Wed May 21 21:31:46 UTC 2014

On 21/05/14 19:08, E-Mail Sent to this address will be added to the 
BlackLists wrote:
> William Unruh wrote:> BlackLists wrote:

>> 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.

I believe it is more like 100 ohms at Ethernet frequencies, but at RS232 
frequencies, it will be very frequency dependent and impossible to match 
across the full bandwidth.

> RS232 was only designed to drive 50ft.
> The capacitance of "RS232 spec cables" prohibits them from working at 1000ft.
> The Cable and Load have to be less than 2500pf
> RS232 Receivers include termination in the 3k to 7k ohms range.
> RS232 Driver impedance must be 300 ohms or greater.

RS232 is designed to operate cables under conditions where the 
reflections make them look capacitive, so both the source and load 
impedance are well above the line impedance.  That means there is no 
ringing, so no false edges.

> RS232 Drivers are limited to 500ma max drive.

I'm fairly sure that is impossible with the specified source impedance 
and open circuit voltage.

RS423, using similar levels to RS232 allows much longer lines for slower 
signalling rates, although it does require explicit slew rate limiting 
at the driver.

RS232 is very conservative.  Also a lot of the limitation are probably 
based on its being unbalanced.

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