[ntp:questions] NMEA ref.clock better than my ISP's timeserver?

David Woolley david at ex.djwhome.demon.co.uk.invalid
Sat Jun 13 09:36:04 UTC 2009

David J Taylor wrote:

> In the RS-232 implementations I have seen, the line is /never/ correctly 
> terminated, as I use the term.  Correct termination implies, if the

I'm not disputing that.  I was really pointing out that by using RS232 
you couldn't follow the earlier advice to use terminated lines.  RS232 
is designed to be used on bandwidth limited, electrically short lines, 
both driven and terminated above the characteristic impedance, so that 
the line behaviour approximates a discrete capacitor.

> cable impedance 50 ohms (such as the often used Belden 8777), that:

RS232 cables tend to have a high frequency impedance closer to 100 ohms, 
although at the frequencies involved, the impedance is variable and far 
from the high frequency limit.

> - the driver output impedance is 50-ohms

Commonly not true, even for controlled impedance systems.  Radio 
transmitters rarely reverse terminate the line properly.

> - the cable impedance is 50-ohms
> - the receiver input impedance is 50-ohms

Whilst optimum for data transmission, for low noise radio receivers it 
can be the wrong thing.


> Checking today's ICs I see that they suggest direct connection to the 
> RS-232 socket, so the static protection must be better than it was some 
> years back, and there is no need for external components.

I think that has been standard practice since the 1488 and 1489 were 
first introduced.

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