[ntp:questions] Convert PPS signal from BNC to RS-232!

David L. Mills mills at udel.edu
Mon Mar 5 16:03:41 UTC 2007


The orignal Bell 103/201/202 modem specification required a bipolar data 
signal +-6V or more. Later, the 113 modem specification allowed unipolar 
signals, but was not widely adopted. In some cases the modem or UART 
chip does work with TTL signals, but not all. It takes two transistors 
and a resistor to fix that, or better yet a MAX232 chip. Details are in 
the NTP documentation on the gadget box page.


Hal Murray wrote:

>>I have a Symmetricom Syncserver S250 as primary NTP-server. 
>>This box has PPS-output via a BNC connector. I need to get this PPS signal 
>>onto a rs232 connector.
> Just wire it up and give it a try.  The PPS output is a good
> solid TTL signal.  It will work fine unless you are going
> very long distances.
> Their data sheet says it will drive 50 ohms.  If you have a scope
> handy, see what it looks like.  Without a 50 ohm terminater, it
> is likely to be 2x the TTL level.  Who knows what "TTL" means
> these days.  It's probably CMOS, either from 5V or from 3.3V.

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