Convert PPS signal from BNC to RS-232!

Convert PPS signal from BNC to RS-232!

Post by Erling Pau » Fri, 02 Mar 2007 21:53:01


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.

Are there any converter kits that can do this, or do I need to make my own
"box" for doing this?

I have setup a secondary server (ntpd on FreeBSD) that will use PPS to
discipline its clock via /dev/pps (on a serial com-port).

Any hints on how to get the PPS from the BNC(coax)-output on the primary
server to the com-port on the secondary server would be appreciated.

- Erling Paulsen

Convert PPS signal from BNC to RS-232!

Post by Jon ?utf-8 » Fri, 02 Mar 2007 23:21:12

Erling Paulsen < XXXX@XXXXX.COM > writes:

I have seen many designs. One is at and uses a
CA3140E op-amt. Another is at ~mills/ntp/html/pps.html, but dated
1992. You can also use a MAX232.

Hope this helps.

Jon Kre


Convert PPS signal from BNC to RS-232!

Post by hal-usene » Sat, 03 Mar 2007 05:48:15

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.

These are my opinions, not necessarily my employer's. I hate spam.

Convert PPS signal from BNC to RS-232!

Post by David L. M » Wed, 07 Mar 2007 01:03:41


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.


Convert PPS signal from BNC to RS-232!

Post by Jonathan B » Thu, 08 Mar 2007 07:16:36

Around 99.9% of all RS232 interfaces on a PC will detect a zero as a
voltage below somewhere in the region of 1-2V and a one as anything above
that. If it where not so my receiver would not work, but it does


Jonathan A. Buzzard Email: jonathan (at)
Northumberland, United Kingdom. Tel: +44 1661-832195