[ntp:questions] PPS on Linux status?

Martin Burnicki martin.burnicki at meinberg.de
Tue May 15 10:05:23 UTC 2007


Martin Karsten wrote:
> Does the need for a level converter depend on the hardware or software
> in use? I.e. will Linux/i386 (with the patch set) or Solaris on Sparc or
> i386 generally require a level converter or work with a GPS clock out of
> the box? This assumes that the GPS receiver is connected via serial
> input. Or do GPS clocks already include this conversion these days?

In most cases the PPS signal is fed via a serial port which expects RS232
voltage levels, i.e -3..-12 V for a logical "high" and +3..+12V for a
logical "low" level. 

The reason for this is mostly historical. On-board logic levels used to be
close to 0V for logical "low" and close to +5V for logical "high" signals.
Those on-board signals were converted to RS232 levels using inverting
driver chips, so you have negative voltages for a logical RS232 "high"
level on the wire.

Normally a PC's serial port expects RS232 levels as inputs, so the PPS pulse
which if normally fed to the Carrier Detect (DCD) pin should also be RS232

However, in practice the RS23 chips built into standard PCs accept also
voltage levels in the range of 0..+5V as input levels.

Anyway you should take care that the signal supplied to the pin has the
correct polarity as expected by the NTP daemon, e.g. if you used an
inverting level converter the leading edge of the original PPS signal
becomes the trailing edge of the inverted/converted signal. 

This would result in a time offset which corresponds to the length of the
PPS pulse. However, you can configure in ntp.conf whether the rising slope
or the falling slope of the PPS signal is the on-time slope, so you can
easily correct this, if necessary.

BTW, some converter chips from 0..+5V to RS232 levels have a limited rise
time of the output signal which introduces delays of up to a couple of
microseconds. Converter boxes which are based on a fast operational
amplifier (or other conventional technique) don't have that delay, so they
are normally better suited for PPS conversion.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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