[LEAPSECS] embedded data loggers?

[Martin Burnicki]

Rob Seaman wrote:
> Don’t know if this got through using my new email address.
> 
> Have been rummaging around looking at Arduino and/or Raspberry Pi hacks,
> but the former seems under-powered to run a full NTP instance and the
> latter is not real-time.

If you want a cheap solution then maybe you should have a look at the
Banana Pi and/or Beaglebone Black.

The Raspberry Pi's NIC chip is internally connected via USB, which
causes latencies and asymmetries for network packets, which in turn can
decrease the accuracy you can yield via NTP.

The Beaglebone Black and AFAIK also the Banana Pi have NIC chips which
are connected directly to the CPU, not via USB, which avoids the
latencies introduced by USB.

Unlike the Beaglebone Black and the Raspberry Pi the Banana Pi also
supports native 1000 MBit link speed. Eventually you don't have to
transfer much data and 100 MBit sounds enough, but if your switch and
your LAN run on 1 GBit and you connect a single 100 MBit device to one
switch port then all packets from or to the slow port need speed
conversion, which often means queuing by the switch, which in turn also
inserts latencies affecting the resulting accuracy.

In any case you need to spend some of your time to configure and test
this properly, so as Warner has already mentioned in his reply, if you
only need one or a few instances of those loggers it may be easier to
buy a solution which is more expensive, but doesn't require lots of your
time.

So just for completeness I'd like to mention that most Meinberg GPS
receivers provide 2 "time capture" inputs which directly capture the
on-board UTC time derived from the stable on-board oscillator which is
directly disciplined by the GPS input signal.

You can apply 2 different hardware trigger signals. The captured time
stamps are saved in an on-board FIFO buffer and can be retrieved via a
serial port, or in case of a GPS PCI cards, directly via the PCI bus.

Take care, I'm biased. ;-)

Martin