[time-nuts] Serial port splitter s/w

[Didier Juges]

A number of people have reported issues with the Raspberry Pi Ethernet hardware when used for NTP, as it is actually a USB-Ethernet bridge and the drivers may not be all they could be. I have not had problems myself but I do not run NTP on it.
The Beaglebone Black (BBB) is supposed to be better in that regard since the Ethernet MAC is directly on the CPU. The BBB is also entirely open source, unlike the proprietary Broadcom CPU on the RPi.

Didier KO4BB


On February 26, 2014 11:32:36 AM CST, Chris Albertson <albertson.chris at gmail.com> wrote:
>It's not going to work.
>
>If the purpose of running the Thunderbolt is only to drive NTP then
>you don't need LH.  NTP's only tags the pulses to the nearest
>microsecond, nano sec on accuracy is lost on NTP.     I'd even say the
>TB is the wrong GPS for NTP.  It costs to much and uses to much power.
>
>But if you are also, or mainly, using the Thunderbolt for it's 10Mhz
>and NTP is a secondary function then a TB makes sense.
>
>Yes you NEED the PPS on the serial cable.   Thunderbolts do not send
>NMEA.  Thunderbolts send their own data format that is unique to
>Trimble.  Don't modify the GPS receiver.  Make a special cable
>adapter.   When you do this pay attention to polarity of the PPS
>signal.  It is easy to get it backwards.  You want the raising edge of
>the TB pulse to interrupt the computer.  It you invert the signal the
>wrong number of times the time will be "off" by the ouse length and I
>don't know if the pulse length is controlled to the level the leading
>edge is.    Remember RS232 uses negative and positive voltage, data
>lines use negative logic, control lines positive.   The TB's PPS is
>TTL level.   Many rs232 ports do accept t/l level if you get the
>polariy correct.
>
>Again don't even bother to run an NTP server without PPS.  You may as
>well just get time from some internet time servers.
>
>You can NOT control a GPS from two ports.  Both NTP and LH will try to
>send commands to the GPS.
>
>Likely, almost certainly you need to build a small circuit board the
>has two connectors that face the TB (PPS and serial) and one that
>faces the computer.  The little perfboard makes a neat way to or
>connect cables but you could solder up a y--cable
>
>The best thing to do is get a cheaper GPS, and one that uses less
>power to drive the NTP server.  The old Motorola Oncore series are
>cheap and the new breed of very small GPSes are good too.  DOn't spend
>more than $40 or $50 on a GPS to drive NTP as ,again NTP record
>microseconds.
>
>You could free up that Windows PC too.  It is not the best platform
>for NTP.  Asmall ARM based system (even the Rasbery Pi) will
>outperform a Windows based NTP server.  and use a LOT less power
>(Power cost for a NTP server is more than you think, it came to about
>$300 a year for me if I used a standard PC and a thunderbolt.
>Switching to a very tiny ARM based system and a smaller GPS gave as
>good performance and power savings paid for the hardware in 1/2 a
>year.  $.21/KWH about 170W and 8760 hours per year comes to a $300
>power bill.  My current system is powered by a 1000 mw plug-in power
>cube and does not need a cooling fan.
>
>On Wed, Feb 26, 2014 at 7:53 AM, David C. Partridge
><david.partridge at perdrix.co.uk> wrote:
>> I'm running Meinberg NTP on the Windows 7 x64 machine to which my
>Thunderbolt is attached.
>>
>> I'd like to be able to share the serial port between LH and NTP so
>that I can run the machine as an NTP Stratum 1 server locked to the TB,
>and also be able to use LH to check things.
>>
>> I looked around the with Google, and saw *numerous" serial port
>splitters.  Which is recommended?
>>
>> Also what's the best way how to configure NTP to lock the the TB on a
>serial port?  Do I need to modify the TB to deliver the PPS down one of
>the serial data lines or will NTP work well by parsing the NMEA time
>messages?
>>
>> Many thanks
>> David Partridge
>>
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>
>
>-- 
>
>Chris Albertson
>Redondo Beach, California
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