[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs. PPS decoding cards

Bob Camp kb8tq at n1k.org
Tue Feb 14 13:49:46 EST 2017


> On Feb 14, 2017, at 10:31 AM, MLewis <mlewis000 at rogers.com> wrote:
> On 14/02/2017 7:26 AM, Bob Camp wrote:
>> Hi
>> A direct port might be a +/- 100 ns sort of thing most of the time and a +/-10 us
>> thing every so often under some OS’s. Most desktop operating systems are not
>> designed to prioritize random pin interrupts. A dirt cheap MCU coded with a few
>> (hundred) lines of assembly code may be a better option than a typical desktop.
>> Complicating this further is the degree to which some OS’s can be directly or
>> indirectly optimized. Install *this* package and it all goes nuts. Install that package
>>  and not much happens ….
>> Bob
> Hence, wouldn't Best Practice be boxes loaded with only the bare OS and software for the time-related tasks?

That would be one approach.

> As in:
> - a dedicated machine/box for unencumbered acceptance of PPS, and
> - for systems with a business need, a dedicated NTP server/box disciplined by the PPS source (with dedicated communication), while maintaining internet NTP sources as backup for when the PPS source fails?
> Is there a better way?

It depends on what you are trying to do. If the objective is to replace a piece of test gear
logging 100% of your events at the 100ns level, the computer likely will not measure up. If the objective is to run
NTP at the 100 us level, there are a lot more things you can get away with. NTP is designed from the 
ground up to be quite tolerant of various issues. 


> Other considerations?
> Michael
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