[ntp:questions] Re: CTS, IRIG-B, and a Sun v210

David L. Mills mills at udel.edu
Sat Apr 16 21:41:45 UTC 2005


Martin,

Several folks have reminded me the PPDAPI is intended for use as 
interval timer, as well as PPS capture. I stand humbled, and I was one 
of the authors. Hooking the IRIG to the CD pin on a serial port might 
just work, although with the typical serial port jitter. Better a source 
like the serial port interrupt pin. The interval timer functionality is 
already in FreeBSD and apparently in Linux.

Yes, you can buy A/D and D/A hardware interfaces for just about any 
machine, including ISA, PCI,... You will not like the price tags.

Dave

Martin Burnicki wrote:
> Dave,
> 
> David L. Mills wrote:
> 
>>Martin,
>>
>>The IRIG audio decoder autodetects IRIG-B (1000 Hz carrier) and IRIG-E
>>(100-Hz carrier. 
> 
> 
> Yes I know. I don't think it's possible to decode unmodulated IRIG signals
> with an audio card.
> 
> 
>>There are no provisions for DC shifts, as there are no 
>>common I/O devices available for PCs and workstations that work with
>>those signals. It would in principle be possible to adapt the audio
>>driver to work with an A/D converter and DC shift signals, but that
>>would seriously degrade accuracy.
> 
> 
> But an A/D converter isn't a piece of standard computer hardware, either.
> Theoretically it might be possible to route an unmodulated IRIG signal to
> the CD pin of a serial port using an appropriate level converter, and let
> it generate interrupts on every slope, as used for PPS. I'm assuming,
> however, that the interrupt latency would make this unreliable at higher
> IRIG data rates and varying CPU load.
> 
> 
>>Once upon a time the IRIG audio decoder was the only driver that
>>seriously challenged the PPS performance with accuracy in the low tens
>>of microseconds. Unfortunately, at least with modern Sun hardware and
>>Solaris kernels, the accuracy has been seriously compromised to the
>>order of a few milliseconds. I would welcome reports on how well it does
>>with other hardware and other kernels.
> 
> 
> I've read about the IRIG audio driver on the mailing lists and this
> newsgroup, but personally I have no experience with it, yet.
> 
> 
> Martin
> 



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