[time-nuts] Of rubidium life and piggy-bank anemia....

[Bruce Griffiths]

Poul-Henning Kamp wrote:
> In message <4751DAF0.3070700 at xtra.co.nz>, Bruce Griffiths writes:
>
>   
>> The thorny issues of avoiding the noisy environment of a PC and its
>> unstable PCI clock whilst still allowing a PC to be synchronised to an
>> external timebase may perhaps be adressed by:
>>
>> Producing a simple PCI (or PCIe, or even ISA bus card - still widely
>> used in industry)
>>     
>
> You don't need that.
>
> If you have an external timestamping device you can just generate
> any relevant signal from the PC (I prefer parallel ports, but they're
> going out of vogue) and time that relative to whatever clock your
> OS uses, then read back from your timestamping device (via USB ?)
> and calibrate accordingly.
>
> Modules cable-length and slope of your generating signal (EMI
> filtering), there is no difference in the resulting precision.
>
> BTW: I belive the NI PCI-66xx series of cards can be used also,
> but I've never actually tried, I only read the low-level doc.
>
>   
That certainly sidesteps the PC bus obsolescence issue until suitable
interfaces such as parallel ports and serial ports vanish never to be
replaced.
It would be relatively easy to include an optically isolated (or
functional equivalent such as the Analog Devices chip scale transformer
isolators) port on the external time stamp device for such an application.

Since USB and Firewire ports may persist a little longer, is it possible
to use similar techniques with these interfaces?

Do you mean that the PCI66XX boards could be used to implement the
multichannel timestamp function?
If so then most of the issues associated with PC noise may be addressed
by using a small external board that incorporates RF transformers and to
break LF ground loops and has clock conditioning circuity to produce
logic level clocks for the PCI66XX board from sinewave inputs.

Bruce