[time-nuts] inexpensive fiber optic distribution

[Warren Kumari]

On Thu, Mar 21, 2019 at 8:00 AM Hal Murray <hmurray at megapathdsl.net> wrote:

>
> jimlux at earthlink.net said:
> > It seems like you could probably figure out how to interface to these
> things
> > and use them to distribute timing signals.
>
> There are two big advantages of fibers:
>   They work well for long distances.
>   No ground loops.
>
> The target market for those contraptions is networking.  The signal is
> digital, and they get to work with a "nice" signal, probably 8B/10B.
> That's 8
> bits of data in 10 bits on the wire.  Roughly, they pick 256 out of 1024
> patterns that have minimal long strings of 0s and 1s and use a few more
> patterns for control - things like idle, and begin/end of packet.  It
> works
> fine if AC coupled.
>
> I don't know how well they will work for something like a PPS.  Somebody
> should try it.  The signal pattern is mostly trying to be friendly to the
> PLL
> that does clock recovery and/or the AGC that sets up the switching level.
>
> If I wanted to send a PPS, I'd use something simple like Manchester
> encoding.
> That assumes you can line your transmit clock up with the PPS.  It won't
> work
> if you just want a link-extender for a PPS.
>
> If you are connecting to a FPGA, use whatever they support for high-speed
> serial interface.  Again, that assumes you can line your signal up with
> the
> transmit clock.
>
> I'm pretty sure the signaling is differential, either PECL or one of the
> newer
> standards like LVDS.
>
>
> > They take a standard singlemode duplex fiber and have a pinout suitable
> for
> > plugging into a Cisco, etc. switch. Apparently, there's some ...
>
> I'm pretty sure none of the programming is on the high speed path.  Maybe
> changing the signal levels.  I don't know how much of it is total BS, aka
> vendor lock in, as compared to actually doing something useful.  I'd be
> surprised (but not much) if they didn't work fine with no programming.


Yup, the majority of the programming is assigning a "personality" (vendor
lock). There is sometimes also a small bit of config (some let you assign
minimum receive before it declares the link up, or (very unusual) the
frequency for tunable optics (usually this is tuned in the device, but some
let you assign in the SFP). AFAIK, all the ones you can easily purchase (eg
FS or flexoptix) come with at least some personality.

W



>
>
>
> --
> These are my opinions.  I hate spam.
>
>
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to
> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.
>
-- 
I don't think the execution is relevant when it was obviously a bad idea in
the first place.
This is like putting rabid weasels in your pants, and later expressing
regret at having chosen those particular rabid weasels and that pair of
pants.
   ---maf