[time-nuts] inexpensive fiber optic distribution

[Gerhard Hoffmann]

There is not that much personality. After all, the governing document is 
called the MSA, which stands for Multi Source Agreement.

I can only speak for our XFPs; these are considered a little bit better, 
bigger and  slightly less exposed to cost pressure than SFP. While it 
may look from the outside as a I2C eeprom, the storage was inside the 
ADuC832 and writing was censored by its 8051 CPU. There were the 
"constants" for laser operating point control that were set as the last 
step on the production line and it would be really dumb to let customers 
/ box movers fondle that. Our modules could also remember if they were 
exposed to excessive temperatures for a long time, and that could be 
read out.

With regard to time nuttery, I would not pin my hopes too high. The 
connection to the circuit board is just a card edge and the interface 
must accommodate some 40 cm of FR-4 transmission line, so the first 
thing when a signal enters the transmitter is the eye opener that 
recreates nice data from the mess. It does not care much about low 
frequencies and dc wander; it uses the loose telecom jitter specs to its 
fullest advantage.

The circuit board swims freely in the box; the ROSA/TOSA are relatively 
tightly connected to the fiber and in case of mechanical stress 
something has to give in. That flexible Kapton strip that connects the 
laser/receiver to the board was a major electromagnetics simulation 
nightmare.


I remember how my decision to use BNC instead of SMA for my BJT 
isolation amplifier was welcomed here ;-)  or how the failure to fasten 
an optical cable fully
led to the discovery of faster-than-light neutrinos at Gran Sasso.

regards, Gerhard


Am 21.03.19 um 17:29 schrieb Mark Sims:
> The "personality" for these modules is stored in a bog standard 3.3V 256 byte I2C EEPROM (address 0xA0).  The I2C bus is n the connector.  An Arduino can reprogram that memory.   For the simplest case you just change the vendor ID info.  I think the kilobuck level programmers is just a simple I2C interface with some software that knows what to change.
>
> --------------
>
>> As far as the reprogramming goes (another comment on the list) - I
> understand that is to make the unit have some particular flavor
> SFP-wise.  If you were plugging them into a Cisco switch and all you had
> was ones configured for Juniper switches, then you'd want to do that. At
> the multikilobuck level to do this, you'd need a LOT of them to make it
> worthwhile.
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