EtherCAT line topology with fibre/fiber converters

[RichardS_Dynex]

Hello,

I have an application where I will be controlling several NI 9144 slave chassis in a line topology from a cRIO master. The chassis need to have 10kV+ isolation from each other and the master so I will be using fibre-optic converters on the ethernet link between each chassis. The link lengths are relatively short (<20m).

Do the fibre converters need to meet a paticular specification, and if so do you have any recommendations?

Would I be better using using standard Ethernet with NI 9149? I may be able to cope without the deterministic timing.

Thanks,

Richard.

[a_clucker · Accepted solution]

Hi Richard,

The EtherCAT protocol normally operates on 100BASE-TX, and can also operate on 100BASE-FX (Sources here and here). I'm guessing you've seen the KB that explains you need different swtiches / hubs for EtherCAT and that has made you wonder if optical converters fall into that same category? 

The need for special switches/hubs is because EtherCAT star junctions work differently than normal Ethernet hubs as the packets need a particular routing. However, optical converters are inline and do not affect the direction packets go. They are just converting bits from voltages to light pulses. The main thing to consider would be how much latency the converters introduce to the network - as this will end up influencing your minimum EtherCAT cycle period.

Having said all that, NI does not test optical converters, but I have heard of customers using optical converters before (sorry, no public-facing reference link available). As I mentioned above, the EtherCAT protocol does specificy 100BASE-TX and 100BASE-FX, so I would make sure the converter complies with those standards.

[RichardS_Dynex]

Thanks for this information, it is very useful.

I'm thinking of using Phoenix Contact FL MC 2000T SC as these have a DIP switch setting to turn off store-and-forward.

Regards,

Richard.