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syncronise multiple ADCs over an Ethernet network?
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max_torque:
As it stands, i don't currently actually have any known fixed distance between nodes, because the layout and packaging of the system isn't actually done yet, were still at concept stage.   All i know, roughly, is that each slave node will be in a 19" rack style box, in some, er, 19" racking (no suprise there!) so some nodes may be as little as 10cm from each other, but others may be (up to) 10's of meters away.  Right now, i can't see any situation where the total length of the network would be longer than something like 50m absolute maximum, so "easy" to wire.

As every slave node is going to have to route back to some form of ethernet switch / hub type arrangement, my current thought was to do a 1U 19" rack type module that sits next to a 19" rack ethernet switch, and that module does clock / sync and power distribution to a number of slave modules?

ie. here is a 24 port switch:



I'd do a "24 port" clock distribution and power module that would take in a clock signal and power (probably 24V DC) and have 24 sets of Clock (BMC) and Power (TBC) output terminals on the front, meaning it's an easy wire up job, just point to point with those cables, which can be bundled with the ethernet cable to each slave device.

One of those Clock/Pwr units could i guess also include the master clock that is then daisy chained out to everything else, if that unit is "smart" and also has an ethernet data link, then the sampling rate and sampling window can be software controlled by the master PC as well.  :-+




mansaxel:
I'd say you want to do a 10MHz clock distribution, if you decide to go separate. And then instruct the end node over a command channel via Ethernet what to do in terms of sampling frequency. The master clock can then be anything from a simple function generator with a quartz crystal in up to a GPS receiver or even a Cæsium normal.

/Måns, borderline time-nut.
dmills:
ALL broadcast engineers are borderline time nuts, comes with the territory.

My boss tells stories of conversations with Cisco when building out the first of the Timeline IP trucks that got 'interesting' on the fact that for this use TCP was right out and they could NOT drop UDP packets because their system wanted to take a port off line for a ms or so to run self tests..... Also on the fact that just adding more buffer to the switch was not a viable fix.

Fun fact, when HFT was just getting going, one of the IT network switch vendors bought a SDI router vendor to find out how to do it in a time sensitive manner, now broadcast buys the switches the HFT guys use.
max_torque:

--- Quote from: mansaxel on December 10, 2019, 12:39:47 pm ---I'd say you want to do a 10MHz clock distribution, if you decide to go separate. And then instruct the end node over a command channel via Ethernet what to do in terms of sampling frequency. The master clock can then be anything from a simple function generator with a quartz crystal in up to a GPS receiver or even a Cæsium normal.

/Måns, borderline time-nut.

--- End quote ---

I don't think there is much margin in going that high, as we will never be sampling at anything approaching that rate.   Perhaps a clock in the low hundreds of kHz would make sense, and would enable simple hardware for the slower sampling slaves (like the ones that read temperature sensors that only need to be sampled at a couple of Hz.  Driving the clock line could be interesting, maybe some from of LVDS could work and be simple / cheap.  If we were really showing off we'd push the clock data over the power lines.......     :-/O
Yansi:
10MHz is what is the de-facto instrumentation standard, that can be interface to almost any existing instruments.

Either word-clock (sampling rate) shall be provided directly or a common 10MHz reference to all, that'd be used to derive sampling frequency.
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