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| syncronise multiple ADCs over an Ethernet network? |
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| mansaxel:
--- Quote from: Yansi on December 10, 2019, 05:17:24 pm ---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. --- End quote --- Exactly, 10MHz can be had from so many sources, and there is distribution hardware that satisfies very tight phase relationship requirements, many measurement devices can consume 10MHz, et c. |
| jbb:
Do you need every sample delivered? Or is missing the occasional sample OK? To open up a completely new front, EtherCAT might be a contender. It’s an industrial Ethernet thing, where the slaves each have 2 ports and daisy-chain off each other. No fancy switch required. If using hardware space drivers (highly recommended), it’s got highly deterministic latency and each additional slave adds only 1.2us latency to the chain. Pretty good (< 300ns if using a hardware timing master and hardware slaves) clock distribution is built in and can do regular timing. We ran an EtherCAT system for the backbone of a megawatt class power conveyer with 100us command cycle timing. |
| Yansi:
--- Quote ---No fancy switch required --- End quote --- Haha... Either very specialized EtherCAT driver IC, or an FPGA with lot of work to make it work. Bute sure, this could work too. You have just moved the fanciness from a simple PTP aware switch to a completely new level ;) |
| jbb:
Oi. I said no fancy switch required. The slaves (especially the timing master which objects timing information) are a bit fancy. We used an FPGA with licensed core from Beckhoff. The license was eye-wateringlh expensive. But I think TI offers ARM chips now with EtherCAT hardware. Actually, I’m being a bit thick. I think Analog Devices have a new series of “A2B” chips for distributing audio over twister pair in a car. Maybe some of that stuff could be repurposed? |
| max_torque:
Ethercat is (probably) out because of the cost / complexity. We have looked at just buying a suitable DAQ system off-the-shelf, and one of the options was a Beckhoff based Ethercat system, and the costs skyrocket (compared to basic home grown ethernet connected slaves) and the hardware becomes non optimum in terms of it's physical layout (ie mostly din rail mounted I/O without suitable environmental sealing, as our system needs to work in a reasonably harsh environment) We are going to have to do a trade-off analysis between the various options, currently the least understood part is the master software that is going to bring all this data together under one roof so to speak, option shere range from DIY home grown software, up to National Instruments based Labview/Veristand etc. Although the home grown option is by far the most work, it offers the most flexibility, but realsitically we will probably plump for something of a halfway house, buying the component parts we can buy, and developing the rest |
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