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According to Beckhoff, the EtherCAT Standard needs correct shielding to work reliable, since EtherCAT cannot tollerate a lot of jitter:

Beckhoff Shielding

Correct shielding means, that the shield is continously throught the whole signal cables (incl. couppling etc.). Now I have here several EtherCAT devices:

  • EtherCAT Master: Has a RJ45 Connector for EtherCAT connection
  • EtherCAT Sensors: Have all two RJ45 connectors, where the shield is shorted together
  • EtherCAT Motor Drives: Also have two RJ45 Connectors (EtherCAT IN and EtherCAT OUT), but the shields are not connected internally between the two RJ45 connectors.

EtherCAT allows a line topology for easy loop the cable through all the slaves, so that for each motor drive, the shielded cable comes in and directly out to the next EtherCAT slave. This will not work when the two RJ45 shields are not connected together.

Now, why would a motor drive manufacturer prevent the shield to be continous? (the Master and the Sensors have a continous connection). Is it because of the fear of GND Loops? Can I use this drive only in Star-Connection then?

HansPeterLoft
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  • But Ethernet is not a loop through interface. If EtherCat implements such a thing, the EtherCat chipset has two individual ports. So even if if has a loop through feature, it does not mean the wires actually loop through physically, it's the data packets that loop - think of it more like an Ethernet switch or router. So even if the chassis of the two connectors are not bonded directly together, both of them may still have proper AC coupled earthing to device chassis or earth ground, done to prevent DC ground loops via EtherCat interface. What does the device manual say? – Justme May 10 '23 at 08:27
  • Not having hands-on experience with EtherCAT, there's much in that Beckoff Shielding document that makes me cringe. If the RJ45 shields connect to _Nothing_ on the EtherCAT _devices_ (they do just 'pass thru"), then no harm, no foul. As soon as a manufacturer decides that the shield on their RJ45 connector in their widget should be (dc) grounded, then all bets are off -especially in an industrial sensor/machine/parallel run environment. Ethernet connections are highly interference-immune. I'd first try unshielded cables, you may be pleasantly surprised. – Chris Knudsen May 10 '23 at 12:42

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