Laser emergency stop over ethernet

**dave** · 03-12-2009, 22:16

Originally Posted by loopee

My understanding of how the isolation is achieved is by using a isolation transformer at the connector connected across the individual pairs, and the isolation is between the cable pairs and the electronics.

Jut looking at a few datasheets, I cant see any AC coupling

http://www.haloelectronics.com/pdf/gigabit.pdf
http://www.midcom-inc.com/Products/L..._1X1_GB_TH.pdf

Maybe im missing something here..

**heroic** · 03-12-2009, 22:18

Originally Posted by dave

Jut looking at a few datasheets, I cant see any AC coupling

http://www.haloelectronics.com/pdf/gigabit.pdf
http://www.midcom-inc.com/Products/L..._1X1_GB_TH.pdf

Maybe im missing something here..

Transformers and capacitors both constitute AC coupling.

**dave** · 03-12-2009, 22:21

and putting 12v at 1a across the primary to the transformer is OK? I would have thought the magic smoke would be released?

**heroic** · 03-12-2009, 22:28

Originally Posted by dave

and putting 12v at 1a across the primary to the transformer is OK? I would have thought the magic smoke would be released?

Some of the transformers have a capacitor (or two) in series with them. Others have a choke to block DC.

**dave** · 03-12-2009, 23:18

Originally Posted by heroic

Some of the transformers have a capacitor (or two) in series with them. Others have a choke to block DC.

Can you post a link to a datasheet? Im interested to get a better grasp of all of this..

**heroic** · 03-12-2009, 23:23

Originally Posted by dave

Can you post a link to a datasheet? Im interested to get a better grasp of all of this..

The datasheet you posted with the ethernet transformers in it shows the choke DC blocking scheme. Capacitors are separate components, you can see them on a lot of ethernet cards. Usually large polystyrene or ceramic types.

This page has a nice explanation of the behaviour of capacitors in these situations, and why 8B10B and MLT-3 coding is important to gigE.

http://www.sigcon.com/Pubs/news/7_09.htm

Ethernet transceivers must be DC isolated because of ground lift problems; moreover, it is possible to connect a structured wiring plant's ethernet ports to a telephone system by mistake. When this happens, it's important that not even the 120 volt AC of a ringing POTS line will kill the ethernet transceiver on the other end; remember that ethernet interfaces used to be really expensive (I remember paying $1600 for one, not too long ago) and you'll realize why they're so well protected.

**dave** · 03-12-2009, 23:43

Chokes block AC. Don't they? I'm sure that's what i learned in school!!

Regardless. I would prefer to implement the most risk free solution.. And running 2 different potentials down different conductors of the same pair is not risk free in my mind.

**heroic** · 03-12-2009, 23:49

Originally Posted by dave

Chokes block AC. Don't they? I'm sure that's what i learned in school!!

Uh, yes they do. Not quite sure what I was thinking. (The chokes in these transformers are intended to improve common-mode noise rejection. The capacitor coupling is external to the magnetics.)

Regardless. I would prefer to implement the most risk free solution.. And running 2 different potentials down different conductors of the same pair is not risk free in my mind.

I'd just use the PoE pinout myself.

**mccarrot** · 03-13-2009, 01:08

Ok thanks for the info, to be sure I now have devided the Key and Kill switch over different pairs:

1 Data pair 1
2 Data pair 1
3 Data pair 2
4 KILL switch
5 KEY switch
6 Data pair 2
7 KILL switch
8 KEY switch

**mccarrot** · 03-13-2009, 04:49

Revision 2.2 of the Laser safety stop.

Thread: Laser emergency stop over ethernet

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