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While searching for RJ45 jacks with magnetics (MagJacks) in Digikey, we found that there is a selection criteria which says "Number of cores per jack". Though we have been using MagJacks previously, this is the first time I noticed that criteria. I compared the datasheets of few jacks with 1 core and 2 cores per jack, but couldn't find any notable difference.

  1. Is there any significance to this parameter?
  2. Which type of connector is better - the one with lesser number of cores or the one with more cores?
  3. For the same application, if I use a jack with more number of cores, what is the advantage which I can obtain?
Avin
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2 Answers2

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It's important if you are doing gigabit ethernet, and for cost/benefit reasons.

Basically, traditional 10/100 ethernet networks only use two of the 4 CAT-5/6 pairs. As such, extremely budget minded magjacks intended for only 10/100 use can simply omit the other two cores (and their associated costs).

The odd numbers seem to relate to additional cores used for filtering purposes. a jack with 1 core basically just has a common-mode inductor, and does not actually provide a ethernet "magjack" in the sense it's normally understood.

A 3-core jack has two-pairs of isolation transformers, and either one common-mode choke for all four pairs, or a choke on one of the two pairs (the TX pair, from the ones I looked at).

It seems similar for the larger numbers - 4 cores would either be all four pairs, or two pairs with individual common-mode inductors (note that digikey has both "4" and "4 (two ports only)" categories for the 4-core option), 5 is 4 pairs with one shared CM choke (or a more exotic topology), etc...

The huge number of cores parts look to be exotic stuff. the 12-pair is a gigabit magjack with per-pair common-mode and DC-tap-off chokes, and the 48 pair is actually a MRJ21 connector, which is an exotic connector that actually carries four separate gigabit ethernet connections (e.g. 16 pairs) over a single connection, and is used for extremely high-density network switches.

Connor Wolf
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  • At the very least, " _5 (chokes) is 4 pairs with one shared CM choke_ " is wrong. MagJacks for 10/100 Mbit Ethernet with **2** pairs typically have either 4 cores (e.g. [belfuse S811-1X1T-06-F](http://belfuse.com/pdfs/S811-1X1T-06-F.pdf)) or 5 cores ([belfuse S811-1X1T-03-F](http://belfuse.com/pdfs/S811-1X1T-03-F.pdf)). – fgrieu Oct 10 '14 at 14:41
  • @fgrieu - Huh, interesting. I wonder why they're going with a separate choke for common-mode components, rather then just using a center tap. – Connor Wolf Oct 10 '14 at 23:33
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If we look at MagJacks for 10/100BaseTX without PoE, there are 4-cores models (e.g. belfuse S811-1X1T-06-F) and 5-cores model (belfuse S811-1X1T-03-F) including from the same manufacturer. In this example, the main difference aside from pinout seems to be that the 4-cores model is documented as suitable for (Auto-)MDIX operation, while the 5-cores model is not. Auto-MDIX performs automatic reversal of the TX and RX pairs (typically: by logic in the PHY) as needed, which allows e.g. to connect two identical devices with a straight cable. Some of this technology is described in these two patents.

All 5-core MagJacks that I found are such that in an application with a straight cable between two such MagJacks, there would be 6 cores on one pair, and 4 cores on the other one, which rings an alarm.

So my tentative answer would be:

  • 4-core MagJacks are for 2-pairs 10/100 Mbit (often without PoE), and generally suitable for Auto-MDIX; they are most common at least in non-PoE 10/100 Mbit uses.
  • 5-core MagJacks are for 2-pairs 10/100 Mbit without Auto-MDIX, and as far as I can tell only without PoE.
  • 6-core MagJacks are for 2-pairs 10/100 Mbit (often with PoE), suitable for Auto-MDIX, and typically more expensive than 4-core MagJacks not rated for PoE.
  • more cores (typically 8 or 12) are used in designs for 4-pairs Gigabit Ethernet.
fgrieu
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