I am attempting to specify a replacement part for a modular jack used in an original design (I basically need something that will stick out farther than the original). However, the part I would like to use specifies different solder tail positions than the original because it is designed for use with Cat 5.

The part is from TE and can be found here.

Here is the difference in the solder tail positions:

enter image description here

1. What is the reason for changing the positions of the solder tails for a category 5 jack?

  • 1
    \$\begingroup\$ I think it's numbering them by CAT5 wire number, not by absolute pin number. \$\endgroup\$ – Majenko Oct 14 '14 at 16:52
  • \$\begingroup\$ @Majenko - It would be great if I could get that verified. I am not using this for Ethernet and the wiring pinouts are custom. \$\endgroup\$ – embedded_guy Oct 14 '14 at 17:12
  • \$\begingroup\$ As the image is for the same part (not two different parts) I think it is safe to assume that @Majenko is entirely correct and that the physical pinout is on the left with an application specific pinout on the right. \$\endgroup\$ – David Oct 14 '14 at 17:40
  • 2
    \$\begingroup\$ @Majenko - I spoke to a representative from TE Connectivity and you are correct. The reference in the drawing is referenced to the wiring and not the actual pins. If you want to put this as an answer I will select it. \$\endgroup\$ – embedded_guy Oct 14 '14 at 17:42

CAT5 cabling, for some reason, doesn't use a simple 1:1 relationship between connector pin numbers and the wire colours and pairs.

CAT5 cables have 4 pairs of wires: Orange/Orange+white, Green/Green+white, Blue/Blue+white and Brown/Brown+white. In CAT5 the colour pairs are assigned backwards (orange+white is 1, plain orange is 2), and the second pair (green) is split around the third pair (blue)*.

So from a CAT5 perspective the wires are numbered according to the second pinout, but from a connector perspective the pins are numbered as per the first pinout.

* Perhaps someone can clarify as to why this is - I have always wondered.

  • 4
    \$\begingroup\$ The pairs are positioned in the same manner as 4 line phone cables that also use RJ-45 connectors. The rather odd middle pair with the split pair around it forms a standard 2 phone line pinout, compatible with RJ-11 cables - the center pair is line 1, the split pair is line 2, and the other two are lines 3 and 4. Because of this, a building can install several runs of Ethernet cable to each room and then decide which ones to patch to phone lines and which ones to patch to an ethernet switch as necessary. However, I have no idea who decided to change the pin numbering. \$\endgroup\$ – alex.forencich Oct 14 '14 at 18:38
  • \$\begingroup\$ So it stems back to the dark ages of analogue telephony then... \$\endgroup\$ – Majenko Oct 14 '14 at 18:40

I wanted to send an update in response to this question.

After contacting TE Connectivity I still didn't have a good feeling about the answer that I received - so I ordered a few connectors to play with in my design. To my surprise, these connectors have a physical crossover within the connector itself. So, pin 1 on the plug side does not match the "Traditional Jack" PCB footprint shown in the drawing - it actually matches the "Category 5 Jack".

Hopefully, this will help someone in their future design. I am glad I did not trust what I was told by TE Connectivity and actually tested the device before I used it in my design. This footprint would not have worked well in my design and, if I hadn't tested it to see what the actual pinout was, wouldn't have worked at all.


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