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I'm trying to use a cat5 network cable and connection jack (what I'm calling the female version of cat5) as a way to quickly connect/disconnect 8 wires for an electrical project. I bought a few cat5 cables and checked them for continuity and all the wires are intact. Then I bought an old D-Link router and unsoldered a cat5 jack from the board.

My issue is when I plug my cat5 cable into the separated jack and check the continuity from the other end of the cat5 and the pins sticking out the bottom of the jack. I get no signal connection. I assumed I may have messed up my soldering so I tried plugging the cat5 cable into one of the other ports still attached to the board and I this time tested continuity from the cable to the still soldered pins showing on the back side of the board.

Is there some kind of gate or diode mechanism in the cat5 receiving jack that keeps it from having signal pass through until enabled?

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Not in any CAT5 that I have worked with. They should be straight through. If the designer wants a diode, it would go on the PCB after the jack. Interesting issue! If you DO NOT connect the CAT5 cable, can you get continuity from the pins IN the jack to the PCB mount pins? – Enemy Of the State Machine Aug 27 '14 at 4:08
Just FYI, it turns out the jack I had did include magnetic couplers (i tore the back of the jack off). So many thanks for that. More to the point if anyone else comes across this, with a flat head screw driver I was able to pop off the metal case, pull all the circuitry off the back of my jack (it was just hotglued there) and then there were 8 tiny solder pins sticking out that I was able to use for my original circuit. So if you have this issue there is still hope! Thanks all! – Mark Schmidt Aug 28 '14 at 1:51
up vote 10 down vote accepted

Most devices in the last decade do not have plain RJ45 jacks. Due to micronization of components and the demand for smarter/smaller parts, RJ45 jacks have combined the Jack with the Magnetic Couplers that used to be separate parts.

Typical schematic for a Mag Jack:
enter image description here

That is a 2 pair, 100Base-T version. Some have the other 2 pairs connected to pins instead of shield. And Gigabit or 4 pair versions simply have 2 more sets of magnetic couplers.

Notice the IC next to this jack. That is an external magnetic coupler/transformer. If the router or device you pull your jack from does not have something similar, it most likely has a MagJack.:

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Although it's obvious to those with experience, you might point out that the transformers in the magjack are what stops there being continuity. – David Aug 27 '14 at 7:02

In all reality the RJ45 jack that you unsoldered is probably something called a MAG JACK. These have the Ethernet coupling transformers built right into the connector body instead of the older style where there was a separate coupling component on the PC board between the PHY and a plane vanilla straight through RJ45 connector.

You will never be able to measure DC continuity through a MAG JACK type connector due to the embedded transformer coils which are optimized to pass AC signals at Ethernet speeds.

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