How to Wire an RJ31X Jack

by Jeff Fisher

Server does not support image manipulation.
This page explains the strange little RJ31X wall-jack. What it's for. What it does. And how to hook one up properly.


A long time ago, there were no "automatic" security systems as we know them and folks that could afford it just hired security guards. Other folks went without security.

Then came the first electronic security systems. No great genius, that. They didn't need security guards all over the place to watch everything. But there was still a problem. The user still needed at least one security guard to do something when the alarm went off.

One company decided that having one guard at each of their offices all over town was a bit silly, and ran wire to link all their security systems directly to a board in the local central police station. When an alarm went off, a light would light up, a buzzer would sound, and the police would be dispatched. This is where the term "central station" came from. The fact that a person was actually "watching" the security system at all times is where the term "monitoring" came from.

Central monitoring was a grand hit and it wasn't long before there was a considerable amount of equipment in the police stations that needed manned 24 hours a day. Naturally, the police rapidly grew tired of all this extra work and forced the user's to move the equipment elsewhere and monitor it themselves.

Thus, the first monitoring company was born. A consortium of business each paid a fee to a new business, whose job it was to simply monitor their alarm system and do something when it went off.

Up until fairly recently, monitored security systems were still "hard wired" with a pair of copper wires to the central station. This is a neat, fail-safe setup. Even if the security system (and building) is completely obliterated in an instant, the central station will know something is wrong due to the change on the other end of the wires. For all I know, banks and such are still hardwired.

We take phones for granted, but it wasn't long ago that using the phone lines for monitoring was simply out of the question; not reliable enough. When the alarm went off, it would have to dial up the central station and report the alarm. There were just too many times that a call could not be completed, and that would result in an alarm not being reported. But stringing copper is expensive, and phones have gotten more reliable. The tremendous growth of security systems in residences has forced homeowners to use the regular phone systems for monitoring.

This actually works pretty well except for one thing: People naturally didn't want to put in an extra phone line for the security system, they wanted the new system to use their existing phone line. OK...but what if the phone is off the hook when the alarm goes off? Or what if the burglar knows about this and simply picks up any phone in the house and interferes with the automated call in some manner?

That's easy, right? Just connect the alarm system ahead of all the phones in the house and build in a relay so it can disconnect house-phones and do its thing. Even if there is a call in progress, it can "seize" the line, hang-up, and dial the monitoring company. There's only one minor "interfacing" problem with this approach. It's almost kinda' silly, but it ended up requiring a unique piece of equipment to solve.

You see...in practice, the guy that puts in the phone wiring, and the guy that puts in the security system, are usually two different people, two different companies, and sometimes, two different planets. They could never agree on where the interconnection between the two systems should be, how it should hook up, etc. And when anything went wrong, they would just point fingers at each other. When the phone installer finished, the phones still wouldn't work because the security system wasn't installed so that its relay could short the inside wiring to the CO (company office) line. If there ever was a problem, there was no easy way to tell if it was the security system's fault. Or, worse yet, if the security system failed, it would take down all the inside phones!

Server does not support image manipulation.
Fortunately, this big complex problem had a very simple solution. Some clever person came up with a jack that could be inserted in the line between the CO and all the house phones. When nothing is plugged into the jack, the house-phones are simply connected to the CO line. (That's what the "shorting bars" do.) But when the security system is plugged into the jack, it is "inserted" into the loop and has separate connections to the inside phones and the CO line.

So the telephone installer is simply told to "put an RJ31X jack here" and he is supposed to know what that means. Even though the security system isn't plugged in yet, the phones will still work, and he can go home.

When the security installer has the security panel up and ready, he plugs the panel into the RJ31X jack and he's in business. If there's ever any question about whether the panel is interfering with the phone lines, or if the panel fails, the homeowner can quickly unplug the panel from the jack.

Wiring in the ELK RJ31X Jack

Server does not support image manipulation.
The Elk RJSET RJ31X jack has an 8 position moduar jack (the actual "RJ31X" jack) on the outside and screw terminals inside. Typically only four screw terminals are used; two for the connection to the outside phone line (T and R), and two for the connection to the inside phone lines (T1 and R1). If you need to "line seize" more than one line, use an additional RJ31X jack for each additional line.

Mount this jack within two feet of the alarm panel so the included cable will reach.

The RJSET jack has 8 screw terminals inside the jack housing. This makes it easy to connect the outside and inside lines. Just connect the lines as shown below. The outside and inside lines can come into the jack from within the wall behind the jack, or into either, or both, sides of the jack.

Server does not support image manipulation.
The color coding may vary depending on how the installer wired the alarm panel to the phones originally, but this is the standard method. When connected as shown (and nothing plugged into the jack), the inside phones should be working. You can place the cover on the jack. It is ready for connection to the security system.

Connect the 2 foot cord to the alarm panel:

  • Red to "R" or "Ring" contact. (Ring, from phone company.)
  • Green to "T" or "Tip" contact. (Tip, from phone company.)
  • Grey to "R1" contact. (Ring, To inside phones)
  • Brown to "T1" contact. (Tip, to inside phones)

Plug the 2 foot cord into the RJ31X jack and test.

Here is the complete hookup diagram.

Server does not support image manipulation.

Connecting Your Security System

Now you need to make a cable to connect the security system to the RJ31X jack (if you aren't using the ELK version). I usually use Belden 1242A quad wire for this cable. (most any cable will work fine for this short single-purpose piece but it's helpful if the color codes match up.) You will also need an eight position modular plug and a modular plug crimp tool.

Note that you can use four conductor flat cable instead of the quad wire, but you'll also have to use a different modular plug.

Server does not support image manipulation.
Crimp the quad cable into the modular plug as shown in the diagram at right. (This is the hardest part of the job!) You'll need to spread the wires out and carefully slide them into the correct slots inside the connector. Make sure the wires are all pushed all the way in before crimping. (It helps to trim the ends flush before inserting since the center two tend to be longer than the outer two.)

Server does not support image manipulation.
Here's another picture of the quad wire, just before insertion. Note that the tab is up on the plug, and thus the yellow wire will go into slot 1, the red into slot 4, green into slot 5, and black into slot 8.

Server does not support image manipulation.
And finally, here's what you do with the other end of the quad wire. The green is CO line (outside line) Tip. Red is CO line Ring. Black is house phone Tip. And yellow is house phone Ring.

Server does not support image manipulation.
When this is all connected, you simply plug the modular plug into the RJ31X jack, and your security system is now "in the loop".


Register (under "Log In" above) to leave a comment.




Newest Blog Posts

  1. A new home.
    2014-10-30 22:44