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RJ12 Pin Assignments?

The 1 Wire Pi Plus and 1 Wire Pi Zero are a 1 Wire interface for the Raspberry Pi


Posted by:

ziesemer Avatar

Just curious as to any background or perspectives around the RJ12 pin assignments used on these 1-Wire boards. Is there any sort of a 1-Wire standard for with RJ plugs that is being followed here?

It would be very economical to use readily-available plugs, cords, and splitters already used for most telecom configurations. However, most such plugs and cords are only 6P2C (using pins 3-4), or 6P4C (using pins 2-5) as the next most-common. However, these boards have the 5V supply on pin 1, which requires an even less common 6P6C plug and cable - while leaving pins 5-6 as no-contact (NC). Why was the 5V supply not put on pin 5? This would have allowed:

Using 6P2C (2 conductors) for 1-Wire (+ ground) configuration (using "parasite power") - also as-is.
Using 6P4C (4 conductors) for 1-Wire, ground, and 5V supply.

This would even seem to follow the "dial light" used on some phones - as described at (though this was 6V AC, vs. 5V DC).

As-is, I'm half tempted to use 6P6C connectors (inexpensive enough) with 4-conductor cable, and just offset the wires into the plugs. However, this would make them nearly worthless of any other standard purpose in the future.

I could also jumper pins 1 and 5 together, allowing me to use 6P4C everywhere else. However, I don't want to crimp 1-Wire temperature sensors, etc. into what I'd later find to be a non-standard configuration.

Just curious as to what other background or standards may be involved here. Thanks!


Posted by:

andrew Avatar

United Kingdom

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The pin assignment on the RJ12 connector was chosen to match the Maxim DS9490 USB to 1-Wire/iButton adapter.

Maxim uses the same pin assignment on most of their 1-Wire accessories and as they are the main manufacturer of 1-wire devices we decided that it would be best to use the same standard as they use.


Posted by:

ziesemer Avatar

Thanks for the reference! I'll just wait for my 6P6C supplies to arrive.


Posted by:

djgriffin Avatar

I had the same thoughts as the original poster (OP) and here is what I did.

I made a custom cable to go from the 6P6C wiring at the HAT to a 6P4C (RJ11) plug using the most common telephone configuration. I included the power supply wire as well as signal and ground so many sensors can be successfully driven. I can use off-the-shelf four wire RJ11 cables, couplers, and splitters. Although you do need to be careful of the manufactured items. Some flat cables and couplers are wired straight through (i.e. pin 1 to pin 1, pin 2 to pin 2, ...) but some are reversed (i.e. pin 1 to pin 4, pin 2 to pin 3, ...). All of the splitters I have found so far are straight through. Using a reversed cable and a reversed coupler yields a straight through "extension cord". Some sensors I just crimp into a RJ11 plug directly and for some I use a modular surface jack and take advantage of the screw terminals to connect a sensor. Due to the modest signaling rate of the 1-wire devices, if you keep the stubs fairly short (less than 2 feet) this all works just fine up to about 100 feet of total cable length. This design allows for the ability to have a very flexible arrangement of sensors at varying locations. If you desire a permanent installation once a suitable geometry has been determined, using lengths of CAT3 cable and soldering the sensors can be implemented.


Posted by:

Slawek Avatar

There is interesting material about using RJ45 here: A_Guide_to_the_1WRJ45_Standard.pdf

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