RJ12 Pin Assignments?
1009 Views - Created 21/08/2020
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 https://en.wikipedia.org/wiki/Registered_jack#Provisioning_of_power (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!
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.
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.
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