Sunday, August 30, 2015

Prototyping a GPS Board for the KS-24361 REF-0

In this post I will describe how to mount a custom GPS board inside the Lucent KS-24361 REF-0 unit. I will also show a small board which I prototyped to neatly add all necessary circuitry for standalone operation of the REF-0 directly inside the housing.

This was my earliest success at getting the REF-0 to run standalone. It involved taking over the breakfast nook, and a lot of wires. What a mess! Time to clean things up.


I have written a few posts on how to make the KS-24361 REF-0 run standalone. My initial write-ups were focused on just getting it working, with connections made externally using the Interface connector. This works great for testing, but for long-term use, it is desirable to get the GPS and all necessary circuitry inside the unit. Getting everything installed neatly in the unit will prevent dangling wires and likely improve reliability.

For standalone operation as I have described previously, several connections need to be made at the Interface connector on the front panel to simulate the presence of a REF-1 unit. This is less than ideal if you want a clean external appearance. I decided to probe around the board and find out where those pins connected to. I hoped that they would terminate at easily accessible pins or pads. Finding them on the top of the board would be even better, because removing the board is a real pain.

To my surprise, I found them in nearly the most convenient spot possible.

Connection Points for Standalone Operation

As you may recall, we need four pins from the Interface connector for standalone operation. One pin (which I have labelled "Present" below) tells the REF-0 that the Interface cable is present and connected. The "Pair A" pin tells it that everything is ok and it should turn its outputs on. Then of course we need the "Serial In" and "PPS In" pins. Technically the Pair B and Pair C pins are also important, but the condition we need them in is floating, so we don't need to connect anything to them. Finally, a regulated 5V supply pin on the board would be great for powering stuff.

All of these can be easily accessed on the board inside the REF-0, underneath the position where the Oncore GPS would sit in the REF-1. The Preset, Pair A, Serial In, and PPS In pins all go to inverter ICs U27 and U32 as shown below. There are also 5V and ground pins on the Oncore GPS header (which is unpopulated in a REF-0). I have labelled these connection points in the image below.

Necessary connection points on the REF-0 mainboard for standalone operation. You can find these where an Oncore GPS would normally be mounted in a REF-1.

It's quite a stroke of luck to find all of the necessary connection points EXACTLY where we need them. The only thing that could have been more convenient was if the pins had gone to the Oncore header. But hey, I'm not complaining. With these connection points located, it is now possible to design a custom GPS board that can mount directly inside the REF-0. This will require some soldering on the board to add jumper wires, but the modifications are simple and reversible.

Connecting to the Points and Installing the Prototype

Get out your good soldering iron, because there is a little fine soldering to be done if you want to access these points. The Present and Pair A pins (9 and 13 on U27) both need to be connected to ground. I decided to bridge them over to the ground pin of that chip (pin 7). I also cut apart some breadboard jumper wires to solder onto the Serial In and PPS In pins (11 and 13) over on U32. The results of this are shown below. Finally, I soldered a couple of wires to 5V and ground on the Oncore header.

Connecting to the necessary pins on U27 and U32.

I then prototyped a little board to install into the REF-0. It houses a Venus GPS, an ATTINY841 microcontroller, the necessary inverter chip, and a 3.3V regulator for the GPS. You can see below that it's quite simple. (There are just some passives underneath the board that you can't see, but the main components are visible) It secures nicely using two of the regular mounting holes for a real Oncore GPS. The final board I design will use all of them for more support.

The only thing that needs to run to the outside of the REF-0 is the GPS antenna. I think I will widen one of the vent holes at the rear of the cover to allow an SMA connector to fit through. I might even be able to find a square rubber grommet to hide the cuts and make it look really clean.

My prototype for a GPS board to install directly inside the REF-0. The only connection that needs to be made externally is to the GPS antenna.

Going Forward

I have run the prototype for over a week now and it works great. I added a few things to the code to increase reliability and monitor lock status as well. My plan is to take this and design a custom board to install in the REF-0. This board will include a modern GPS unit made for timing, such as a U-blox LEA-6T.

So that's it! I hope this information is useful in creating a long-term solution for REF-0 standalone operation. Having everything mounted neatly inside the case makes a world of difference, especially if you are running multiple units.

I tried to keep this from being horribly long, so let me know in the comments below if you need clarification on anything I described above.

Thanks for reading!

- Dan W.


  1. Very, very nice, Dan. I'm excited at the possibilities. I hope you will make your board available for us to purchase. I do like the idea of a ublox LEA-6T in there. I can hardly wait. Thanks.

  2. I've started to poke around the pcb with a view to understanding the schematic and what the code is doing. This sort of thing is much faster if everyone contributes their discoveries and knowledge. Is there a wiki where findings can be aggregated ?

    1. I do not know of a Time Nuts wiki, though such a thing would be *really* cool. There are so many projects that could be documented on such a site.

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