[time-nuts] GPS for Nixie Clock

John Swenson johnswenson1 at comcast.net
Sun Jul 10 23:55:14 EDT 2016

I'm thinking about converting a Nixie clock I built years ago into using 
GPS for the time base. No real NEED, just for fun.

The clock uses an FPGA for formatting and display, using the 60Hz line 
frequency as the time base. The case is a single hollowed out block of 

I'm looking into a TU36-D400-020 receiver. This seems to be optimized 
for timing purposes rather than navigation, it has 1PPS and 10KHz outputs.

I'd be getting it from RDR Electronics, which says it it uses the 
Motorola command set. This seems fine for me, it has the information I 
need, specifically UTC time so I don't have to worry about leap seconds.

I have a few questions about this receiver:
The data sheet lists two serial ports, but I don't see any information 
about which to use. Are they identical, do I have to use one for some 
functions and the other for other purposes?

What are the serial port parameters? 9600-8-N-1? Or something else?

Which is better to use, the 1PPS or the 10KHz? I can easily go either 
way. The clock display just goes down to seconds so 1PPS would work. I 
could also re-clock the 1PPS with the 10KHz.

What antenna to use? I would prefer something mounted inside the case. 
It is wood so an internal antenna will hopefully work. The board comes 
with a pigtail but it is not SMA.

Any other hints for using this?

I've never done a GPS interface before so I'm not sure about how I 
calibrate the time coming from the message over the serial port. Is it 
something like "the time is such and such at the rising edge of the next 
PPS, or the previous one? Or is there some other mechanism for 
calibrating when the second changes on the display to something close to 

I previously toyed with the idea of using an X72 rubidium oscillator 
just for the bragging rights, but I would still need the GPS to get the 
time, I decided the TU36 on its own is probably just fine.


John S.

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