Radio clock sync circuit identification
That looks like one of the modules for receiving the radio time signals transmitted from various places including:

UK - Time from the NPL (was MSF from Rugby)
Frankfurt, Germany - DCF77
Colorado, USA - WWVB
And others

Receiver modules might receive the time signal from only one or a few sources, not all, as they are transmitted on a range of different frequencies.

I tried bringing up one of the middle contacts and listening on the other, but it stayed on the voltage.

It's not clear what you mean by &quot;bringing up&quot; and &quot;on the voltage&quot;, but hopefully you haven't damaged the module by whatever you did.

I assume the communication is digital.

Yes. The protocols are explained on the different Wikipedia pages above.

The two in the middle, labelled T and P2

If you search for similar modules, you will find that they typically have 2 signal connections (in addition to the two power connections):

one signal (input to the module) to switch the module between &quot;standby&quot; (effectively &quot;off&quot;) and &quot;on&quot;;

the other signal (output from the module) is the encoded time signal output.
That fits with you having 2 unknown connections.


Based on the letters used, it would make sense if T is the digital time signal output, and P2 is the power control input. However you will need to investigate further, to see whether that hypothesis is correct.
Different modules have different specifications for those signals. If you still have the original clock, you will reduce your reverse-engineering work by reconnecting that module to the clock and monitoring those 2 signals with an oscilloscope, when the clock is in use. It will be useful if you can determine that a device (probably MCU) elsewhere in the clock, is actively driving one of those signals (e.g. the P2 signal), but is not driving the other one (because this one is an input to the MCU in the clock).
If you cannot connect that module back to the original clock, then my approach would be something like the following (but since I can't see or test your specific module, I cannot guarantee that this will work for you - use it as a basis for your own analysis):

I would pull P2 low via a 10k resistor to 0V (don't connect it directly to 0V or 3V, in case it is actually an output on your specific module), and then monitor the T signal with an oscilloscope. If T does not seem to be driven, then move that 10k resistor to pull P2 high to 3V and re-test. (The idea is to try with P2 low and then high to try to enable (switch-on) the module, so that it drives the T signal, which I suspect to be the time output signal).

If T still seems not to be driven, then the T signal might be open collector and require a pull-up resistor itself (as I explained, different modules have different signal specifications). To allow for that possibility, add another 10k resistor between T and 3V, then repeat the trial-and-error process with P2 as above.