This information is for Phase3A probes, or Phase3B probes used with the Kintex FPGA. (which version do I have?)
Synchronizing other signals with electrophysiological data from Neuropixels probes can be achieved by providing up to 16 signals of your choice to the basestation. These signals will be treated as digital TTL inputs and their value (high or low) will be recorded alongside the electrophysiological data stream.
Note that the FPGA boards have independent clocks, so if you are using more than one Neuropixels probe simultaneously, you should take care to ensure that they are each synchronized appropriately, since they will drift relative to each other.