Interesting, in ex. USSR and Czechoslovakia i am aware only about "2 wire" systems, but i heard also about "IBM" one with a 3 wires, so now i know what it was for, thanks :) Here master clock was producing 1m or 30s duration impulses. I also heard about "hybrid" systems where both digital timestamp and 1m impulse were delivered, so clock was able to choose what to use. What is funny that there were no common standard. 6V, 12V, 24 and 60 - all were common! So few times i saw converters from one to another. One of the last "master clock" devices from Pragotron was based on IC circuits, but principle was the same - https://aukro.cz/matecni-hodiny-pragotron-eh40-6960978702