Some radio clocks and related timekeeping gear have a pulse-per- second (PPS) signal that can be used to discipline the local clock oscillator to a high degree of precision, typically to the order less than 50 us in time and 0.1 PPM in frequency. ... Either way requires conversion of the PPS signal, usually at TTL levels, to RS232 levels, which can be done using a circuit such as described in the Gadget Box PPS Level Converter and CHU Modem page. ...