2002 - cesnet

generated by the NTP server. Its output is the PPS (pulse per second) signal forthe NTP server and two serial ports connected to the KPC and NTP server. TheFK system has the following functions:• generation of the PPS signal on the TTL level for the NTP server• measurement of the NTP second pulse shift comparing to second pulsesfrom the standard• possibility to set date and time using buttons and a display• possibility to enter information that a leap second will occur at the end ofthe current month• generation of the output sentence containing the current UTC second label,measured shift of the second generated by the NTP server and the leapsecond flag20.5 Special NTP Server Hardware20.5.1 PPS Signal Processing CardThe standard way of working with PPS is to transfer the signal on the RS-232 levelto the DCD serial port input and process interrupts generated by changes of theDCD input. In the interrupt handling procedure, a timestamp is assigned to thisinput signal edge. Problems are caused by the delay in the interrupt processing,which results in timestamp errors depending on the operating system type,processor speed, and current utilization. A typical delay is 10–25 microseconds.This issue is described in detail in the technical report No. 18/2001.We designed a special PCI card, which can record the exact PPS signal arrivaltime, and had it manufactured. In this way, we obtain timestamps with the accuracyof 50 ns.20.5.2 Temperature-Compensated OscillatorIn contemporary PCs, all frequencies are derived from a single 14.318 MHz oscillator.However, standard quartz is usually used, the temperature dependence ofwhich is high even on the level of normal operating temperatures. The oscillatorcircuitry fortunately allows connecting directly a source of a TTL signal withthe voltage of 3.3 V instead of the quartz. This enabled us to replace the quartzwith a temperature compensated oscillator.High-speed National Research Network and its New Applications 2002203