Chapter 4. CPU Operation158 PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222AD4.8.2 Time-of-Day ClockA hardware time-of-day clock maintains the time-of-day (TOD) in the CPU. The time-of-day clockmaintains the following time functions:▪ Year (two digits)▪ Month▪ Day of month▪ Hour▪ Minute▪ Second▪ Day of weekThe TOD clock is battery-backed and maintains its present state across a power failure. The time-of-dayclock handles month-to-month and year-to-year transitions and automatically compensates for leapyears through year 2036.You can read and set the hardware TOD time and date through the application program using ServiceRequest function #7. For details, refer to PACSystems RX7i, RX3i and RSTi-EP CPU Programmer’sReference Manual, GFK-2950 Chapter 6.High-Resolution Time of Day Software ClockA high-resolution software TOD clock is implemented in firmware to provide nanoseconds resolution.When the high-resolution software TOD clock is set, the hardware TOD clock is set with the YYYY: Mon:Day: Hr: Min: Sec fields in the POSIX72 time, the RTC is read, and the delta between the POSIXtime andthe value read from the RTC is computed and saved. Thus, if 1-second resolution is desired the hardwareTOD clock is read. Otherwise, the high-resolution software TOD clock is read to provide greaterresolution. When the latter occurs, the hardware RTC is read and the saved delta added to the valueread.When the SNTP Time Transfer feature is implemented, all SNTP time updates received at the CPU willcause the high-resolution software TOD clock to be updated.Synchronizing the High-resolution Time of Day Clock to an SNTP NetworkTime ServerIn an SNTP system, a computer on the network (called an SNTP server) sends out a periodic timingmessage to all SNTP-capable Ethernet Interfaces on the network, which synchronize their internal clockswith this SNTP timing message. If SNTP is used to perform network time synchronization, the time-stampinformation typically has ±10ms accuracy between controllers on the same network.Synchronizing the CPU TOD clock to an SNTP server allows you to set a consistent time across multiplesystems. Once the CPU TOD clock has been synchronized with the SNTP time, all produced EGDexchanges will use the CPU TOD current value for the time-stamp.The CPU TOD clock is set with accuracy within ±2ms of the SNTP time-stamp.TOD clock synchronization is enabled on an Ethernet module by the advanced user parameter (AUP),ncpu_sync. The CPU must also use a COMMREQ in user logic to select an Ethernet module as the timemaster. For additional information, refer to Time-stamping of Ethernet Global Data Exchanges inPACSystems RX7i, RX3i and RSTi-EP TCP/IP Ethernet Communications User Manual, GFK-2224 Chapter4.72 RSTi-EP CPE100/CPE115 does not allow the Time-of-Day clock to be set older than 1st Jan, 2001 when POSIX format is usedalong with SVC_REQ 7.
