Motorola V186 User Manual - Page 148

Clock Functions and Synchronization The accuracy of other clocks is judged according to how "close" a clock is to a reference clock (the stratum of the clock, the network latency to the clock, and the claimed accuracy of the clock. The accuracy of NTP thus depends on the network environment. Because NTP uses UDP packets, traffic congestion could temporarily prevent synchronization, but the client can still self-adjust, based on its historic drift. Under good conditions on a LAN without too many routers or other sources of network delay, synchronization to within a few milliseconds is normal. Anything that adds latency, such as hubs, switches, routers, or network traffic, will reduce this accuracy. The synchronization accuracy on a WAN is typically within the range of 10-100 ms. For the Internet/GPRS synchronization accuracy is unpredictable, so special attention is needed when configuring a client to use public NTP servers. Testing with the ACE3600 connected with the Internet gains accuracy of 20-30ms, but theoretically it may be even 100ms. NTP uses UTC time base (Coordinated Universal Time). UTC evolved from Greenwich Mean Time (GMT). GMT is based on the earth's rotation, which is not constant enough to be used for detailed time measurements. UTC is based on a standard second length determined by the quantum phenomena. There is a difference of a few seconds between the two (14seconds in 2006), so every several years add one more second (called leap second) to UTC. This is built in NTP protocol. To translate the UTC time into local time, user can configure Time zones and Daylight Savings in RTU. Note however, that if setting NTP server to another stand alone ACE3600, which has no time zone, both will operate with the same local time if no time zone set. If that ACE3600 is connected to a GPS or to another NTP server then there is a need to set a time zone. Global Positioning System (GPS) The ACE3600 system can use a GPS receiver precise time measurement application for synchronization purposes, to synchronize the RTU with other SCADA systems. The ACE3600 RTUs use GPS timing receivers equipped with a 1 Pulse per Second (PPS) output. The receivers are connected to an RTU port. In case of a satellite failure, the time is manufactured internally and the receiver indicates its inability to trace the satellite. The recommended GPS receiver is the Synergy Systems SynPaQ/E GPS Sensor with M12+ Timing Receiver which must be purchased from a Synergy vendor. Along with the timing receiver, a data/power cable and antenna should be purchased. For details on connecting to the GPS receiver, see Appendix A: RS232/RS485 Adaptor Cables in the ACE3600 STS Advanced Features manual. 144