Clock Synchronization with the NTP Protocol
Original instructions
About us
Features of the Service
The clock synchronization via NTP offers:
Controller projects use a function block to read the clock, a feature that allows events or variables in the project to be time stamped.
Time stamping is accurate to:
Clock Synchronization and Time Stamps
The BMXNOR0200H module sends a source clock synchronization signal to the CPU. The module firmware includes an NTP client, which provides clock synchronization. The synchronization process occurs as follows:
The NTP Client...
Result
1
... requests a clock synchronization signal from the NTP server over an Ethernet network.
The NTP server sends a signal.
2
... stores the time.
3
... sends a message to the clock system counter in the CPU.
The CPU updates its internal clock. The CPU clock is now typically within 5 ms of the NTP server, with a worst case of 10 ms.
Before the next clock synchronization signal, the CPU clock is updated each ms by an internal timer.
Use the R_NTPC function block in either MAST, FAST, or Interrupt sections to read the clock from the PLC application.
The CPUs on an Ethernet network should be synchronized with the same NTP server.
Clock Synchronization Terms
Term
Description of Service
local clock offset
Accurate local time adjustments are made via a local clock offset. The local clock offset is calculated as:
((T2 - T1) + (T4 - T3))/2
where:
  • T1 = time when NTP request is transmitted from the module
  • T2 = time when NTP server receives the request (provided by the module in response)
  • T3 = time when the NTP server transmits the response (provided to the module in the response)
  • T4 = time when NTP response is received by the module
time accuracy
The local time margin is < 10 ms compared to the referenced NTP server time.
  • typical: 5 ms
  • worst case: <10 ms
settling time
Maximum accuracy is obtained after 2 updates from the NTP server.
polling period dependency
Accuracy depends on the polling period. Less than 10 ms of margin is achieved for polling periods of 120 ms or less. To obtain a high degree of accuracy (when your network bandwidth allows), reduce the polling period to a small value—for example, a polling time of 5 s provides better accuracy than a time of 30 s.
time zone
The default format is universal time, coordinated (UTC). Optionally you may configure the service to use a local time zone—e.g., GMT+1 for Barcelona or Paris
daylight saving time
The module automatically adjusts the time change in the spring and fall.
leap second
To compensate for the deceleration of the earth rotation, the module automatically inserts a leap second in the UTC time every 18 months via an international earth rotation service (IERS).
Leap seconds are inserted automatically as needed. When needed, they are inserted at the end of the last minute in June or December, as commanded by the NTP server.
Obtaining and Maintaining Accuracy
The time service clock starts at 0 and increments until the Ethernet network time is fully updated from the module.
Model
Starting Date
M340 with Control Expert
January 1, 1980 00:00:00.00
Clock characteristics:
Re initializing the Time Service Register
After a download or an NTP server swap, the status clock value associated with the time service register in the CPU is re initialized.
Two polling periods elapse before an accurate time is re-established.