GE GFK-1541B Simple Network Time Protocol Sntp, Timestamping EGD Exchanges, Normal Sntp Operation

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Simple Network Time Protocol (SNTP)

SNTP is a method of synchronizing the internal clocks in a network's Ethernet Interfaces to an external source. In an SNTP system, a computer on the network (called an SNTP server), sends out a periodic timing message to all of the SNTP-capable Ethernet Interfaces on the network, which keep their internal clocks synchronized with this common SNTP timing message.

Timestamping EGD Exchanges

Each EGD exchange contains a timestamp that pinpoints when the associated data was transferred from the producing PLC's CPU to its Ethernet Interface for transmission over the network. The timestamp value is based on a built-in clock in the Ethernet Interface. It sends this value to the PLC CPU, which uses it to timestamp its data exchanges. Its time clock is synchronized to one of the following two sources:

ƒPLC's Time Clock: If the Network Time Sync parameter of the Ethernet Interface is set to None or DISABLED, the Ethernet Interface’s built-in time clock is synchronized once, at power-up or restart, to the clock in the local PLC CPU. The disadvantage of this source is that the time clocks in the various PLCs on the network are not synchronized, so timestamps produced by different PLCs cannot be compared accurately.

ƒSNTP Server's Time Clock: If the Ethernet Interface's Network Time Sync parameter is set to SNTP or ENABLED, the Ethernet Interface’s built-in clock is periodically synchronized to the clock on the SNTP server. This method has the advantage that all Ethernet Interfaces configured to use SNTP will have updated, synchronized timestamps, because they will all be synchronized to the SNTP server's clock. Therefore, accurate timing comparisons between exchanged data can be made. For example, if alarm data were sent over the network from several PLCs, it might be advantageous for troubleshooting purposes to know the order in which the alarms were generated. Using SNTP, the data's synchronized timestamps could help you determine this.

Notes

With SNTP operation, the time value passed from the Ethernet Interface to the PLC CPU is used in the PLC only for timestamping EGD exchanges. It does not affect the time of the CPU's internal time clock.

SNTP server dates before January 1, 1989 are not supported by the Ethernet

Interface.

Configuring an Ethernet Interface for SNTP

SNTP operation must be configured for each Ethernet Interface individually. For more information for your particular Ethernet Interface, refer to the installation chapter.

Normal SNTP Operation

The Ethernet Interface will synchronize to a remote SNTP time server after receiving two broadcast clock values within a 150-second period. The Station Manager can be used to view server status information.

GFK-1541B Chapter 5 Ethernet Global Data 5-25