GE GFK-1541B manual Time Stamp

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Status Word	A variable that identifies the memory location where the status value for
	the consumed exchange will be placed. Valid memory types are listed
	later in this chapter.
Time Stamp	A variable that identifies the memory location where the timestamp of
	the last data packet will be placed. The timestamp is not an actual date, it
	is an 8-byte value representing the time elapsed since midnight, January
	1, 1970. The first four bytes contain a signed integer representing
	seconds and the next four bytes contain a signed integer representing
	nanoseconds. This value represents the time in the producer when the
	data sample originated. It can be examined to determine if a new packet
	received from the network has a new data sample or if it is the same data
	received previously.
	The timestamp information produced by the PLC currently has a
	resolution of 100 microseconds if no network synchronization is used. If
	SNTP is used to perform network time synchronization, the timestamp
	information has a resolution of 1 millisecond and has ±10 millisecond
	accuracy between PLCs on the same LAN.
	SNTP is enabled in Module Configuration for the Ethernet Interface.
	Once SNTP time synchronization is configured, the Ethernet Interface
	will synchronize itself to an external SNTP time server. Only EGD
	exchanges produced through an SNTP-synchronized Ethernet Interface
	will carry network-synchronized timestamps. EGD exchanges produced
	through other Ethernet Interfaces in the same PLC that are not
	configured for network synchronization will use the PLC’s timestamp
	information.

Exchange Variables A list of data elements making up the snapshot of internal memory that will be consumed from the network. Each exchange has its own variable list, which will be received from the producing device as a contiguous set of bytes. The elements in the list are defined as variables. A maximum of 100 variables can be defined in one exchange definition. The data types BOOL and BOOL_STATE are not allowed for exchange variables and the total size of all variables cannot exceed 1400 bytes. For consumed exchanges, %S memory types and override references are not allowed. Valid memory types are listed in this chapter.

GFK-1541B Chapter 5 Ethernet Global Data 5-15