GE GFK-1541B manual Asynchronous Operation of EGD, PLC Producing EGD Via LAN PLC Consuming EGD

5

Asynchronous Operation of EGD

The production and consumption of EGD is asynchronous. The period timers for the producer and consumer are independent of each other. In addition, the initiation of the production and consumption of EGD can occur at different times.

The figure below illustrates the asynchronous aspect of EGD in all Ethernet Interfaces except the Series 90-30 CPU374. In the Series 90-30 CPU374, there is no consumer period. When data is received, it is made available to the application on the next PLC sweep.

PLC Producing EGD via LAN PLC Consuming EGD

EGD Production Is Initiated

• Producer period timer starts.

•Exchange Status word is set to 0.

•First exchange data sample is sent onto the network.

Producer	EGD Production Repeats
period

•Producer period timer expires and is restarted.

•Exchange data sample is sent onto the network.

•Exchange Status Word set to 1.

Producer

period

•	Producer period timer expires and is
	restarted.

•Exchange data sample is sent onto the network.

•Exchange Status Word set to 1.

EGD Consumption is Initiated

• Consumer period timer starts. *

•Exchange Status word is set to 0.

Consumer

period *

EGD Consumption Repeats

• Consumer period timer expires and is restarted. *

•Exchange data sample received

from the network is updated in	Consumer
consumer PLC memory.
	period *

•Exchange Status word is set to 1 (or 3 if SNTP timestamping is used).

• Consumer period timer expires and is restarted. *

•Exchange data sample received from the network is updated in consumer PLC memory.

•Exchange Status word is set to 1

(or 3 if SNTP timestamping is used).

* Does not apply to CPU374

The figure presents a simplified overview of EGD. There are other timing issues regarding data transferred from the Ethernet Interface to the PLC CPU that could be an issue depending on your application. See the topic, “Configuration Planning,” for more information.

5-4 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual– May 2002 GFK-1541B