Chapter 5. Ethernet Global DataGFK-2224Q January 2017 91Modifying an Exchange’s Variable ListsWhen modifying the variable list for an exchange, the operation differs depending on whether EGD signaturesare enabled or not. The use of EGD signatures is strongly recommended when doing run mode stores of EGD.Modifying Exchange Variable Lists with EGD Signatures EnabledModifying the variable list with signatures enabled results in either a major signature change or a minorsignature change.A major signature change in a run mode store will cause incompatibility between a producer and consumer(s).When a consumer that supports dynamic rebinding recognizes a major signature change, the consumer willrequest a new configuration from an EGD configuration server without user intervention.A minor signature change in a run mode store to a producer will cause the exchange not to be produced for ashort time, but will not cause the consumer(s) to stop consuming.Type of Change Resulting Signature ChangeAdding a variable to the end of the variable list MinorAdding a variable at the beginning or middle of the list MajorDeleting or modifying a variable MajorChanging a variable’s name, type, or array dimensions MajorChanging other variable properties such as reference address andpublish stateNoneModifying Exchange Variable Lists without EGD Signatures EnabledIn applications without EGD signatures, a consumer determines compatibility solely by the number of bytes ofdata in the exchange. Modifying an exchange so that the length of the produced data does not match theexpected length by the consumer(s) causes the consumer(s) to no longer consume that exchange. A store toupdate the corresponding producer/consumer is required to resume consumption of the exchange(s).CautionWith signatures disabled, it is possible for an RMS to a producer or consumer tocause an incompatibility that cannot be detected by the consumer. For example,replacing an exchange variable with a different variable of the same size doesnot change the size of the exchange. Since the size of the exchange is the same,the consumer will continue to consume that exchange when the new definitionis run-mode stored to either the producer or the consumer.
