GE PACSystems RX7i, User Manual

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Chapter 5. Ethernet Global Data
GFK-2224Q January 2017 91
Modifying an Exchange’s Variable Lists
When modifying the variable list for an exchange, the operation differs depending on whether EGD signatures
are 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 Enabled
Modifying the variable list with signatures enabled results in either a major signature change or a minor
signature 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 will
request 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 a
short time, but will not cause the consumer(s) to stop consuming.
Type of Change Resulting Signature Change
Adding a variable to the end of the variable list Minor
Adding a variable at the beginning or middle of the list Major
Deleting or modifying a variable Major
Changing a variable’s name, type, or array dimensions Major
Changing other variable properties such as reference address and
publish state
None
Modifying Exchange Variable Lists without EGD Signatures Enabled
In applications without EGD signatures, a consumer determines compatibility solely by the number of bytes of
data in the exchange. Modifying an exchange so that the length of the produced data does not match the
expected length by the consumer(s) causes the consumer(s) to no longer consume that exchange. A store to
update the corresponding producer/consumer is required to resume consumption of the exchange(s).
Caution
With signatures disabled, it is possible for an RMS to a producer or consumer to
cause an incompatibility that cannot be detected by the consumer. For example,
replacing an exchange variable with a different variable of the same size does
not 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 definition
is run-mode stored to either the producer or the consumer.