PUBLISH_n/SUBSCRIBE_n function blocks encapsulate UDP multicast services as prescribed in subclause 5.5 of the Compliance Profile. The datagram payload is limited to 1472 octets of encoded data so that each PUBLISH/SUBSCRIBE transaction can be accomplished in a single Ethernet packet under IPv4. STATUS values of "OVERFLOW" and "UNDERFLOW" indicate when this limit is exceed by encoding in a PUBLISH block or decoding in a SUBSCRIBE block, respectively.
If a PUBLISH or SUBSCRIBE block is instantiated in an environment that does not support UDP communication (e.g., in an Applet), the block automatically switches into the local multicast design pattern, with a channel whose name corresponds to the ID input of the block, and whose content consists of an instance of the class containing the ASN.1 encoded data. See CODEC_TEST2 for an example of one such Applet in action.
The FB type definitions below illustrate the service sequences and interfaces of this service for n = 3 .
Last updated: 2015-02-09.
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