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, 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.

The FB interfaces shown above and the service sequences below illustrate the operation of this service for n = 3.