Unconnected Messaging

The Unconnected Message Manager transport class supports unconnected messaging. It consists of a client side and a server side; clients initiate messaging transactions while servers respond to client requests. The UCMM protocol contains provisions for retries and acknowledgements to guarantee delivery of both the request and response packets of each transaction. UCMM is relatively inefficient when compared to the class 1 and class 3 connected transport classes, but unlike the class 1 and 3 transports, UCMM requires no pre-negotiation to transmit request and response packets.

UCMM clients can initiate any number of concurrent request/response transactions to one or more UCMM servers, bounded only by the number of UCMM client transaction records configured in the client device.

UCMM servers can accept multiple concurrent request/response transactions from any number of different clients, bounded only by the number of UCMM server transactions records configured in the server device.

UCMM messages are used to initiate operation of class 1 and class 3 connected transports. The initiation of connections is handled by the Connection Manager Object. See the CM component section for more details.

Connected Messaging

Transport class 3 supports connected messaging. Like UCMM, class 3 transports consist of a client side and a server side. Clients initiate messaging transactions while servers respond to client requests. Messaging connections are useful for the transfer of event-triggered data in a timely manner, with higher efficiency than with UCMM transactions.

An example of the use of transport class 3 is the method used by PLC-5 processors to implement message rungs. When the PLC processor transitions to "run" mode, a transport class 3 connection is opened from the PLC-5 processor to the destination device's message router object. Once opened, the connection is used as a "message pipeline" to efficiently send message requests and receive message responses. The connection is kept open until the PLC-5 processor transitions to program mode.

Connected Real-time Data Transfer

Transport class 1 supports connected, real-time data transfers. Unlike UCMM and class 3, class 1 connections consist of producers and consumers rather than clients and servers. This allows both nodes involved in the connection to both send and receive data concurrently. There is no request/response sequencing.

Class 1 transports are repeatable and are the primary means of delivering real-time control data.