A unit of data significant for inter-system communication is called a logical message. A logical message consists of one or more segments. A segment is a unit of information which can be processed by a single call to a library function from a UAP process.

When a logical message consists of one segment, the message can be processed by a single call to a function. When a logical message consists of multiple segments, the message should be processed by calling the same number of functions as the segments.

A segment consists of the header area used by the MCF and segment data. The length of the header area determines buffer format 1 or buffer format 2. The user can decide which format should be used. However, only buffer format 2 is available with TP1/NET/XMAP3.

The length of the header area varies by the communication protocol product. For more information, see the explanation of message exchange APIs in the OpenTP1 Protocol manual.

The figure below shows the relationship between a logical message and segments.

Figure 3-7 Relationship between logical message and segments