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$95 HD/LINKer - Invensys Rail

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8.3 COMMUNICATIONS SESSIONS<br />

Document No.: SIG-00-97-08 Version: C<br />

8-3<br />

VIRTUAL LINE CIRCUITS<br />

A standardized ATCS message format known as a datagram is employed during communications<br />

sessions. Each datagram consists of five major fields as shown in figure 8-2.<br />

Destination Source M# Label Data<br />

Figure 8-2. ATCS Datagram<br />

The Destination field is the address of the recipient I/O module.<br />

The Source field is the address of the sending I/O module.<br />

The M# (message number) field is sequentially allocated by the source I/O module, enabling the<br />

recipient I/O module to detect duplicate, missing, or out of order messages.<br />

The Label field identifies the type of information transmitted within the Data field.<br />

The Data field comprises vital instructions for the recipient I/O module.<br />

8.3.1 Communications Components<br />

Communications components (figure 8-3) appear in the Communications View as a combination of<br />

Termination Communications Server icons, Termination Communications Client icons, Cut<br />

Communications Clients/Servers icons, and Communications Session lines.<br />

Termination<br />

Communications<br />

Cut<br />

Communication<br />

Client/Server<br />

Groups<br />

Figure 8-3. Communications Components<br />

Termination<br />

Communications<br />

Client<br />

Communications<br />

Session<br />

Communications icons are identified by two two-digit numbers separated by a dash (see figure 8-4).<br />

The first two-digit number corresponds to the assigned module designation within the group. The<br />

second two-digit number corresponds to the assigned Server/Client designation.

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