The segment grammar of an HL7 message is the sequence of segments that forms the 'signature' of the message. The segment grammar specifies the order in which the segments appear, and whether the segments are optional or repeating.

When you use Chameleon to generate HL7 messages, you must define a segment grammar for each message type that your application will produce. In this tutorial, your application will be producing an ORU^R01 message containing the following segments, in order:

• A segment of type MSH (Message Header);
• A segment of type PID (Patient Identification);
• A segment of type OBR (Observation Request);
• One or more OBX (Observation/Result) segments.

	In the sample message, two OBX segments are defined. It is best to assume that any number of OBX segments could be specified in an ORU^R01 message.

Your segment grammar must be defined to conform to these requirements.

To set up the segment grammar:

• In the Workspace, locate the LabResult message, and double-click its Message Grammar link. The Message Grammar window appears. The MSH segment is already present, since it was added when the message was defined; this segment is always the first segment in the segment grammar.

Message Grammar Window, Showing Segment Grammar Panel

• Add the PID, OBR and OBX segments by dragging them from the Workspace and dropping them into the Segment Grammar panel:

	Order is important in a segment grammar: segments must appear in the grammar in the order in which they appear in the message.

Next, you will need to indicate that the OBX segment repeats. To do this:

• Select the segment, and check the Repeating checkbox.
• In the Repeats list box (located to the right of the Repeating checkbox), leave the value at 0. This indicates that the number of segments is unknown (and can be unlimited).

When you select the Repeating checkbox, the OBX segment in the Segment Grammar panel is enclosed in curly brackets. This is the HL7 notation for a repeating segment.