Once you have a sample of the HL7 message type you want to generate, you can use Chameleon to define the message structure and specify the data to include. This information is summarized in a Visual Message Definition (VMD) file.

In this section of the tutorial, you will set up a VMD file for your application. The following HL7 message will be the sample for the application:

MSH|^~\&|TEST CLIENT|ACME, INC.|||200502251127||ORU^R01|MESA3b781ae8|P|2.5||||||||
PID|||CR3||CASTORP^HANS||19050817120000||||||||||||21643900|||||||||||
OBR||||B101Z^CHEST X-RAY|||20010501141500||||||||||||||||||F||||||||||||||||||
OBX|1|TX|SR Text||^^^^^11.2||||||F||||||
OBX|2|TX|SR Text||^^^^^100.3||||||F||||||

An HL7 message is a sequence of segments, each of which begins with a three-letter label that identifies the segment type. In this sample message, each of the segments is written on a separate line to make it easier to read.

Each segment in a message is divided into composites, or fields, and the fields are separated by pipe characters ('|'). As you can see, many of the fields in the sample message are empty, and this is frequently the case in the real world as well. Fields may be further subdivided into subfields, delimited by the caret ('^'), and subsubfields, delimited by the ampersand ('&').

Every HL7 message begins with a MSH segment that contains basic information about the message, such as the time it was issued and the message type. In the sample message shown above, the MSH segment indicates that the message type is ORU^R01 (Unsolicited Observation Result), which is the message type that your application will be generating.

As you can see, HL7 messages consist of readable characters. However, they are not really designed to be read by people. It is easy to lose your place when counting pipe characters, and it is impossible to determine the purpose of any information contained in the message. For example, if a field contains a name, there is no way to tell whether it is the name of the patient, the next of kin, the physician, or someone else. This purpose could be determined by consulting the appropriate segment definition in the HL7 standard, but this quickly becomes tedious.

With Chameleon, these problems disappear. The Chameleon Message Browser is a handy utility that makes both the structure and the content of HL7 messages plainly evident.

	For more information on HL7 messages, see HL7 Overview.