HL7

Overview
HL7 is the Health-Level-Seven protocol standardized with the website http://hl7.org by a group of health organizations. It is a specialized set of communications rules on the Application layer (the seventh level) of the ISO Network Stack, focused on Healthcare specific information to be sent between computers providing interchange of health application data.

The typical HL7 message contains several HL7 segments.

Each segment (line of text) starts with a header consisting of several letters, such as MSH or PID. There are one or more segments making up a message, and the segments are made up of more than one composite. Typically segments are separated by carriage returns ($C(13) or hex 0D ), which is defined in the MSH segment, so it may be changed if necessary. There is a VistA File #771.1 (named "HL7 FIELD") which defines many of these headers.

The exact definition of the composites that make up a segment depend on the particular segment that is used. The composite separator is also defined in the MSH segment, so it may actually be changed if necessary as well, but typically, composites are separated by $C(124) or hex 7C which some folks call the "pipe" or "bar" symbol (i.e.: | ).