For these who have a need, the HL7 v 2.3.1 standard document is available online at as at February 24, 2010. The site does not seems to have been updated for a long time so the standard document may well be available for a while longer.

HL7 v 2.3 document is available at, again, as at February 24, 2010.

IHE Europe has an interesting "interactive" site where HL7 can be "explored" at