The first step when defining an LLP Listener source component that selectively generates either NACK messages or ACK messages is to create the VMD file that generates the messages. To do this:

• In the directory in which Iguana is installed, make a copy of autoack.vmd. This is the default VMD file that is used to generate ACK messages. Rename the copy ackcustom.vmd (or whatever name you like), and make a note of the directory in which you have saved this file.

• Use Chameleon to open this VMD file.

• In the toolbar, select Options. The Options window appears with the Message Delimiters tab visible.

• In the Configuration list box, select Custom.

• In the table at the bottom left of the Message Delimiters tab, in the first row of the table, change the value in the Character column from 0x0D to 0x0B (where each 0 is the number zero, not the letter O). This changes the value of the segment delimiter for the incoming message.

Setting the Segment Delimiter to 0x0B

• Click OK to close the Options window.

You must change the segment delimiter to make it possible for Iguana to examine the entire message when deciding whether to generate an ACK or a NACK. Here is why this works:

• By default, Iguana only examines the first segment, MSH, when determining whether to generate an ACK or a NACK.

• To determine where the first segment ends, Iguana looks for the segment delimiter. In most HL7 messages, this is 0x0D.

• However, the segment delimiter has been set to be 0x0B. Since no 0x0B character exists in the message, Iguana does not find any segment delimiter. Therefore, Iguana uses the entire message to decide whether to generate an ACK or a NACK.

In the following section, you will create a Python module that defines the criteria to use when deciding whether to generate an ACK message or a NACK message.

	0x0B is the vertical tab character. Because this character is never contained in an HL7 message, it can be specified as the segment delimiter without any risk of causing a conflict.