SIP generally communicates over a TCP connection (either raw sockets or over telnet), but can also communicate via serial connections and other methods. In Evergreen, the most common deployment is a RAW socket connection on port 6001.

SIP communication consists of strings of messages, each message request and response begin with a 2-digit “command” - Requests usually being an odd number and responses usually increased by 1 to be an even number. The combination numbers for the request command and response is often referred to as a Message Pair (for example, a 23 command is a request for patron status, a 24 response is a patron status, and the message pair 23/24 is patron status message pair). The table in the next section shows the message pairs and a description of them.

For clarification, the “Request” is from the device (selfcheck or otherwise) to the ILS/ACS. The response is… the response to the request ;).

Within each request and response, a number of fields (either a fixed width or separated with a | [pipe symbol] and preceded with a 2-character field identifier) are used. The fields vary between message pairs.

01<card retained><date>[fields AO, AL, AA, AC]