1. External Interface Update 1 External Interface Update April 2, 2007 Daryl Shing

2. External Interface Update 2 Specification is published Is available for download from http://nodal.ercot.com. We will seek approval for the specification on April 22 nd. Includes feedback from the API Subgroup meeting. We also plan to organize all of the interface specifications to give a clear picture of how Market Participant will interact with ERCOT.

3. External Interface Update 3 Feedback from the API working group MRID & MMS keys questions –There is a 3 step validation of submissions Submitted message conforms with the message format (XSD Schema). MMS applies business rules and may reject certain submissions. The rules and timeline will be published separately by the MMS team. MMS sequentially processes the submissions against the credit limit. Submissions that cause the credit limit to be exceeded are rejected. –MRID will be generated as an optional element and not stored in EIP. –MMS supplied the following definition of market intervals Valid submission Invalid submission

4. External Interface Update 4 Feedback from the API working group Timestamps –Agreed to use ISO 8601 and timestamps would use the prevailing time in Texas rather than Zulu or other convention –Another time format will be used to reflect market hours and intervals that is suitable for use with MMS. –The specified interfaces may change once the rules from receiving systems e.g. MMS are finalized Web services will support some MIS Portlets but not all –Excludes user interface Portlets. –Some data interfaces will be JMS rather than Web Services. –The External Interface Specification is the complete list of available Web Services. Organizing all interface specifications to give Market Participants a clear picture –There are a number of separate specifications and lists of Market Information being sourced from various efforts within the Nodal program –We are working with the relevant projects to produce a single interface overview document that will refer to: The External Interface Specification MIS published content Market information reports ICCP Other interfaces such as long term wind power forecast that Wind Generators will supply to ERCOT

5. External Interface Update 5 Feedback from the API working group Incremental & Decremental energy offer curves –Version 0.9 was published on 3-29 with a single curve. –Version 0.91 published 3-30 with curves for incremental and decremental energy offer curves. This was an oversight rather than deliberate omission –Identified that both curves will be necessary from protocols. –Discussions on this part of the MMS detailed design indicated a single curve. –Integration team assumed the input on detailed design was the latest and correct view. –The integration team is using protocols and all available design input, however design documentation is incomplete and there are frequent instances of changes in source systems. –The integration team tracks these changes and responded appropriately to this event within a couple of days by amending the specification.