The central interface (CI) is the core program that handles the communication between the subsystems, and maintains a global database for the current design and a history of previous designs.
The CI is the implementation of the communication protocols described in Section 3.2. Some features and enhancement to the protocols have been added to the CI. For example, when the CI receives a change message from an SSI, it directly sends lock messages to the other subsystems so that no more changes can be sent from any SSI until they receive a steady message. This solves the concurrency problem if more than one system send changes to the CI at the same time. The first message received by the CI will be handled and the others will be ignored. If an SSI receives a lock message after it sent a change message, that means its message was ignored. Another feature added to the CI is the ability to detect if an SSI is working or not by tracing the SSI_Start and SSI_Stop messages.