Central integration usually involves the use of a Communicator or similar centralised integration product. Integration takes place by means of the use of drivers or protocol converters. With the central overlay integration system, the drivers reside as software stored on the Communicator's hard disk instead of in a network node or point such as a Compass point or Commander. Connections onto the subsystems take place via an RS232 / RS485 connection, LON Works connection or some sort of an IP connection (i.e. BACnet).

Each system has a respective driver or protocol converter which converts all the subsystem protocols into the overlay system protocol on a request/response basis. Data is then transferred across the systems in the Communicator which forms the overlay network. Due to the limitations in PC hardware platforms, serial connections are usually limited up to 8 COM Ports.  However further connections can be achieved utilising more modern technologies such as IP virtual serial ports.

Full integration can take place whilst allowing subsystem manufacturers to supply their own systems as they see best.

Viewing the system
With a centrally integrated overlay system the graphical user interface can be provided in several different ways.  With internet technologies becoming more and more reliable there is a trend to provide the graphical user interface via a web server rather than a fixed function application.  Providing a web server as the user interface has several advantages over a fixed stand-alone top end, the main advantage being that any PC with a web browser (Internet Explorer) that is connected to the same network as the web serving device can access the graphics, data logs and alarms.  A fixed client application is also available on the web based system this allows a PC station to be turned into a dedicated alarm receiving station.  Add this functionality to a broadband connection and this allows the graphics system, data logs and alarms to be accessed from anywhere in the world.

Advantages

• Subsystems created in commercial and intellectual safety which lend themselves well to their particular application

• Any loss of the CPU will still allow the subsystems to continue operation

• Development and use of experience in a certain field, not promoting inexperienced engineers crossing over into other disciplines

• No restrictions on development and creativity, and also flexibility for the subsystem suppliers in not having to conform to any particular standard

• An overlay system would be developed to handle integrated traffic, not all-systems traffic

Disadvantages

• Single point of failure, as with subsystems

• More cable installations with multi-network installations, usually one per system

• Additional cabling requirements to connect all the systems into one location, ie: the Communicator

• No interchangeability of devices either on the same type of system or between different systems

• To enable integration to take place, the protocols need to be supplied to the overlay system provider, together with hardware, to enable driver development. This can sometimes be difficult to arrange with certain subsystems providers.