**5. Origins and features of the standard elevator information schema**

#### **5.1 Origins**

The origins of SEIS can be traced back to a refurbishment project in 1980 of several groups of lifts in a classical (11 storey) building in central London – Bush House. The traffic demand was non-standard and continued throughout 24 hours of every day of the week. At the time there were virtually no computer controlled lifts anywhere in the world, and this provided the opportunity to design computer software from first principles for both individual car and group supervisory controllers.

#### *5.1.1 Interface abstraction*

Working from the available published literature [34], the Bush House central controller design was developed around an abstract model of the primary elements of information that are essential to the control of a group of lifts. Each control element is linked to its external environment – electrical sensors, relays, indicators and other control computers – through an interface software layer which communicates via very short (less than 6 characters) textual event messages. Even delay timers are implemented as messages that the controller sends to itself, which are held for a specified period before being delivered. This architecture has a number of significant advantages:


The great benefit of the interface abstraction layer is that the controller is not predicated on a specific configuration of equipment nor a set of electrical connections to the hardware of the lifts. The design made it very straight forward to port the controller to a different building where components from other lift manufacturers might be used. An example of this abstraction is the use of a 64-bit absolute position encoder providing sub-millimetre resolution of the lift position in the shaft. From the point of view of the lift drive control, it is important to know very accurately the position of landing levels, slow-down points, etc., the thousands of positions in between however, are ignored. On the other hand, from the perspective of the logical control element of the lift, it is most important to know the floor position of the lift, including when the lift is travelling between floors and taking into account the direction of travel when a slow-down point is reached – hence the 'Next Possible Stopping Floor' of the lift was defined (now <CarDynamicData><Floor> in SEIS).
