The approach to implementing Scenarios is guided by two general factors: this facility needs to connect to the control system; we have limited time and personnel to work on this facility. To get something done and at the same time allow for future enhancement we will use both top-down and bottom-up approaches.
A special class of ACNET devices, called "States Devices", provide the hooks into the control system, and a place where top-down and bottom-up design and implementation can meet. Think of the collection of States Devices like a large multi-dimensional honeycomb: each device is a cell in this honeycomb. The States front-end (which owns the States Devices) provides a method for viewing the devices (cells). The loading of the devices and their organization is imposed from outside the States front-end.
The States devices, front-end, and some of the methods for setting the devices are bottom-up components. This still allows, and invites, ways to organize the devices. This will happen in two main ways: the consumers (users) of the information contained in a set of devices will impose an organization; and a model (or several models) will impose some structure to sets of devices. These organizations or structures can be of different natures depending on what is needed in each context of their use. Some devices will be simply used as tags; others as enumeration; still others could be used to represent states of a finite-state-machine. Some devices may be used in several different ways, depending on the need of the user involved.
Read on for definitions of some of the key components of the Scenarios facility.
