System Controllers
For most HA'ers, one or more system controllers form the heart of their system. The system controller is typically at the hub of the house - often literally, in the sense that it is located in Node Zero - and co-ordinates the activities of various HA enabled devices according to user defined rules. Examples might include simulating occupancy when the house is empty, forwarding a message left on the answering machine by e-mail if the house holder is at work, or co-ordinating the audio, visual and lighting systems involved in a home cinema.
The system controller - or controllers - have to perform two basic tasks. Firstly, they have to be interfaced to the systems they control - whether X-10, e-mail, hardwired lighting systems, or telemetry units such as temperature sensors. Secondly, they have to apply house-holder defined rules to data arriving from those systems on a case-by-case basis.
The trade-off in selecting a system controller boils down to trade-offs between flexibility, reliability and cost. A dedicated PC provides excellent flexibility, and is increasingly cost effective. Since most home automation tasks do not require great horse-power, an entry level, redundant or retired PC is often up to the task. The flexibility offered by PC can come at the cost of reliability, however. The moving parts in a PC such as hard-disks are prone to mechanical failure over time, PC software is not always 100% reliable, and unexpected power failures can cause problems. It is possible to address these issues, however, and implement a stable and reliable PC based system.
If reliability is the primary concern over and above flexibility and cost, then one or more dedicated controllers offers a good solution. A dedicated controller such as Home Vision offers the reliability of an embedded controller, but at the expense of long-term expansion and with some limitations on functionality. It is unable, for example, to provide the speech synthesis and recognition features of a dedicated PC. Sooner or later I think most Home Vision owners end up with a two-tiered strategy, with Home Vision performing essential tasks with more complex activities delegated to a dedicated PC.
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