Control Paths

When designing a whole-house automation system it is important to have a good understanding of the paths that sensor and control signals will follow so as to make sure that nothing is missed from the intended design. Missed links will mean rewiring and most probably substantial re-work.

The term "logical control paths" refers to the conceptual route followed by your command or signal. The term "physical control path" refers to the actual route followed by the electrical signal. It goes without saying that the shorter the routes, the better.

Logical control paths

An illustration will help to visualise a typical logical control path. Say for instance, we want to follow the sequence to turn a light on (in the automation lingo, to set a scene):

1. A button is pressed on a control panel a signal travels to the main control unit.
2. The main control unit detects the user action and issues the appropriate command using the right protocol to the control subsystem (in this case a lighting module).
3. The subsystem switches the light on or alters its power (dimming).

Obviously, the example above is quite trivial, but you get the idea. Much more complex control paths are established when there are interactions between say, the security subsystem, the main control unit and other subsystems. For instance:

1. A presence sensor detects activity and notifies to the security subsystem.
2. The security subsystem is unarmed so no action is taken, but the event is notified to the main control unit.
3. The main control unit analyses the sensor event and concludes that presence is detected in a room which is set to auto-light, it therefore sends a lighting command to the appropriate subsystem (in this case a X10 controlled switch). The main control unit can also trigger other actions such as issuing a welcome greeting through the PA system, play some ambient music, reroute incoming calls to the nearest extension, log the activity or start recording from the relevant CCTV.
4. The lighting command is interpreted by the lighting subsystem and routed to the appropriate device(s): lights are turned on and curtains closed.

Logical control paths normally follow the [device]-[subsystem]-[main control unit]-[subsystem]-[device] or [panel]-[main control unit]-[subsystem]-[device] sequence. This arrangement ensures that subsystems can be operated using native commands as well as super-commands issued by the main control unit.

There are however exceptions that need to be taken into account. The most common exception is managing signals from subsystems' proprietary panels. The sequence will be something like [panel]-[subsystem]-[main control unit]-[subsystem]-[device].

Physical control paths

This term refers to the physical path followed by the electrical signal. Roughly speaking there are three routes a signal can travel: wireless, low voltage connection and/or powerline carrier (PLC).

Wireless paths require proximity and a path which is relatively free of interferences. In residential environments, interferences come from transformers, microwave devices, CRT-based TVs, and anything with an engine. Sometimes a poorly insulated CD player or computer becomes a source of interference - thus not all wireless routes are available. Various protocols have been defined but the leading ones appear to be IEEE 802.11a/b/g (there are three variants) from the computing world and Bluetooth from the mobile phone world. But there are other proprietary protocols in use.

Low voltage wired connections carry the lion share of the control signals and its associated data. It uses unshielded twisted pair (UTP) , shielded twisted pair (STP) cables or similar cables to communicate signals between devices. Protocols are mostly proprietary running on RS232, RS485 or Ethernet standards.

In recent years there has been an effort to agree on common "Building Management Standard" protocols (C-BUS, CEBUS, EIB (now referred as KNX) and LonWorks ). In practice these protocols are supported by particular vendors and thus difficult to integrate with other equipment. They support both low voltage wired and power line carrier connectivity.

Despite careful design, low voltage cabling is also sensitive to EMI and thus should be routed away from potential sources of EMI as well as mains cable (experts differ, but we recommend at least 2ft distance, 4ft best).

Power line carrier (PLC) uses the mains voltage wiring to distribute signals around the house using an agreed protocol. The most common are CEBUS, EIB/KNX, Lonworks and X10, (but there are others around). While it may sound great that you can re-use your existing mains wiring for control purposes there are limits on the number of devices that you can connect (256 for X10) and reliability - mains voltage lines are very noisy and subject to spikes and other interferences with a large percentage of lost commands. PLC signals also do not travel well between fused rings (but you can bridge circuits to allow the signal to cross but not the main power, if necessary).

'Node Zero' (aka Wiring closet)

The location of the bulk of the connections is a critical decision as it will determine the length of the wiring runs and the ease of installation and maintenance. Refer to our discussion on Wiring for further details.

Top

[Prev] | [Next]