ROANOKE TIMES
Copyright (c) 1996, Roanoke Times
DATE: Tuesday, October 22, 1996 TAG: 9610220040
SECTION: EXTRA PAGE: 3 EDITION: METRO
COLUMN: at home with technology
SOURCE: DAVID BUTLER
Q: I'd like to learn more about the power line carrier products you often write about in your column. Can you explain how PLC works and describe some of the things it can do?
A: Power line carrier is a signaling technique supported by virtually all home control systems. Because it operates over standard household wiring, PLC offers a convenient way to control lights and other electrical loads without having to install dedicated wiring.
A PLC system consists of one or more controllers and one or more switch modules. Controllers usually take the form of a push-button console, a simple timer or a programmable computer interface. Plug-in switch modules can be used for controlling lamps and appliances. Replacement light switches and switched receptacles are also available.
Here's how PLC works: The controller inserts a high-frequency signal on the power line via its own electrical cord. This signal carries two pieces of information - a command (on, off, dim/brighten) and a module address. The address is further broken down into house code (A through P) and unit code (1 through 16).
This address uniquely identifies the intended module - a lamp in the den might be A4. When a carrier signal appears on the power line, the appropriate module recognizes its address and responds to the command. The entire process takes less than a half-second.
PLC systems are popular because they're both inexpensive - modules start at about $12 - and easy to set up. Each module has a pair of rotary address selectors: one for unit code; one for house code. Whenever you add a module, all you have to do is set the dials to the desired address. Controllers have only a house code selector. The unit code follows from which button you press.
The purpose of the house code is to prevent nearby PLC systems from interfering with each other. If your neighbor's PLC system turns on your lights, simply adopt a different house code. Interference only occurs between houses fed by the same utility transformer, and even then, only if both houses are relatively close to the transformer.
Most PLC controllers can address as many as sixteen modules. Four- and eight-button controllers have a range switch that effectively doubles the number of unit codes they can address. Sixteen-button panels are also available. If you need more than sixteen addresses, you can use two or more controllers, each set to a different house code.
One of the most popular controllers is the PLC timer. Its digital clock can be programmed to issue commands at a specified time. This is referred to as event scheduling. The timer can store four daily events (on or off) for each of four modules. In addition, it includes four manual control buttons.
Many special-purpose modules and controllers are available, greatly expanding the power and flexibility of PLC-based home control. One miniature control console has a built-in light sensor that can be set up to turn selected lights on at dusk and off at dawn. You can also get a PLC-compatible garage door opener. It automatically turns on indoor or outdoor lights as you raise the door.
No home control system would be complete without a hand-held remote. A top consumer electronics manufacturer recently introduced a universal remote capable of controlling lights and appliances via PLC. It has two modes - infrared for operating A/V gear and radio frequency (RF) for whole-house control. It comes with a plug-in RF-to-PLC signal converter.
To receive a list of manufacturers as well as retail and mail order outlets for power line carrier components, please send $2 and a self-addressed envelope to David Butler F-624, Department TWN, P.O. Box 36352, Cincinnati, Ohio 45236-0352. The list includes toll-free phone numbers and a brief description of the most popular controllers and modules.
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PLC also works with low-voltage loads such as sprinkler valves and thermostats. The 'universal' module features a dry contact relay output (momentary or latched operation). Four- and eight-channel relay modules are available for more demanding projects.
Send questions or comments to Butler at 14713 Pleasant Hill Road, Charlotte, N.C. 28278.
LENGTH: Medium: 81 lines ILLUSTRATION: GRAPHIC: A PLC signal includes a command along with the addressby CNB(house code and unit code) of the intended module.