PLC functions are programmed codes that define a connection between an input and a corresponding output.

Programmable Logic Controllers - Functions

Eugene Kowch | P.I.D. Consultants Inc.

Programmable Logic Controller
---- Functions ----

By Eugene Kowch, P.I.D. Consultants Inc.

PLC functions are programmed codes that define a connection between an input and a corresponding output.


For programmable logic controllers (PLCs) to be useful in a home automation application they need to be programmed with various functions. PLC functions are programmed codes that define a connection between an input and a corresponding output. Functions discussed in this article are;

  1. Toggle
  2. Pulse
  3. All On/All Off
  4. Select
  5. Time
  6. Security

One very practical function is the toggle function. To toggle means to switch between two options. The toggle function is very useful when control of a device is required from multiple locations. Wiring a momentary button to a PLC input, and applying a toggle function, will turn a light 'on' if it is 'off' and will also turn a light 'off' if it is 'on'. Wiring a second momentary button to a different PLC input and applying the same toggle function will emulate a typical electrical 'three-way' switch, but without the nonsense of the 'three-way' switch being in an 'off' position when the light is 'on'. Momentary buttons do not have 'on' or 'off' positions.

Pulse functions are required whenever motors are involved. Windows in hard to reach places may have motors installed to open and close the windows. Wiring one momentary button to a PLC input and applying an 'open' pulse function will open the window. Wiring a second momentary button to a PLC and applying a 'close' pulse function will close the window. The duration of the pulse is set in the PLC to match the desired timing of the window controller, so no matter how long the momentary button is held closed, the pulse signal to the window controller is a constant.

The all on/all off function is practically self-explanatory. This function allows the user to turn 'on' or 'off' a group of outputs from one input. Landscape lights are a good example. There will probably be individual momentary buttons using the toggle function for every lighting circuit in the back and front yards. With the all on/all off function, wiring one momentary button to a PLC allows for all the landscape lights to be turned 'on' or 'off' with only one button.

The select function allows the user to combine other functions by using a two-button control sequence. First, a series of maintained switches are assigned to be the select buttons. For example, let's say there are four motor controlled drapes in a room. Four maintained switches will be assigned as select buttons: Blackout1, Sheer1, Blackout2, and Sheer2. Second, two momentary buttons are assigned as 'open' and 'close'. Wiring the four maintained switches and two momentary buttons to PLC inputs and applying the select function allows the home owner to first select one drape type and then either 'open' or 'close' the drape with the two momentary buttons (using the pulse function). By using the select function for the above example, we only need six buttons instead of eight buttons.

The time function is required when lights need to be turned on at a specified time, like sunset for example. This function does not require a button to be wired to the PLC, but a time is entered when lights are to be turned 'on' and when the lights are to be turned 'off'. Also, the time function should have an astronomical time clock to allow for changes in sunset and sunrise times.

The security function is a very nice feature that integrates security systems and lighting in a home. An output from a security panel, which is programmed to activate when the security system is set to armed and away, is wired to an input on the PLC. Applying the security function, lights in the home will come 'on' and 'off' automatically at various times at night to give the home a lived in look. When the security system is disarmed, the input to the PLC is deactivated and control of the lights is returned to the homeowner.

The PLC installed in a home and programmed with the above functions, gives the homeowner added control and flexibility over conventional electrical switching installations.


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