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In short, it depends on whether this element of output load device is activated by a positive supply pole or a negative supply pole. Still, most sensors are of a switch type. One such example where outputs with normally closed contacts are used is a safety wall with trimming machines. This system could work, but there would be some problems with this, too. Industry has to number its products, determine a needed action in time, etc.

Humans achieve full activity only with processing of information from a sensor eyes, ears, touch, smell and by taking action through hands, legs or some tools. Good examples for both situations are the doorbell and a house alarm.

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Let's suppose that alarm system is intended for surveillance of the front door to the house. Sensor outputs can be different depending on a sensor itself and also on a particular application. These considerations are even more important in industrial environment where a failure could cause injury at work. Basic elements of a relay diagram can be seen in the following picture.

Conditions which lead to instructions positioned at the right edge of a diagram are stored along instruction lines. Second input is a reset input. If it is defined as an input with normally open contact, pushing a key will set off an instruction found after the condition. Based on a program and based on the status of that input as obtained in the preceding step, an appropriate action is taken.

Let's suppose that switch is not working, that a wire is somehow disconnected, or a switch is broken, etc. Typically, timers have two inputs.

When product B appears, a variable can change value to what is required by product B. In order to function it needs sensors to obtain information from environment, and it also needs execution devices so it could turn the programmed change into a reality. Operand can be some memory location, one memory location bit, gate exam preparation materials or some numeric value -number.

If a variable is used, timer will use a real time value of the variable to determine a delay. Vertical line pairs are called conditions. If a key is attached to a corresponding screw terminal, you can change bit status from a logic one status to a logic zero status, and vice versa.

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Under scanning we mean running through all conditions within a guaranteed period. Example is a system that has produced two different products, each requiring different timing during process itself. Logical combination of these conditions determines when and in what way instruction on the right will execute.

First is timer enable, or conditional input when this input is activated, timer will start counting. For instance, one type of sensors can be used to detect presence of a box on an industry transfer belt. Example of a simple instruction is activation of some bit in memory location.

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System should be set up in such a way so the alarm is activated by a burglar, but also by its own dysfunction, or if any of the components stopped working. Concepts normally open and normally closed can apply to sensors as well. Then, if the door was opened, this would close the switch, and an alarm would be activated. When a condition is fulfilled, instruction is executed, and that's all!


Value set in advance or ahead of time is a number of increments that timer has to calculate before it changes the output status. Values set in advance can be constants or variables. If a normally closed switch is selected, bell will work continually until someone pushes the switch. Input where a key is connected can be defined as an input with open or closed contacts. Example of a delay-off timer would be staircase lighting.

Having a brain is simply not enough. Right section of a ladder diagram is an instruction which is executed if left condition is fulfilled. Instead, we use a system of connecting several inputs to one return line as in the following picture. Example can be connection of external device such as proximity sensor.

Let's take for instance a sensor made to feel metal when a metal object passes by the sensor. This reaction can be defined as activation of a certain output, or results can be put off and stored in memory to be retrieved later in the following step.

Both terms apply to words such as contacts, input, output, etc. Single box type generally applied for small fairish controller of which can be programmed and marketed in the form of inwrought packaging, complete with energy?

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This way bell wouldn't work until someone pushed the switch button and thus informed of his or her presence at the entrance. There are two types of timers delay-off and delay-on.