Timers and counters are indispensable in PLC programming. Industry has to number its products, determine a needed action in time, etc. Timing functions is very important, and cycle periods are critical in many processes.

There are two types of timers delay-off and delay-on. First is late with turn off and the other runs late in turning on in relation to a signal that activated timers. Example of a delay-off timer would be staircase lighting. Following its activation, it simply turns off after few minutes.

Each timer has a time basis, or more precisely has several timer basis. Typical values are: 1 second, 0.1 second, and 0,01 second. If programmer has entered .1 as time basis and 50 as a number for delay increase, timer will have a delay of 5 seconds (50 x 0.1 second = 5 seconds).

Timers also have to have value SV set in advance. 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 variable is used, timer will use a real time value of the variable to determine a delay. This enables delays to vary depending on the conditions during function. Example is a system that has produced two different products, each requiring different timing during process itself. Product A requires a period of 10 seconds, so number 10 would be assigned to the variable. When product B appears, a variable can change value to what is required by product B.

Typically, timers have two inputs. First is timer enable, or conditional input (when this input is activated, timer will start counting). Second input is a reset input. This input has to be in OFF status in order for a timer to be active, or the whole function would be repeated over again. Some PLC models require this input to be low for a timer to be active, other makers require high status (all of them function in the same way basically). However, if reset line changes status, timer erases accumulated value. It can measure from 0 to 999.9 seconds with precision of 0.1 seconds more or less.

author: Nebojsa Matic

Introduction 
Connecting external devices to a PLC controller regardless whether they are input or output is a special subject matter for industry. If it stands alone, PLC controller itself is nothing. 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. Similar concept is seen in how human being functions. Having a brain is simply not enough. 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. Unlike human being who receives his sensors automatically, when dealing with controllers, sensors have to be subsequently connected to a PLC. How to connect input and output parts is the topic of this chapter.

Sinking-Sourcing Concept

PLC has input and output lines through which it is connected to a system it directs. Input can be keys, switches, sensors while outputs are led to different devices from simple signalization lights to complex communication modules.

This is a very important part of the story about PLC controllers because it directly influences what can be connected and how it can be connected to controller inputs or outputs. Two terms most frequently mentioned when discussing connections to inputs or outputs are "sinking" and "sourcing". These two concepts are very important in connecting a PLC correctly with external environment. The most brief definition of these two concepts would be:

SINKING = Common GND line (-)
SOURCING = Common VCC line (+)

First thing that catches one's eye are "+" and "-" supply, DC supply. Inputs and outputs which are either sinking or sourcing can conduct electricity only in one direction, so they are only supplied with direct current. According to what we've said thus far, each input or output has its own return line, so 5 inputs would need 10 screw terminals on PLC controller housing. Instead, we use a system of connecting several inputs to one return line as in the following picture. These common lines are usually marked "COMM" on the PLC controller housing.

 

Input lines
Explanation of PLC controller input and output lines has up to now been given only theoretically. In order to apply this knowledge, we need to make it a little more specific. Example can be connection of external device such as proximity sensor. Sensor outputs can be different depending on a sensor itself and also on a particular application. Following pictures display some examples of sensor outputs and their connection with a PLC controller. Sensor output actually marks the size of a signal given by a sensor at its output when this sensor is active. In one case this is +V (supply voltage, usually 12 or 24V) and in other case a GND (0V). Another thing worth mentioning is that sinking-sourcing and sourcing - sinking pairing is always used, and not sourcing-sourcing or sinking-sinking pairing.

If we were to make type of connection more specific, we'd get combinations as in following pictures (for more specific connection schemas we need to know the exact sensor model and a PLC controller model).

Output lines 

PLC controller output lines usually can be:
-transistors in PNP connection
-transistors in NPN connection
-relays

The following two pictures display a realistic way how a PLC manages external devices. It ought to be noted that a main difference between these two pictures is a position of "output load device". By "output load device" we mean some relay, signalization light or similar.

How something is connected with a PLC output depends on the element being connected. In short, it depends on whether this element of output load device is activated by a positive supply pole or a negative supply pole.

author: Nebojsa Matic

There are two mechanic design types used occasionally for system-sistem PLC : Single box Type , and Type Modular and Rak. 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?power allowance, processor, memory, and units input/output. See picture under

Usually, a kind of this PLC type can have 40 point of input/output and an memory unit which storage can around 300 to 1000 instructions.

Modular type consisted of modules that is separate, his(its each to ration energy?power, processor, other modules, often stuck into at rail lines in a base. Rack Type can be utilized for all controller measure program and has multifarious tidy functional unit for independent modules of which can be stuck into socket-socket at a base is in the form of Rak. See picture under ;

Combination of Module needed to a the application of certain determined by user and modules selected then stuck at rack. Because of item relative easy for we to add number of extensions input/output with simply sticking additional modules or enlarge memory measure with simply sticking addition memory units.

Programs packed into one of memory PLC by using a programming peripheral generally is not jointed permanently to PLC and movable out of one controllers to other controllers without disturbing operations is being implemented. Oparation to PLC , programming peripheral is not necessarily be jointed to PLC because this peripheral will only remove program which we are create to memory PLC.

Supplier PLC : Mitsubishi, Keyence, Omron, Siemens, Atos, Festo, Honeywell, Schneider Electric, Allen Bradley, IDEC, ABB, Rockwell Automation, General Electric, and etc.

Basis of a PLC function is continual scanning of a program. Under scanning we mean running through all conditions within a guaranteed period. Scanning process has three basic steps:

Step 1.
Testing input status. First, a PLC checks each of the inputs with intention to see which one of them has status ON or OFF. In other words, it checks whether a sensor, or a switch etc. connected with an input is activated or not. Information that processor thus obtains through this step is stored in memory in order to be used in the following step.

Step 2. 
Program execution. Here a PLC executes a program, instruction by instruction. Based on a program and based on the status of that input as obtained in the preceding step, an appropriate action is taken. 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.

Step 3. 
Checkup and correction of output status. Finally, a PLC checks up output status and adjusts it as needed. Change is performed based on the input status that had been read during the first step, and based on the results of program execution in step two. Following the execution of step 3 PLC returns to the beginning of this cycle and continually repeats these steps. Scanning time is defined by the time needed to perform these three steps, and sometimes it is an important program feature.