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Connecting sensors and execution devices Programmable Logic Controllers ( PLC )

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 PLCmanages 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

resource : plc-pograming.blogspot.com

PLC Leader Diagram

PLC Leader Diagram

Introduction Programmable controllers are generally programmed in ladder diagram (or "relay diagram") which is nothing but a symbolic representation of electric circuits. Symbols were selected that actually looked similar to schematic symbols of electric devices, and this has made it much easier for electricians to switch to programming PLC controllers. Electrician who has never seen a PLC can understand a ladder diagram. Ladder diagram There are several languages designed for user communication with a PLC, among which ladder diagram is the most popular. Ladder diagram consists of one vertical line found on the left hand side, and lines which branch off to the right. Line on the left is called a "bus bar", and lines that branch off to the right are instruction lines. Conditions which lead to instructions positioned at the right edge of a diagram are stored along instruction lines. Logical combination of these conditions determines when and in what way instruction on the right will execute. Basic elements of a relay diagram can be seen in the following picture. 

Most instructions require at least one operand, and often more than one. Operand can be some memory location, one memory location bit, or some numeric value -number.In a case when we wish to proclaim a constant as an operand, designation # is used beneath the numeric writing (for a compiler to know it is a constant and not an address.) Based on the picture above, one should note that a ladder diagram consists of two basic parts: left section also called conditional, and a right section which has instructions. When a condition is fulfilled, instruction is executed, and that's all! 

Picture above represents a example of a ladder diagram where relay is activated in PLC controller when signal appears at input line 0. Vertical line pairs are called conditions. Each condition in a ladder diagram has a value ON or OFF, depending on a bit status assigned to it. In this case, this bit is also physically present as an input line (screw terminal) to a PLC controller. 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. Status of logic one is usually designated as "ON", and status of logic zero as "OFF". 

Right section of a ladder diagram is an instruction which is executed if left condition is fulfilled. There are several types of instructions that could easily be divided into simple and complex. Example of a simple instruction is activation of some bit in memory location. In the example above, this bit has physical connotation because it is connected with a relay inside a PLC controller. When a CPU activates one of the leading four bits 10, relay contacts move and connect lines attached to it. In this case, these are the lines connected to a screw terminal marked as 0 and to one of COM screw terminals. Normally open and normally closed contacts Since we frequently meet with concepts "normally open" and "normally closed" in industrial environment, it's important to know them. Both terms apply to words such as contacts, input, output, etc. (all combinations have the same meaning whether we are talking about input, output, contact or something else). Principle is quite simple, normally open switch won't conduct electricity until it is pressed down, and normally closed switch will conduct electricity until it is pressed. Good examples for both situations are the doorbell and a house alarm. If a normally closed switch is selected, bell will work continually until someone pushes the switch. By pushing a switch, contacts are opened and the flow of electricity towards the bell is interrupted. Of course, system so designed would not in any case suit the owner of the house. A better choice would certainly be a normally open switch. This way bell wouldn't work until someone pushed the switch button and thus informed of his or her presence at the entrance. Home alarm system is an example of an application of a normally closed switch. Let's suppose that alarm system is intended for surveillance of the front door to the house. One of the ways to "wire" the house would be to install a normally open switch from each door to the alarm itself (precisely as with a bell switch). Then, if the door was opened, this would close the switch, and an alarm would be activated. This system could work, but there would be some problems with this, too. Let's suppose that switch is not working, that a wire is somehow disconnected, or a switch is broken, etc. (there are many ways in which this system could become dysfunctional). The real trouble is that a homeowner would not know that a system was out of order. A burglar could open the door, a switch would not work, and the alarm would not be activated. Obviously, this isn't a good way to set up this system. 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. (A homeowner would certainly want to know if a system was dysfunctional). Having these things in mind, it is far better to use a switch with normally closed contacts which will detect an unauthorized entrance (opened door interrupts the flow of electricity, and this signal is used to activate a sound signal), or a failure on the system such as a disconnected wire. These considerations are even more important in industrial environment where a failure could cause injury at work. One such example where outputs with normally closed contacts are used is a safety wall with trimming machines. If the wall doors open, switch affects the output with normally closed contacts and interrupts a supply circuit. This stops the machine and prevents an injury. Concepts normally open and normally closed can apply to sensors as well. Sensors are used to sense the presence of physical objects, measure some dimension or some amount. For instance, one type of sensors can be used to detect presence of a box on an industry transfer belt. Other types can be used to measure physical dimensions such as heat, etc. Still, most sensors are of a switch type. Their output is in status ON or OFF depending on what the sensor "feels". Let's take for instance a sensor made to feel metal when a metal object passes by the sensor. For this purpose, a sensor with a normally open or a normally closed contact at the output could be used. If it were necessary to inform a PLC each time an object passed by the sensor, a sensor with a normally open output should be selected. Sensor output would set off only if a metal object were placed right before the sensor. A sensor would turn off after the object has passed. PLC could then calculate how many times a normally open contact was set off at the sensor output, and would thus know how many metal objects passed by the sensor. Concepts normally open and normally closed contact ought to be clarified and explained in detail in the example of a PLCcontroller input and output. The easiest way to explain them is in the example of a relay. 

Normally open contacts would represent relay contacts that would perform a connection upon receipt of a signal. Unlike open contacts, with normally closed contacts signal will interrupt a contact, or turn a relay off. Previous picture shows what this looks like in practice. First two relays are defined as normally open , and the other two as normally closed. All relays react to a signal! First relay (10) has a signal and closes its contacts. Second relay (11) does not have a signal and remains opened. Third relay (12) has a signal and opens its contacts considering it is defined as a closed contact. Fourth relay (13) does not have a signal and remains closed because it is so defined. 

Concepts "normally open" and "normally closed" can also refer to inputs of a PLC controller. Let's use a key as an example of an input to a PLC controller. Input where a key is connected can be defined as an input with open or closed contacts. If it is defined as an input with normally open contact, pushing a key will set off an instruction found after the condition. In this case it will be an activation of a relay 0. 

If input is defined as an input with normally closed contact, pushing the key will interrupt instruction found after the condition. In this case, this will cause deactivation of relay 0 (relay is active until the key is pressed). You can see in picture below how keys are connected, and view the relay diagrams in both cases. 

Normally open/closed conditions differ in a ladder diagram by a diagonal line across a symbol. What determines an execution condition for instruction is a bit status marked beneath each condition on instruction line. Normally open condition is ON if its operand bit has ON status, or its status is OFF if that is the status of its operand bit. Normally closed condition is ON when its operand bit is OFF, or it has OFF status when the status of its operand bit is ON. 

When programming with a ladder diagram, logical combination of ON and OFF conditions set before the instruction determines the eventual condition under which the instruction will be, or will not be executed. This condition, which can have only ON or OFF values is called instruction execution condition. Operand assigned to any instruction in a relay diagram can be any bit. This means that conditions in a relay diagram can be determined by a status of I/O bits, operational bits, timers/counters, etc. 

Resource : program-plc.blogspot.com

Concept of Program Logic Controller

A PLC consits of a Center Processing Unit (CPU) containing an application program and Input and out put interface modules, with is directly connectedto the field I/O devices. The program controls the PLC so that when an input signal from an input device turns ON, the appropriate response is made. The response normally involves turning ON an out put signal to some sort of output devices.

Center Processing Unit

The central Processing Unit (CPU) is a microprocessor that co-ordinates the activities of the PLC system. It executes the program, processes I/O signal and communicatea with external devices.

Memory

The are various types of mamory unit. It is the area that hold the operating system and user memory. The operating system is actually a system software that co-ofdinates the PLC. Ladder program, timer and counter Values are stored in the user memory, Depending on user’s need various types of memory are available for choice.

1. Read-only Memory (ROM)

ROM is non – volatile memory that can be programmed only once. It is there fore unsuitable. It is least popular as compared with others memory type.

2. Random Access Memory (RAM)

RAM is commonly used memory type for storing the user program and data. The data in the volatile RAM would normally be lost if the power source is removed. Hoever, this problem is solved by backing up the RAM with a battery.

3. Erasable Programmable Read Only Memory (EPROM)

EPROM holds data permanently just like ROM. It does not require battery backup. However, its content can be eased by exposing it to ultraviolet light. A prom writer is required to reprogram the  memory.

4. Electrically Erasable Programmable Read Only Memory (EEPROM)

EEPROM combines the access flexibility of RAM and the non-volatility of EPROM in one. Its contents can be eased and reprogrammed electrically, however, to a limit number of times.

Scan time

The process of reading the inputs, executing the program and updating the outputs is known as scan. The scan time is normally a continous and sequential process of reading the status of inputs, evaluating the control logic and updating outputs. Scan time specification indicates how fast the controller can react to the field inputs and correctly solve the control logic.

Factors fluencing Scan Time

The time required to make a single scan ( scan time) varies from 0,1ms to tens of ms depending on its CPU processing speed and the length of the user program. The user of remote I/O subsystems increase the scan time as a result of having to transmit the I/O updates to remote subsystem. Monitoring of the control program also adds overhead time to the san because the controller’s CPU has to send the status of coils and contacts to the CRT or other monitoring device.

The Role of the PLC

In autosystem, the PLC is commonly regarede as the heart of the control system. With a control application program ( store within the PLC mem.) in execution, the PLC consutaly monitors the state of the system through the field input devices feedback signal. It will the base on the program logic to determine the course of action to be carried out at the field output devices.

The PLC may be use to control a simple and repetitive task, or a few of then may be interconnected together with other host controllers or host controllers of host computers through a sort of communication network, in order to intergrate the control of a complete process.

Input devices

Interlligence of an automated system is greatly depending on the ability of a PLC to read in the sign from varius type of automatic sensing and manual input fiel devices

Keypad, push button and togge switches, witch form the basic ma-machine unterface, are types of manual in put device. On the other hand, for detection of workpice, monitoring of moving mechanism, checking on pressure and or liquid level and many others, the PLC will have to tap the signal from the specific automatic sensing devices like proximity switch, limit switch, level sensor, photo sensor and …Type of input signal to the PLC would be of ON/OFF logic or analogue. These input signals are interfaced to PLC through various types of PLC input module.

Output device

An automatic system is incomplete and the PLC system is virtually paralysed without means of interface to the field output devices Some of the most commonly controlled devices are motor, solenolds, relay indicator, buzzers and etc. Through activation of motors and solenoids the PLC can control from a simple pick and place system to a much complex servo positioning system. These type of output devices are the mechanism of an automated system and so its direct effect on the system performance.

However some output devices is only alarm and notting for the man, which is pilot lamp,buzzes. Like input signal interface, signal from output devices are interfaces to the PLC through the wide range of the PLC output module.

Out put module

Output Modules

As with input modules, output modules rarely supply any power, but instead act as

switches. External power supplies are connected to the output card and the card will

switch the power on or off for each output. Typical output voltages are listed below, and

roughly ordered by popularity.

120 Vac

24 Vdc

12-48 Vac

12-48 Vdc

5Vdc (TTL)

230 Vac

These cards typically have 8 to 16 outputs of the same type and can be purchased

with different current ratings. A common choice when purchasing output cards is relays,

transistors or triacs. Relays are the most flexible output devices. They are capable of

switching both AC and DC outputs. But, they are slower (about 10ms switching is typical),

they are bulkier, they cost more, and they will wear out after millions of cycles. Relay

outputs are often called dry contacts. Transistors are limited to DC outputs, and Triacs are

limited to AC outputs. Transistor and triac outputs are called switched outputs.

- Dry contacts - a separate relay is dedicated to each output. This allows mixed

voltages (AC or DC and voltage levels up to the maximum), as well as isolated

outputs to protect other outputs and the PLC. Response times are often greater

than 10ms. This method is the least sensitive to voltage variations and spikes.

- Switched outputs - a voltage is supplied to the PLC card, and the card switches it

to different outputs using solid state circuitry (transistors, triacs, etc.) Triacs are

well suited to AC devices requiring less than 1A. Transistor outputs use NPN or

PNP transistors up to 1A typically. Their response time is well under 1ms.

WARNING - ALWAYS CHECK RATED VOLTAGES AND CURRENTS FOR PLC’s

AND NEVER EXCEED!

Caution is required when building a system with both AC and DC outputs. If AC is

accidentally connected to a DC transistor output it will only be on for the positive half of

the cycle, and appear to be working with a diminished voltage. If DC is connected to an

AC triac output it will turn on and appear to work, but you will not be able to turn it off

without turning off the entire PLC.

ASIDE: A transistor is a semiconductor based device that can act as an adjustable valve.

When switched off it will block current flow in both directions.While switched on it

will allow current flow in one direction only. There is normally a loss of a couple of

volts across the transistor. A triac is like two SCRs (or imagine transistors) connected

together so that current can flow in both directions, which is good for AC current.

One major difference for a triac is that if it has been switched on so that current flows,

and then switched off, it will not turn off until the current stops flowing. This is fine

with AC current because the current stops and reverses every 1/2 cycle, but this does

not happen with DC current, and so the triac will remain on

A major issue with outputs is mixed power sources. It is good practice to isolate all

power supplies and keep their commons separate, but this is not always feasible. Some

output modules, such as relays, allow each output to have its own common. Other output

cards require that multiple, or all, outputs on each card share the same common. Each output

card will be isolated from the rest, so each common will have to be connected. It is

common for beginners to only connect the common to one card, and forget the other cards

then only one card seems to work!

The output card shown in Figure 3.5 is an example of a 24Vdc output card that has

a shared common. This type of output card would typically use transistors for the outputs.

In this example the outputs are connected to a low current light bulb (lamp) and a

relay coil. Consider the circuit through the lamp, starting at the 24Vdc supply. When the

output 07 is on, current can flow in 07 to the COM, thus completing the circuit, and allowing

the light to turn on. If the output is off the current cannot flow, and the light will not

turn on. The output 03 for the relay is connected in a similar way. When the output 03 is

on, current will flow through the relay coil to close the contacts and supply 120Vac to the

motor. Ladder logic for the outputs is shown in the bottom right of the figure. The notation

is for an Allen Bradley PLC-5. The value at the top left of the outputs, O:012, indicates

that the card is an output card, in rack 01, in slot 2 of the rack. To the bottom right of the

outputs is the output number on the card 03 or 07. This card could have many different

A major issue with outputs is mixed power sources. It is good practice to isolate all

power supplies and keep their commons separate, but this is not always feasible. Some

output modules, such as relays, allow each output to have its own common. Other output

cards require that multiple, or all, outputs on each card share the same common. Each output

card will be isolated from the rest, so each common will have to be connected. It is

common for beginners to only connect the common to one card, and forget the other cards

- then only one card seems to work!

The output card shown in Figure 3.5 is an example of a 24Vdc output card that has

a shared common. This type of output card would typically use transistors for the outputs.

voltages applied from different sources, but all the power supplies would need a single

shared common.

The circuits in Figure 3.6 had the sequence of power supply, then device, then PLC

card, then power supply. This requires that the output card have a common. Some output

schemes reverse the device and PLC card, thereby replacing the common with a voltage

input. The example in Figure 3.5 is repeated in Figure 3.6 for a voltage supply card.

In this example the positive terminal of the 24Vdc supply is connected to the out-

put card directly.When an output is on power will be supplied to that output. For example,

if output 07 is on then the supply voltage will be output to the lamp. Current will flow

through the lamp and back to the common on the power supply. The operation is very similar

for the relay switching the motor. Notice that the ladder logic (shown in the bottom

right of the figure) is identical to that in Figure 3.5. With this type of output card only one

power supply can be used.

We can also use relay outputs to switch the outputs. The example shown in Figure

3.5 and Figure 3.6 is repeated yet again in Figure 3.7 for relay output.

In this example the 24Vdc supply is connected directly to both relays (note that

this requires 2 connections now, whereas the previous example only required one.) When

an output is activated the output switches on and power is delivered to the output devices.

This layout is more similar to Figure 3.6 with the outputs supplying voltage, but the relays

could also be used to connect outputs to grounds, as in Figure 3.5. When using relay outputs

it is possible to have each output isolated from the next. A relay output card could

have AC and DC outputs beside each other.

(resource : on internet Hugh Jack's book)

Input module

Inputs

In smaller PLCs the inputs are normally built in and are specified when purchasing

the PLC. For larger PLCs the inputs are purchased as modules, or cards, with 8 or 16

inputs of the same type on each card. For discussion purposes we will discuss all inputs as

if they have been purchased as cards. The list below shows typical ranges for input voltages,

and is roughly in order of popularity.

PLC input cards rarely supply power, this means that an external power supply is

needed to supply power for the inputs and sensors. The example in Figure 3.2 shows how

to connect an AC input card.

Note: inputs are normally high impedance. This means that they will

use very little current.

In the example there are two inputs, one is a normally open push button, and the

second is a temperature switch, or thermal relay. (NOTE: These symbols are standard and

will be discussed in chapter 24.) Both of the switches are powered by the hot output of the

24Vac power supply - this is like the positive terminal on a DC supply. Power is supplied

to the left side of both of the switches. When the switches are open there is no voltage

passed to the input card. If either of the switches are closed power will be supplied to the

input card. In this case inputs 1 and 3 are used - notice that the inputs start at 0. The input

card compares these voltages to the common. If the input voltage is within a given tolerance

range the inputs will switch on. Ladder logic is shown in the figure for the inputs.

Here it uses Allen Bradley notation for PLC-5 racks. At the top is the location of the input

card I:013 which indicates that the card is an Input card in rack 01 in slot 3. The input

number on the card is shown below the contact as 01 and 03.

Many beginners become confused about where connections are needed in the circuit

above. The key word to remember is circuit, which means that there is a full loop that

the voltage must be able to follow. In Figure 3.2 we can start following the circuit (loop) at

the power supply. The path goes through the switches, through the input card, and back to

the power supply where it flows back through to the start. In a full PLC implementation

there will be many circuits that must each be complete.

A second important concept is the common. Here the neutral on the power supply

is the common, or reference voltage. In effect we have chosen this to be our 0V reference,

and all other voltages are measured relative to it. If we had a second power supply, we

would also need to connect the neutral so that both neutrals would be connected to the

same common. Often common and ground will be confused. The common is a reference,

or datum voltage that is used for 0V, but the ground is used to prevent shocks and damage

to equipment. The ground is connected under a building to a metal pipe or grid in the

ground. This is connected to the electrical system of a building, to the power outlets,

where the metal cases of electrical equipment are connected. When power flows through

the ground it is bad. Unfortunately many engineers, and manufacturers mix up ground and

common. It is very common to find a power supply with the ground and common mislabeled

Remember - Don’t mix up the ground and common. Don’t connect them together if the

common of your device is connected to a common on another device.

One final concept that tends to trap beginners is that each input card is isolated.

This means that if you have connected a common to only one card, then the other cards are

not connected. When this happens the other cards will not work properly. You must connect

a common for each of the output cards.

There are many trade-offs when deciding which type of input cards to use.

• DC voltages are usually lower, and therefore safer (i.e., 12-24V).

• DC inputs are very fast, AC inputs require a longer on-time. For example, a 60Hz

wave may require up to 1/60sec for reasonable recognition.

• DC voltages can be connected to larger variety of electrical systems.

• AC signals are more immune to noise than DC, so they are suited to long distances,

and noisy (magnetic) environments.

• AC power is easier and less expensive to supply to equipment.

• AC signals are very common in many existing automation devices.

In put and Out put on PLCS

Inputs to, and outputs from, a PLC are necessary to monitor and control a process.

Both inputs and outputs can be categorized into two basic types: logical or continuous.

Consider the example of a light bulb. If it can only be turned on or off, it is logical control.

If the light can be dimmed to different levels, it is continuous. Continuous values seem

more intuitive, but logical values are preferred because they allow more certainty, and

simplify control. As a result most controls applications (and PLCs) use logical inputs and

outputs for most applications. Hence, we will discuss logical I/O and leave continuous I/O

for later.

Outputs to actuators allow a PLC to cause something to happen in a process. A

short list of popular actuators is given below in order of relative popularity.

Solenoid Valves - logical outputs that can switch a hydraulic or pneumatic flow.

Lights - logical outputs that can often be powered directly from PLC output

boards.

Motor Starters - motors often draw a large amount of current when started, so they

require motor starters, which are basically large relays.

Servo Motors - a continuous output from the PLC can command a variable speed

or position.

Outputs

from PLCs are often relays, but they can also be solid state electronics

such as transistors for DC outputs or Triacs for AC outputs. Continuous outputs require

special output cards with digital to analog converters.

Inputs come from sensors that translate physical phenomena into electrical signals.

Typical examples of sensors are listed below in relative order of popularity.

Proximity Switches - use inductance, capacitance or light to detect an object logically.

Switches - mechanical mechanisms will open or close electrical contacts for a logical

signal.

Potentiometer - measures angular positions continuously, using resistance.

LVDT (linear variable differential transformer) - measures linear displacement

continuously using magnetic coupling.

Inputs for a PLC come in a few basic varieties, the simplest are AC and DC inputs.

Sourcing and sinking inputs are also popular. This output method dictates that a device

does not supply any power. Instead, the device only switches current on or off, like a simple

switch.

Sinking -When active the output allows current to flow to a common ground. This

is best selected when different voltages are supplied.

Sourcing - When active, current flows from a supply, through the output device

and to ground. This method is best used when all devices use a single supply

voltage.

This is also referred to as NPN (sinking) and PNP (sourcing). PNP is more popular.

This will be covered in more detail in the chapter on sensors.
(resource : Hugh Jack's ebook: thanks)

Base Introduction PLC

Many PLC configurations are available, even from a single vendor. But, in each of

these there are common components and concepts. The most essential components are:

Power Supply - This can be built into the PLC or be an external unit. Common

voltage levels required by the PLC (with and without the power supply) are

24Vdc, 120Vac, 220Vac.

CPU (Central Processing Unit) - This is a computer where ladder logic is stored

and processed.

I/O (Input/Output) - A number of input/output terminals must be provided so that

the PLC can monitor the process and initiate actions.

Indicator lights - These indicate the status of the PLC including power on, program

running, and a fault. These are essential when diagnosing problems.

The configuration of the PLC refers to the packaging of the components. Typical

configurations are listed below from largest to smallest as shown in Figure 3.1.

Rack - A rack is often large (up to 18” by 30” by 10”) and can hold multiple cards.

When necessary, multiple racks can be connected together. These tend to be the

highest cost, but also the most flexible and easy to maintain.

Mini - These are similar in function to PLC racks, but about half the size.

Shoebox - A compact, all-in-one unit (about the size of a shoebox) that has limited

expansion capabilities. Lower cost, and compactness make these ideal for small

applications.

Micro - These units can be as small as a deck of cards. They tend to have fixed

quantities of I/O and limited abilities, but costs will be the lowest.

Software - A software based PLC requires a computer with an interface card, but

allows the PLC to be connected to sensors and other PLCs across a network.

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