Advanced plc programming & scada system design

1 Advanced PLC Programming & SCADA System Design| APOGEE AUTOMATION SYSTEMS
ADVANCED PLC PROGRAMMING &
SCADA SYSTEM DESIGN
Conducted by:

MODULE-1
Introduction
A Programmable Logic Controller, PLC or Programmable Controller is a digital computer used
for automation of electromechanical processes, such as control of machinery on factory
assembly lines, amusement rides, or light fixtures. The abbreviation "PLC" and the term
"Programmable Logic Controller" are registered trademarks of the Allen-Bradley Company
(Rockwell Automation). PLCs are used in many industries and machines. Unlike general-
purpose computers, the PLC is designed for multiple inputs and output arrangements, extended
temperature ranges, immunity to electrical noise, and resistance to vibration and impact.
Programs to control machine operation are typically stored in battery-backed-up or non-volatile
memory. A PLC is an example of a hard real time system since output results must be produced
in response to input conditions within a limited time, otherwise unintended operation will
result.
History
Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles
was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop
controllers. Since these could number in the hundreds or even thousands, the process for
updating such facilities for the yearly model change-over was very time consuming and
expensive, as electricians needed to individually rewire relays to change the logic.
Digital computers, being general-purpose programmable devices, were soon applied to control
of industrial processes. Early computers required specialist programmers, and stringent
operating environmental control for temperature, cleanliness, and power quality. Using a
general-purpose computer for process control required protecting the computer from the plant
floor conditions. An industrial control computer would have several attributes: it would tolerate
the shop-floor environment, it would support discrete (bit-form) input and output in an easily
extensible manner, it would not require years of training to use, and it would permit its
operation to be monitored. The response time of any computer system must be fast enough to
be useful for control; the required speed varying according to the nature of the process.
In 1968 GM Hydra-Matic (the automatic transmission division of General Motors) issued a
request for proposals for an electronic replacement for hard-wired relay systems based on a
white paper written by engineer Edward R. Clark. The winning proposal came from Bedford
Associates of Bedford, Massachusetts. The first PLC, designated the 084 because it was Bedford
Associates' eighty-fourth project, was the result. Bedford Associates started a new company
dedicated to developing, manufacturing, selling, and servicing this new product: Modicon, which
stood for MOdular DIgital CONtroller. One of the people who worked on that project was Dick
Morley, who is considered to be the "father" of the PLC.The Modicon brand was sold in 1977 to
Gould Electronics, and later acquired by German Company AEG and then by French Schneider
Electric, the current owner.

Programming
Early PLCs, up to the mid-1980s, were programmed using proprietary programming panels or
special-purpose programming terminals, which often had dedicated function keys representing
the various logical elements of PLC programs.Some proprietary programming terminals
displayed the elements of PLC programs as graphic symbols, but plain ASCII character
representations of contacts, coils, and wires were common. Programs were stored on cassette
tape cartridges. Facilities for printing and documentation were minimal due to lack of memory
capacity. The very oldest PLCs used non-volatile magnetic core memory.
More recently, PLCs are programmed using application software on personal computers, which
now represent the logic in graphic form instead of character symbols. The computer is
connected to the PLC through Ethernet, RS-232, RS-485 or RS-422 cabling. The programming
software allows entry and editing of the ladder-style logic. Generally the software provides
functions for debugging and troubleshooting the PLC software, for example, by highlighting
portions of the logic to show current status during operation or via simulation. The software
will upload and download the PLC program, for backup and restoration purposes. In some
models of programmable controller, the program is transferred from a personal computer to the
PLC through a programming board which writes the program into a removable chip such as an
EEPROM or EPROM.
Functionality
The functionality of the PLC has evolved over the years to include sequential relay control,
motion control, process control, distributed control systems and networking. The data handling,
storage, processing power and communication capabilities of some modern PLCs are
approximately equivalent to desktop computers. PLC-like programming combined with remote
I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain
applications. Regarding the practicality of these desktop computer based logic controllers, it is
important to note that they have not been generally accepted in heavy industry because the
desktop computers run on less stable operating systems than do PLCs, and because the desktop
computer hardware is typically not designed to the same levels of tolerance to temperature,
humidity, vibration, and longevity as the processors used in PLCs. In addition to the hardware
limitations of desktop based logic, operating systems such as Windows do not lend themselves
to deterministic logic execution, with the result that the logic may not always respond to
changes in logic state or input status with the extreme consistency in timing as is expected from
PLCs. Still, such desktop logic applications find use in less critical situations, such as laboratory
automation and use in small facilities where the application is less demanding and critical,
because they are generally much less expensive than PLCs.

The Architecture of a PLC, Function, Scan cycle and the Speed of a PLC
Main Parts of a PLC
 Microprocessor
 RAM
 EPROM
 Base I/O Modules
 Power Supply
 Data BUS
 Extensions
PLC modes
Basically there are two modes in a PLC.
1. Programming Mode
2. Run Mode
Monitoring mode/ Stop mode also available in some of brands.

SCAN CYCLE
A PLC works by continually scanning a program. We can think of this scan cycle as consisting of
3 important steps. There are typically more than 3 but we can focus on the important parts and
not worry about the others. Typically the others are checking the system and updating the
current internal counter and timer values.
Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is on or
off. In other words, is the sensor connected to the first input on? How about the second input?
How about the third... It records this data into its memory to be used during the next step.
Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time.
Maybe your program said that if the first input was on then it should turn on the first output.
Since it already knows which inputs are on/off from the previous step it will be able to decide
whether the first output should be turned on based on the state of the first input. It will store
the execution results for use later during the next step.
Step 3-UPDATE OUTPUT STATUS-Finally the PLC updates the status of the outputs. It updates
the outputs based on which inputs were on during the first step and the results of executing
your program during the second step. Based on the example in step 2 it would now turn on the
first output because the first input was on and your program said to turn on the first output
when this condition is true.
After the third step the PLC goes back to step one and repeats the steps continuously. One scan
time is defined as the time it takes to execute the 3 steps listed above
WatchDog Timer
A timer that monitors how long it takes the CPU to complete a scan. Watchdog timers output an
error message if the CPU scan takes too long.
Speed of a PLC
Depending on,
1. Processor Speed
2. Program length and number of modules.
3. Network speed

Combinational Designs for PLCs
All programmable controllers have standard logic instructions – AND, OR, NOT etc. and
these may be combined to create logic networks. Boolean algebra may be used as a tool
to assist in the design of logic networks. E.g. Y5 = X1.X2.X3.
There are three steps in combinational design method:
1. Functional Description
2. Truth Table
3. Boolean Expression
Karnaugh Maps method (Simplification of Boolean Expression)
A Karnaugh map provides a pictorial method of grouping together expressions with common
factors and therefore eliminating unwanted variables. The Karnaugh map can also be described
as a special arrangement of a truth table.

Inputs and Outputs of a PLC and their Connection Methods
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.
 12-24 Vdc
 100-120 Vac
 10-60 Vdc
 12-24 Vac/dc
 5 Vdc (TTL)
 200-240 Vac
 48 Vdc
 24 Vac
PLC input cards rarely supply power, this means that an external power supply is needed to
supply power for the inputs and sensors.
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.

Selection of a Programmable Logic Controller
When an engineer is embarking on any program of automation, it must be remembered
that the controller is only a tool used to perform the necessary tasks. The actual task of
implementing automation is therefore paramount, often involves many methodologies in its
system design phase. Once the specifications are established, the job of selecting a suitable
controller will then become most important as this would determine how at ease the
automation program might continue.
There is a massive range of PLC systems available today, with new additions or
replacements continually being produced with enhanced features of one types or another.
Advances in technology are quickly adopted by manufacturers in order to improve the
performance and market status of their products. However, irrespective of make, the majority
of PLC in each size range are very similar in term of their control facilities. Where significant
differences are to be found is in the programming methods and languages, together with
differing standards of manufacturer support and backup.
Considerations in choosing a suitable PLC
With the vast choice of equipment now available, the engineer can usually obtain
similar systems from several original equipment manufacturers (OEM). When the specification
requires certain types of function or input/output, it is possible that one system from a single
manufacturer standing out as more superior or cost effective than the other; but normally this is
rarely the case. To determine the the most suitable PLC to be used in the automation task,
there are several basic considerations to be made:
_ Necessary input/output capacity;
_ Types of I/O required;
_ Size of memory required;
_ Speed and power required of the CPU and instruction set
_ Manufacturer's support and backup.
All these topics are to a large extent interdependent, with the memory size being
directly tied to the amount of I/O as well as program size. As the I/O and memory size rises,
this takes longer to process and requires a more powerful, faster central processor if scan times
are to remain acceptable.
Introduction to Programming Methods
1. Ladder Diagram
2. Instruction List & Statement List (STL)
3. FBD (Function Block Diagram)
4. SFC (Sequential Function Chart)

Standard Contacts
The Normally Open contact instructions (LD, A, and O) and Normally Closed contact instructions
(LDN, AN, ON) obtain the referenced value from the memory or from the process-image
register. The standard contact instructions obtain the referenced value from the memory (or
process-image register if the data type is I or Q). The Normally Open contact is closed (on) when
the bit is equal to 1, and the Normally Closed contact is closed (on) when the bit is equal to 0. In
FBD, inputs to both the And and Or boxes can be expanded to a maximum of 32 inputs. In STL,
the Normally Open instructions Load, AND, or OR the bit value of the address bit to the top of
the stack, and the Normally Closed instructions Load, AND, or OR the logical NOT of the bit value
to the top of the stack.
Immediate Contacts
An immediate contact does not rely on the S7-224 scan cycle to update; it updates immediately.
The Normally Open Immediate contact instructions (LDI, AI, and OI) and Normally Closed
Immediate contact instructions (LDNI, ANI, and ONI) obtain the physical input value when the
instruction is executed, but the process-image register is not updated. The Normally Open
Immediate contact is closed (on) when the physical input point (bit) is 1, and the Normally
Closed Immediate contact is closed (on) when the physical input point (bit) is 0. The Normally
Open instructions immediately Load, AND, or OR the physical input value to the top of the stack,
and the Normally Closed instructions immediately Load, AND, or OR the logical NOT of the value
of the physical input point to the top of the stack.

Positive and Negative Transition Instructions
The Positive Transition contact instruction (EU) allows power to flow for one scan for each off-
to-on transition. The Negative Transition contact instruction (ED) allows power to flow for one
scan for each on-to-off transition. For the Positive Transition instruction, detection of a 0-to-1
transition in the value on the top of the stack sets the top of the stack value to 1; otherwise, it is
set to 0. For a Negative Transition instruction, detection of a 1-to-0 transition in the value on the
top of the stack sets the top of the stack value to 1; otherwise, it is set to 0.
NOT Instruction
The Not instruction (NOT) changes the state of power flow input (that is, it changes the value on
the top of the stack from 0 to 1 or from 1 to 0).
Output Instructions
The Output instruction (=) writes the new value for the output bit to the process-image register.
When the Output instruction is executed, the CPU turns the output bit in the process-image
register on or off. For LAD and FBD, the specified bit is set equal to power flow. For STL, the
value on the top of the stack is copied to the specified bit.

The Output Immediate instruction (=I) writes the new value to both the physical output and the
corresponding process-image register location when the instruction is executed. When the
Output Immediate instruction is executed, the physical output point (Bit) is immediately set
equal to power flow. For STL, the instruction immediately copies the value on the top of the
stack to the specified physical output bit (STL). The “I” indicates an immediate reference; the
new value is written to both the physical output and the corresponding process-image register
location when the instruction is executed. This differs from the non-immediate references,
which write the new value to the process-image register only.
Set Reset Bits
The Set (S) and Reset (R) instructions set (turn on) or reset (turn off) the specified
number of points (N), starting at the specified address (Bit). You can set or reset from 1
to 255 points. If the Reset instruction specifies either a timer bit (T) or counter bit (C),
the instruction resets the timer or counter bit and clears the current value of the timer
or counter.
TIMER Instruction
There are four fundamental types of timers shown in below figure. An on-delay timer will wait
for a set time after a line of ladder logic has been true before turning on, but it will turn off
immediately. An off-delay timer will turn on immediately when a line of ladder logic is true, but
it will delay before turning off. Consider the example of an old car. If you turn the key in the
ignition and the car does not start immediately, that is an on-delay. If you turn the key to stop
the engine but the engine doesn’t stop for a few seconds, that is an off delay. An on-delay timer
can be used to allow an oven to reach temperature before starting production. An off delay
timer can keep cooling fans on for a set time after the oven has been turned off.
On delay
Operands: 0-255
PT Value: VW,T,C,IW,QW,MW, ALW,
constant

On-Delay Timer, Retentive On-Delay Timer, Off-Delay Time
The On-Delay Timer and Retentive On-Delay Timer instructions count time when the enabling
input is ON. When the current value (Txxx) is greater than or equal to the preset time (PT), the
timer bit is ON. The On-Delay timer current value is cleared when the enabling input is OFF,
while the current value of the Retentive On-Delay Timer is maintained when the input is OFF.
You can use the Retentive On-Delay Timer to accumulate time for multiple periods of the input
ON. A Reset instruction (R) is used to clear the current value of
the Retentive On-Delay Timer.
Both the On-Delay Timer and the Retentive On-Delay Timers continue counting after the Preset
is reached, and they stop counting at the maximum value of 32767. The Off-Delay Timer is used
to delay turning an output OFF for a fixed period of time after the input turns OFF. When the
enabling input turns ON, the timer bit turns ON immediately, and the current value is set to 0.
When the input turns OFF, the timer counts until the elapsed time reaches the preset time.
When the preset is reached, the timer bit turns OFF and the current value stops counting.
If the input is OFF for a time shorter than the pre-set value, the timer bit remains ON. The TOF
instruction must see an ON to OFF transition to begin counting. If the TOF timer is inside an SCR
region and the SCR region is inactive, then the current value is set to 0, the timer bit is turned
OFF, and the current value does not count.

Understanding the Timer Instructions
You can use timers to implement time-based counting functions. The S7-200 instruction set
provides three types of timers as shown below. Following Table shows the actions of the
different timers.
On-Delay Timer (TON) for timing a single interval
Retentive On-Delay Timer (TONR) for accumulating a number of timed intervals
Off-Delay Timer (TOF) for extending time past a false condition (in other words,
such as cooling a motor after it is turned off)
Counter Instructions

-The Count Up instruction counts up to the maximum value on the rising edges of the Count Up
(CU) input. When the current value (Cxxx) is greater than or equal to the Preset Value (PV), the
counter bit (Cxxx) turns on. The counter is reset when the Reset (R) input turns on.
-The Count Up/Down instruction counts up on rising edges of the Count Up (CU) input. It
counts down on the rising edges of the Count Down (CD) input. When the current value (Cxxx) is
greater than or equal to the Preset Value (PV), the counter bit (Cxxx) turns on. The counter is
reset when the Reset (R) input turns on.
-The Count Down Counter counts down from the preset value on the rising edges of the Count
Down (CD) input . When the current value is equal to zero, the counter bit (Cxxx) turns on. The
counter resets the counter bit (Cxxx) and loads the current value with the preset value (PV)
when the load input (LD) turns on. The Down Counter stops counting when it reaches zero.
Counter ranges: Cxxx=C0 through C255 In STL, the CTU Reset input is the top of the stack value,
while the Count Up input is the value loaded in the second stack location. In STL, the CTUD Reset
input is the top of the stack value, the Count Down input is the value loaded in the second stack
location, and the Count Up input is the value loaded in the third stack location. In STL, the CTD
Load input is the top of stack, and the Count Down input is the value loaded in the second stack
location.

Compare Instructions
The Compare Byte instruction is used to compare two values: IN1 to IN2.
Comparisons include:
Equal: IN1 = IN2
Greater than or equal : IN1 >= IN2
Less than or Equal: IN1 <= IN2
Greater than: IN1 > IN2
Less than IN1 < IN2
or IN1 <> IN2.
Also can use for integer, Word, double words.
Ex:

Math Instructions
Add and Subtract:
The Add Integer and Subtract Integer instructions add or subtract two 16-bit integers and
produce a 16-bit result (OUT).
The Add Double Integer and Subtract Double Integer instructions add or subtract two 32-bit
integers, and produce a 32-bit result (OUT).
The Add Real and Subtract Real instructions add or subtract two 32-bit real numbers and
produce a 32-bit real number result (OUT).
In LAD and FBD:
IN1 + IN2 = OUT
IN1 - IN2 = OUT
Multiply and Divide:

Increment & Decrement
 The Increment Byte and Decrement Byte instructions add or subtract 1 to or from the
input byte (IN) and place the result into the variable specified by OUT. Increment and
decrement byte operations are unsigned.
 The Increment Word and Decrement Word instructions add or subtract 1 to or from
the input word (IN) and place the result in OUT. Increment and decrement word
operations are signed (16#7FFF > 16#8000).
 The Increment Double Word and Decrement Double Word instructions add or
subtract 1 to or from the input double word (IN) and place the result in OUT. Increment
and decrement double word operations are signed (16#7FFFFFFF > 16#80000000).
In LAD and FBD:
IN + 1 = OUT
IN - 1 = OUT

Special Memory (SM) Bits
Special memory bits provide a variety of status and control functions, and also serve as a means
of communicating information between the CPU and your program. Special memory bits can be
used as bits, bytes, words, or double words.

CPU Memory Areas
Using the Memory Address to Access Data.
To access a bit in a memory area, you specify the address, which includes the memory area
identifier, the byte address, and the bit number. Figure 5-1 shows an example of accessing a bit
(which is also called “byte.bit” addressing). In this example, the memory area and byte address
(I = input, and 3 = byte 3) are followed by a period (“.”) to separate the bit address (bit 4).
You can access data in many CPU memory areas (V, I, Q, M, S, L, and SM) as bytes, words, or
double words by using the byte-address format. To access a byte, word, or double word of data
in the CPU memory, you must specify the address in a way similar to specifying the address for a
bit. This includes an area identifier, data size designation, and the starting byte address of the
byte, word, or double-word value, as shown in following figure. Data in other CPU memory
areas
(such as T, C, HC, and the accumulators) are accessed by using an address format that includes
an area identifier and a device number.

Representation of Numbers
Below image shows the range of integer values that can be represented by the different sizes of
data. Real (or floating-point) numbers are represented as 32-bit, single-precision numbers
whose format is: +1.175495E-38 to +3.402823E+38 (positive), and -1.175495E-38 to
3.402823E+38 (negative). Real number values are accessed in double-word lengths. Refer to
ANSI/IEEE 754-1985 standard for more information about real or floating-point numbers.
Addressing the Process-Image Input Register (I)
As described in Section 4.6, the CPU samples the physical input points at the beginning of each
scan cycle and writes these values to the process-image input register. You can access the
process-image input register in bits, bytes, words, or double words.
Format:
Bit I[byte address].[bit address] I0.1
Byte, Word, Double Word I[size][starting byte address] IB4
Addressing the Process-Image Output Register (Q)
At the end of the scan cycle, the CPU copies the values stored in the process-image output
register to the physical output points. You can access the process-image output register in bits,
bytes, words, or double words.
Format:
Bit Q[byte address].[bit address] Q1.1
Byte, Word, Double Word Q[size][starting byte address] QB5

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