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Introduction
• Today programmable controllers are found in almost all areas of life.
• In the past control tasks were solved with switch and relay controls.
The function of controller was defined through the wiring and
combination of switching elements.
• For PLC, the wiring effort is considerably less. The function of
controller is defined by program.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
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•
•
•
•
•
Outline
Section 1: Brief Introduction of Relay Control Panels
Section 2: Basic Components of PLC System
Section 3: Program Execution In PLC
Section 4: Basic PLC Programming
Section 5: Extending Ladder Logic Beyond Relay Logic
George W. Woodruff School of Mechanical Engineering, Georgia Tech
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George W. Woodruff School of Mechanical Engineering, Georgia Tech
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Before the invention of the Programmable Logic Controller
(PLC), most industrial control was done using relay control
panels.
Logical decisions are made on relay control panels by
wiring several to thousands of :
Switches
Relays
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Switch
Pin 2
Pin1
Movable
Contact
Released
Pin 1 and 2 connected
Pressed
Pin 1 and 3 connected
Pin 3
Switch is a type of binary state device.
Take push button switch as example, it has two state: released and pressed
Depending on the wiring, switch has two different usages:
“normally closed switch” :
Pins 1 and 2 are wired to the circuit.
When the switch is released, current can pass through (closed circuit).
“normally open switch” :
Pins 1 and 3 are wired to the circuit.
When the switch is released, current cannot pass through (open circuit).
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Relay
A relay is an electrically operated switch, which consists of coil and switch.
When no current is passed through the coil, pin 1 and 2 are connected.
When current is passed through the coil, the contact is pulled by
electromagnetic force and pin 1 and 3 are connected.
1
3
1
2
Coil off: Pin 1 and 2
connected
3
2
Coil on: Pin 1 and 3
connected
Combinations of switches and relays can realize simple to extremely
complicated logical operations.
Control function is defined through wiring switching elements.
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“NOT” operation
Input connected to one relay, whose switch is wired as a normally closed switch
V+
Input
3
2
Input
Output
0
1
1
0
Output
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“AND” operation
Two relays in series
Input 1 connected to one relay, whose switch is wired as a normally open switch
Input 2 connected to the other relay, whose switch is wired as a normally open
switch
V+
Input 1
2
3
Input 2
3
2
Input 1
Input 2
Output
0
0
0
1
0
0
0
1
0
1
1
1
Output
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“OR” operation
Two relays in parallel
Input 1 connected to one relay, whose switch is wired as a normally open switch
Input 2 connected to the other relay, whose switch is wired as a normally open
switch
V+
V+
Input 1
Input 2
3
2
3
2
Input 1
Input 2
Output
0
0
0
1
0
1
0
1
1
1
1
1
Output
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“XOR” operation
Four relays, two paths in parallel, on each path two in series
Input 1 connected to two relays, one on each path, whose switches are wired as a
normally open switch and normally close switch, respectively
Input 2 connected to the other two relays, whose switches are wired opposite to
Input 1 on each path
Input 1
V+
V+
1
1
2
3
1
Input 2
3
2
3
Input 1
Input 2
Output
0
0
0
1
0
1
0
1
1
1
1
0
1
2
3
2
Output
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Problems with relay control panels:
Mechanical Relays and switches failed regularly (coil failure, contact
wear and contamination, etc.)
Difficult to diagnose problems and replace relays and switches
Difficult to change hardwired logic (example: changing an “OR” circuit
to “XOR”)
Consumed a lot of power
To address these problems, Richard E. Morley of Bedford
Associates invented the first PLC as a consulting project for
General Electric in 1968. Bedford Associates is currently named
Modicon and is a supplier of PLCs.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
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George W. Woodruff School of Mechanical Engineering, Georgia Tech
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Basic PLC System Components include:
Power Supply
CPU
Signal Modules (expandable I/Os)
Communication Modules
HMI(Human Machine Interface)
Programming Device (e.g. PC)
Siemens SIMATIC S7-1200 PLC system will be used as an example, which
is installed in the Mechatronics Laboratory in 2012.
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Power Supply
Every PLC has an external or internal Power Supply.
Typically 24 Volts for industrial PLCs.
Power Supplies convert 110V AC to 24V DC.
Power Supplies may have more than one isolated outputs.
One isolated output is reserved for the PLC CPU. The rest are
reserved for other components such as communication module.
The S7-1200 PLC uses the Siemens
A&D PS307 5A power supply. The
PS307 5A can source 5 amps of current
at 24 volts. The PS307 5A has 3
isolated outputs.
Siemens 07 5A
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CPU
Every PLC system has at least one CPU
The SIMATIC S7-1200 system comes in four different models, with
CPU 1211C, CPU 1212C, CPU 1214C and CPU 1215C, that may
each be expanded to exactly fit the application requirements.
In our lab PLC system, CPU 1214C
DC/DC/DC is used , which accepts up to
eight signal modules at the right side of
the CPU.
The digital and analog I/Os can easily
be expanded without affecting the
physical size of the controller by
installing a signal board inside the front
of the CPUs.
CPU 1214C DC/DC/DC
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CPU ( Continued)
User Memory Areas on a CPU :
LOAD Memory
•
Non-volatile storage for the user program, data and configuration
•
For CPU 1214C: 2MB integrated Load Memory + SIMATIC Memory Card
(optional)
WORK Memory
•
Volatile storage for some elements of the user project while executing.
•
When PLC starts, The CPU copies some elements of the project from
load memory into work memory.
•
For CPU 1214C: 50 Kbytes of WORK memory
Retentive Memory
•
non-volatile storage for a limited quantity of work memory values
•
For CPU 1214C: 2 Kbytes of Retentive memory
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CPU ( Continued)
Bit Memory (M)
•
Bit memory is a free area of RAM that can be used by the programmer
•
For CPU 1214C: 8192 bytes of WORK memory
Process Image
•
Process Image input (I) is memory location for each physical input pin
•
Process Image output (Q) is memory location for each physical output pin
•
To immediately access the physical inputs and physical outputs, append a
":P" to the address (e.g. I0.1:P).
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Signal Modules (expandable I/Os)
CPU 1214C has 14 integrated digital inputs and 10 digital outputs
The number of input/output pins can be increased by adding additional
signal modules to the right side of CPU.
Up to 8 digital or analog signal modules can be added to CPU 1214C.
In our lab, we don’t have
expanded signal modules.
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Communication Module
CPU 1214C has an integrated Ethernet interface, which support the
PROFINET communication with PCs, HMIs, other CPUs , etc.
Communication Module works like a hub or router, which provide
multiple Ethernet ports to support multi-point communication.
Up to 3 communication modules can be added to any of the SIMATIC
S7-1200 CPUs.
The RS485 and RS232 communication modules can also be added,
which we don’t’ use in our lab and will not talk about in this lecture.
PROFINET is the open standard based on Ethernet
widely used in industrial communications. You can
find a lot of information on line.
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HMI(Human Machine Interface)
SIMATIC HMI KTP600 Basic Color Panel is connected with PLC CPU.
It has a 6 inch touch screen.
It has an Ethernet interface.
The user interface is programmable using TIA Portal.
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Wiring of basic PLC components in lab
CPU
Communication
Module
PC
Power
Supply
Power Cable
Ethernet Cable
HMI
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George W. Woodruff School of Mechanical Engineering, Georgia Tech
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Initially PLC was invented to directly replace relay
control panels based on mechanical relays.
However, the power of PLC CPU make it possible to
realize much more complicated controls than relay
control panels.
The function of controller is defined by user program
instead of wiring in the case of relay control panels.
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PLC Operating System
The operating system is a program that is needed for the basic
operation of PLC.
It is located on the CPU of a PLC.
It serves as a bridge linking between the user program and the
hardware.
It also controls the communication between PLC and programming
device (e.g. uploading user program, online debugging).
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User Program
User program is a program which is created by PLC programmer to
solve certain control tasks.
It is created with the help of programming device and uploaded to the
PLC after compiling.
The Siemens TIA Portal (Totally Integrated Automation Portal ) is the
programming environment where we can configure hardware and do
the programming.
Three programming languages:
Function Block Diagram (FBD)
Statement List (STL)
Ladder Logic (LAD)
It’s easy to switch between different programming languages based
on preference.
Ladder logic is a visual programming language, we will discuss it in
detail later in this lecture.
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Cyclical Program Execution
After the user program is uploaded to PLC, the operating system
cyclically processes the user program
User programs and data are arranged in program blocks.
TIA portal will create a program block Main [OB1] automatically.
We start to program in OB1 and can create other blocks for better
organization of programs.
OB1 is like the main function in C and is the entrance point of user
programs.
The operating system cyclically execute the OB1 in a continuously
running repeat loop.
This process is called cyclical program execution and each loop is
a program cycle.
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Cyclical Program Execution cont’
Within each program cycle:
•
Reads Input pins and updates Process
Input Image
•
Executes User Program Once (OB1)
Update Process Input
Image
Execute User Program
•
Writes Process Output Image to Output
pins
•
Take care of system processes ( such as
communications with other PLCs, updating
user program, checking for STOP
condition, etc..)
•
Loop Back
Update Output Pins
Operating System
program cycle
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Cyclical Program Execution cont’
PLC loads the states of all inputs in its memory (Process Input Image)
first in each program cycle
----Guarantee a constant signal state of inputs during the entire
program cycle
After completion of execution of user program, all output pins are
updated simultaneously based on Process Output Image
----Guarantee all output states are resulting from the same set of
input states
If the user program is revised, the new program is written first to LOAD
memory. In each program cycle, after updating output pins, the
operating system will copy the new program to WORK memory from
LOAD Memory. During the next scan cycle, the new user program will
be executed
----Make it possible to upload new program to PLC during its running
and without interrupting current program
The minimum response time for changing inputs is one program cycle,
the
maximum
response
timeofisMechanical
two cycles.Engineering, Georgia Tech
George
W. Woodruff
School
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George W. Woodruff School of Mechanical Engineering, Georgia Tech
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PLC’s logic control is defined by user program instead of
hard wiring of relay control panels.
Ladder Logic is a graphical programming language. The
programming process involves dragging and dropping
elements, arranging them and specifying their corresponding
memories and parameters.
In this section, we will first talk about how to address
memories in PLC CPU. Then we will learn how to realize
simple logical operations in PLC using Ladder Logic.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
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Memory Addressing
To address a bit of memory
___ ___ . ___
Memory Area
Notation
Byte Address
Bit Number
To address a byte, word, or double word
___ ___ ___
Memory Area
Notation
Size of Addressed
Memory Notation
Byte Address
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Memory Addressing (continued)
Memory Area Notations:
Notation
Memory Area
I
Process Image Input
Q
Process Image Output
M
Bit Memory
PI
Peripheral Input ( Actual Input Pins)
PQ
Peripheral Output ( Actual Output Pins)
T
Timer Storage Area
C
Counter Storage Area
L
Local Memory of current Data Block
DB
Data Block Memory
(Note: Advanced features such as Timers, Counters, Data
Blocks
not beSchool
discussed
in this lecture)
George
W.will
Woodruff
of Mechanical
Engineering, Georgia Tech
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Memory Addressing (continued)
Size of Addressed Memory Notations:
Notation
Size of Addressed Memory
B
Byte (8 bits)
W
Word (16 bits)
D
Double Word (32 bits)
Byte Address:
Each Memory Area is addressed in one byte increments
starting at byte 0.
Bit Number:
MSBit is 7 and LSBit is 0
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Memory Addressing (continued)
Examples:
Bit Memory Area
Byte 0
M1.3
(Note: only bit 3 of
Marker Area byte 1)
MB0
Byte 1
Byte 2
MW1
Byte 3
Byte 4
MD3
Byte 5
Byte 6
MD4
Byte 7
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Ladder Logic
A ladder logic program has a “ladder” look.
The program is mapped in one or more networks.
On the left edge, the network contains one power rail.
Current flows from power rail to ground through each rung.
A rung is ended with a coil which is the result of logic operation of
upstream elements on the same rung.
Network 1
%I0.2
“Button2”
%Q0.1
“LED1”
Ground
Power Rail
%I0.1
“Button1”
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Ladder Logic
In a network, there is at least one rung from the power rail.
The power rail can be extended with several rungs and
branches can be started from or close to any rung.
Inputs and outputs are stored in CPU memories.
Tags make the memory location meaningful to programmer.
Elements are dragged and dropped to rungs from tool box.
Network 1
%I0.1
“Button1”
%I0.2
“Button2”
%Q0.1
“LED1”
%I0.3
“Button3”
%I0.4
“Button4”
%Q0.3
“LED1”
Ground
Power Rail
%Q0.2
“Motor”
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Ladder Logic : Basic Elements on Rungs
<address> Normally Open Contact Current can be passed when the corresponding
memory bit has value “1”. Corresponding memory bit
is specified above it.
<address>
Normally Closed Contact Current can pass through it when the corresponding
memory bit has value “0”. Corresponding memory bit is
specified above it.
<address>
Coil
Corresponding memory bit will be set to “1” when
current passes through it. Corresponding memory bit
will be set to “0” when no current passes through it.
Corresponding memory bit is specified above it.
Open Branch
Start a parallel branch from a rung.
Close Branch
Close a branch to a rung.
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Ladder Logic: “NOT” operation
Network 1
%I0.0
“Button”
%Q0.0
“LED”
I0.0
Q0.0
0
1
1
0
Drag and drop a Normally Closed Contact. Designate the corresponding
memory bit as I0.0 (put “%” in front of global memory). “Button” is an
example tag entered for this memory bit when a button is wired to PLC
input pin associated with I0.0
Drag and drop a Coil to end the rung. Designate the corresponding
memory bit as Q0.0. “LED” is an example tag entered for this memory bit
when a LED is wired to PLC output pin associated with Q0.0
When I0.0 is set to 1, no current can flow to the coil, Q0.0 is set to 0.
When I0.0 is set to 0, current can flow to the coil, Q0.0 is set to 1.
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Ladder Logic: “AND” operation
Network 1
%I0.0
“Button1”
%I0.1
“Button2”
%Q0.0
“LED”
I0.0
I0.1
Q0.0
0
0
0
0
1
0
1
0
0
1
1
1
Drag and drop two Normally Open Contacts and put them in series on a
rung. Designate the corresponding memory bit as I0.0 and I0.1,
respectively. Create any meaningful tag based on your application.
Drag and drop a Coil to end the rung. Designate the corresponding
memory bit as Q0.0. Create any meaningful tag based on your application.
Only when both I0.0 and I0.1 are set to 1, current can flow to the coil and
Q0.0 will be set to 1.
Otherwise no current can flow to the coil, Q0.0 is set to 0.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
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Ladder Logic: “OR” operation
Network 1
%I0.0
“Button1”
%I0.1
“Button2”
%Q0.0
“LED”
I0.0
I0.1
Q0.0
0
0
0
0
1
1
1
0
1
1
1
1
Drag and drop two Normally Open Contacts and put them in parallel and
merging to one rung . Designate the corresponding memory bit as I0.0 and
I0.1, respectively. Create any meaningful tag based on your application.
Drag and drop a Coil to end the rung. Designate the corresponding
memory bit as Q0.0. Create any meaningful tag based on your application.
Only when both I0.0 and I0.1 are set to 0, no current can flow to the coil
and Q0.0 will be set to 0.
Otherwise current can flow to the coil, Q0.0 is set to 1.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
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Ladder Logic: “XOR” operation
Network 1
%I0.0
“Button1”
%I0.1
“Button2”
%I0.0
“Button1”
%I0.1
“Button2”
%Q0.0
“LED”
I0.0
I0.1
Q0.0
0
0
0
0
1
1
1
0
1
1
1
0
Both I0.0 and I0.1 are controlling one Normally Open Contact and one
Normally Closed Contact. The four elements are arranged as shown
above, and finally merges to one Coil.
Only when I0.0 and I0.1 are set differently, current can flow to the coil and
Q0.0 will be set to 1.
Otherwise no current can flow to the coil, Q0.0 is set to 0.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
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George W. Woodruff School of Mechanical Engineering, Georgia Tech
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PLC’s CPU can be used for more than relay logic as relay control
panels. Ladder logic has advanced components that take
advantage of the CPU. These components can be categorized as
follows: Bit logic, Counters, Comparator, Convert, Math, Program
control, Shift/Rotate, Timers, and so on.
It is impossible to cover all of these components in this lecture.
This section will first explain data types supported by PLC S71200. Then, only a few categories and examples of advanced
components will be presented.
All advanced components can be found in the instruction window
of TIA portal and pressing <F1> will open the help window
showing the details of the component and how to use them.
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PLC support a wide variety of
data types. The complete
description is located at
Programming a PLC->Creating a user
program->Programming basics->Data
types under
the Contents tab in
Help window. A screen capture
is shown here.
We will extract some frequently
used data types and present
them here.
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BOOL:
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BYTE:
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WORD:
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DOUBLE WORD:
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SIGNED INTEGER:
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REAL:
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TIME
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Additional Bit Logic: Negate assignment
<address>
--( / )--
Description:
The "Negate assignment" instruction inverts the result of logic operation (RLO)
and assigns it to the specified operand. When the RLO at the input of the coil is
"1", the operand is reset. When the RLO at the input of the coil is "0", the
operand is set to signal state "1".
The instruction does not influence the RLO. The RLO at the input of the coil is
sent directly to the output of the coil.
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Additional Bit Logic: Set Output
<address>
--( S )--
Description:
You can use the "Set output" instruction to set the signal state of a specified
operand to "1".
The instruction is only executed if the result of logic operation (RLO) at the input
of the coil is "1". If power flows to the coil (RLO = "1"), the specified operand is
set to "1". If the RLO at the input of the coil is "0" (no signal flow to the coil), the
signal state of the specified operand remains unchanged.
Executing the instruction does not influence the RLO. The RLO at the input of the
coil is sent directly to the output of the coil.
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Additional Bit Logic: Reset Output
<address>
--( R )--
Description:
You can use the "Reset output" instruction to reset the signal state of a specified
operand to "0".
The instruction is only executed if the result of logic operation (RLO) at the input
of the coil is "1". If power flows to the coil (RLO = "1"), the specified operand is
reset to "0". If the RLO at the input of the coil is "0" (no signal flow to the coil), the
signal state of the specified operand remains unchanged.
Executing the instruction does not influence the RLO. The RLO at the input of the
coil is sent directly to the output of the coil. (can be used to reset timer)
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Additional Bit Logic: Example
Button 1 connected to I0.0
Button 2 connected to I0.1
Button 3 connected to I0.2
LED connected to Q0.0
Motor connected to Q0.1
Button 1 is pressed, LED is off; Button is released, LED is on.
Button 2 is pressed, turn on Motor and keep it on even if Button 2 is released.
Button 3 is pressed, turn off Motor and keeps it off even if Button 3 is released.
%I0.0
“Button1”
%Q0.0
“LED”
%I0.1
“Button2”
%Q0.1
“Motor”
S
%I0.2
“Button3”
%Q0.1
“Motor”
R
There are other bit logic components available in PLC. If you are interested,
you can explore them under the Bit logic category of the Instruction
Window.
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Comparator Operations
Equal
Not equal
Greater or equal
Less or equal
Greater than
Less than
Value within range
Value outside range
…
You can explore more under the Comparator Operations category of the
Instruction Window.
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Equal operation:
<Operand1>
<Operand2>
Description:
You can use the "Equal" instruction to determine if a first comparison value (<Operand1>) is
equal to a second comparison value (<Operand2>).
If the condition of the comparison is fulfilled, the instruction returns the result of logic operation
(RLO) "1". If the comparison condition is not fulfilled, the instruction returns RLO "0". The RLO
of the instruction is logically combined with the RLO of the entire current path as follows:
By AND, when the comparison instruction is connected in series.
By OR, when the comparison instruction is connected in parallel.
Specify the first comparison value (<Operand1>) in the operand placeholder above the
instruction. George
Specify W.
the Woodruff
second comparison
(<Operand2>)
in theGeorgia
operandTech
placeholder
School of value
Mechanical
Engineering,
below the instruction.
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Equal operation example
Q0.0 is set to 1 only when both conditions are satisfied:
I0.0 is 1; And MB0 equals 15.
%I0.0
MB0
Q0.0
INT
15
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Program control operations
Jump if RLO=1
Jump if RLO=0
Jump label
Define jump list
Jump distributor
…
You can explore more under the Program control Operations category of
the Instruction Window.
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Jump if RLO=1
LABEL
Description:
You can use the "Jump if RLO = 1" instruction to interrupt the linear execution of
the program and resume it in another network. The destination network must be
identified by a jump label (LABEL). The name of this jump label is specified in the
placeholder above the instruction.
The specified jump label must be in the same block in which the instruction is
executed. The name you specify can only occur once in the block.
If the result of logic operation (RLO) at the input of the instruction is "1", the jump
to the network identified by the specified jump label is executed. The jump
direction can be towards higher or lower network numbers.
If the condition at the input of the instruction is not fulfilled (RLO = 0), execution
of the program continues in the next network.
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Jump label
Description
You can use a jump label to identify a destination network, in which the program
execution should resume when a jump is executed.
The jump label and the instruction in which the jump label is specified must be
located in the same block. The name of a jump label can only be assigned once
in a block.
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Jump if RLO=1 and Jump label example
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Math functions
Add
Subtract
Multiply
Divide
Return remainder of division
Create 2’s complement
Increment
Decrement
…
You can explore more under the Math functions category of the Instruction
Window.
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Add
Description:
You can use the "Add" instruction to add the value at input IN1 and the value at input IN2 and query the
sum at output OUT (OUT = IN1+IN2).
In its initial state, the instruction box contains at least 2 inputs (IN1 and IN2). The number of inputs can
be extended. The inserted inputs are numbered in ascending order in the box. When the instruction is
executed, the values of all available input parameters are added. The sum is stored at the OUT output.
The instruction is only executed if the signal state is "1" at the EN enable input. If the instruction is
executed without errors, the ENO enable output also returns the signal state "1".
The ENO enable output has the signal state "0" if one of the following conditions is fulfilled:
Enable input EN has the signal state "0".
The result of the instruction is outside the range permitted for the data type specified at the OUT output.
George
W.has
Woodruff
A floating-point
number
an invalidSchool
value. of Mechanical Engineering, Georgia Tech
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Add example
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Move operations
Move value
Read field
Write field
Move Block
…
You can explore more under the Move operations category of the
Instruction Window.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Move value
Description:
You use the "Move value" instruction to transfer the contents of the operand at
the IN input to the operand at the OUT1 output. The transfer is always made in
the direction
of the
ascending
address.
George
W. Woodruff
School
of Mechanical Engineering, Georgia Tech
ME4447/6405
Move value example:
George W. Woodruff School of Mechanical Engineering, Georgia Tech
ME4447/6405
Timer operations
Generate pulse
Generate on-delay
Generate off-delay
…
You can explore more under the Timer operations category of the
Instruction Window.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
Generate
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on-delay
Description:
The instruction "Generate on-delay" delays setting of the output Q by the programmed duration PT. The
instruction is started when the result of logic operation (RLO) at input IN changes from "0" to "1" (positive
signal edge). The programmed time PT begins when the instruction starts. When the duration PT
expires, the output Q has the signal state "1". Output Q remains set as long as the start input is still "1".
When the signal state at the start input changes from "1" to "0", the Q output is reset. The timer function
is started again when a new positive signal edge is detected at the start input.
The current time value can be queried at the ET output. The time value starts at T#0s and ends when the
value of duration PT is reached. The ET output is reset as soon as the signal state at the IN input
changes to "0".
The execution of the "Generate on-delay" instruction requires a preceding logic operation. It can be
placed within
or atW.
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end of the network.
George
School of Mechanical Engineering, Georgia Tech
ME4447/6405
Generate on-delay example
I0.0 is set to “1”;
1 second later, M0.1 is set to “1”.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
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Word Logic operations
“AND”
“OR”
“XOR”
Create 1’s complement
…
You can explore more under the Word Logic operations category of the
Instruction Window.
George W. Woodruff School of Mechanical Engineering, Georgia Tech
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“AND” word logic operation
Description:
You can use the "AND logic operation" instruction to combine the value at the IN1 input and the value at the
IN2 input bit-by-bit by AND logic and query the result at the OUT output.
The number of inputs can be expanded in the instruction box. The added inputs are numbered in ascending
order in the box. When the instruction is executed, the values of all available input parameters are combined
with AND logic. The result is stored in the OUT output.
The instruction is only executed if the signal state is "1" at the EN enable input. In this case, the ENO output
also has the signal state "1".
If the signal state at the EN enable input is "0", the ENO enable output also has the signal state "0".
George W. Woodruff School of Mechanical Engineering, Georgia Tech
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“AND” word logic operation example
George W. Woodruff School of Mechanical Engineering, Georgia Tech