01 introduction to_plc-pac_rev01_fa16

EMEC240
Industrial Control Systems I
Programmable Logic and Automation Controllers

Introduction
Introduction to Programmable Logic
Controllers

PLC Definition
 Programmable Logic Controller (PLC)
 A PLC is a member of the computer family.
They are special-purpose industrial
computers that
 monitor the status of devices wired to it as inputs
 solves the logic of a user-written program based
upon the status of the input devices, then
 turns on or off devices that are wired to it as
outputs.
 The Rockwell Automation/Allen Bradley
PLC5, SLC and micrologix platforms and the
Siemens S7-300 and S7-1200 are examples
of PLCs.

PC-Based Control
Definition
 PC-based control replaces the traditional and proprietary
controllers such as PLCs with standard PC-based
hardware and software.
 PC-based control runs on personal computers and
provides answers to initiatives for lean control programs.
 Machine control programs with PC-based control provide
maximum control with minimum hardware configuration.
 PCs provide end-users and machine builders with a
platform to dramatically reduce control system design
time.
 Maintenance costs are reduced by the addition of:
 Built-in diagnostics
 Real-time simulation
 Data being stored in a single database
 Examples would be: National Instruments Lab View,
Rockwell Automation SoftLogix5800, etc.
Plant Services.Com http://www.plantservices.com/articles/2007/064.html Retrieved: June 27, 2007

PAC Definition
 Programmable Automation Controller
(PAC)
 A PAC is a compact controller that combines
the features and capabilities of a PC-based
control system with that of a PLC-based
control system.
 The Rockwell Automation ControlLogix
platform, Siemens Simatic S7-1200 series
and the National Instruments Industrial
Controllers are an example of a PAC.

Advantages of a PLC/PAC
Control System
 The PLC/PAC
eliminates much of
the hard wiring that
is associated with
conventional relay
control circuits and
replaces it with a
user written
program.
 The program takes
the place of much
of the external
wiring required for
controlling a
process.
Graphic from: Petruzella, Frank D. (2005). Programmable Logic Controllers (3rd ed.). New York, NY: McGraw-Hill
Pg. 6

Control System
 Increased reliability
 Once a program is written,
tested and commissioned, it
can, in most cases, be
downloaded to other
PLC/PACs that perform the
same machine control
function with only minor
tweaking of the code.
 Since all the logic is
contained in the PLC/PAC
memory, there is no chance
of making a logic wiring
error.
 Failures:
 85% of all failures occur in the
external wiring or field device.
 10% occur in the I/O modules.
 5% occur in the processor.
Percentage of Failures
I/O modules
10%
Processor module
5%
External wiring or
field device
85%
As of 2003, the estimated downtime due to PLC/PAC
failure is: 0.0082% to 0.025% per year of total
machine operating hours
Data from 1999

Control System
 More flexibility
 Original equipment manufacturers
(OEMs) can usually provide
system updates for a process by
simply sending out a new
program.
 It is easier to create and change a
program in a PLC/PAC than to
wire and/or rewire a circuit.
 End-users can modify the
program in the field.
http://dic.academic.ru/pictures/wiki/files/82/Relay_room27.jpg

Control System
 Lower costs
 PLCs were originally
designed to replace relay
control logic. The cost
savings using PLC/PACs
have been so significant
that except for high power
applications, relay control
has become obsolete.
 Generally, if an application
requires more than about
six control relays, it will
usually be less expensive
to install a PLC/PAC.
Pg. 130

Control System
 Communications
capability
 A PLC/PAC can
communicate with other
controllers or computer
equipment.
 They can be networked to
perform functions such as:
 Supervisory control
 Data gathering
 Monitoring devices and
process parameters
 Downloading and uploading
of programs

Control System
 Faster response time
 PLC/PACs are electronic
devices, therefore they can
respond quicker than
electromechanical relays.
This means that an event
taking place in the field will
result in an operation or
output taking place in what
looks to be “real-time”.
 Machines that process
thousands of items per
second, and objects that
spend only a fraction of a
second in front of a sensor
require the quick response
capability of a PLC/PAC.
New high-speed line for San Pellegrino
Krones AG
https://www.youtube.com/watch?v=9lY0cY0RVv8

Control System
 Easier to troubleshoot
 PLC/PACs have built-in
diagnostic indicators and
override functions that
allow maintenance
personnel to easily trace
and correct hardware and
software problems.
 A maintenance technician
can watch the control
program as it executes.
Instructions viewed on the
computer screen highlight
to show changing states
(on/off). This feature aids
greatly in locating and
repairing problems.
Fault
Indicators
Status
Indicators

Processor Architecture
 Personal computers, PACs and PLCs share
similar architectures:

PLC/PAC Architecture
 There are two types of PLC/PAC and
computer architectures:
 Open Architecture
 A design that allows the system to be easily connected
to devices and run programs made by other
manufactures.
 The IEC1 61131-3 standard is attempting to make
PLC/PAC manufactures standardize.
 Closed Architecture
 A design, also referred to as a proprietary system, that
makes it more difficult, if not impossible, to connect
devices and run programs made by other
manufactures. (Current platforms)

PLC/PACs Program in RLL
 PLCs program in Relay Ladder Logic (RLL).
 PACs program in RLL, Sequential Function
Chart (SFC), Function Block Diagram (FBD),
Structured Text, or a mix of all. National
Instrument PACs program using a program
called: LabView®
 The next couple of slides show very small
‘snippets’ of program ‘code’ as an example of
how the different code types look. These
snippets are for reference only.

Relay Ladder Logic
Example (AB ControlLogix)

Relay Ladder Logic
Example (Siemens S7-300)
The Siemens S7-300 does not have a Compute or Calculate instruction therefore, discrete math instruction are used.

Relay Ladder Logic
Example (Siemens S7-1200)
The Siemens S7-1200 Calculate instruction therefore, discrete math instructions are not used.

Relay Ladder Logic
Example (Automation Direct)
All math in Automation Direct works out of the accumulator, therefore values must
be loaded (LD) into the accumulator before any math can be done.

Relay Ladder Logic
Example (Automation Direct)

Function Block Diagram

Sequential Function Chart

A VERY SIMPLE EXAMPLE
Relay Control System

Relay Control Systems
 Consider the following control problem.
 A tank (vessel) is to be filled through an
electrically operated valve and emptied by
a motor driven pump. The following
conditions must be satisfied:
 The pump can operate only when the input
valve to the tank is open.
 The input valve can be opened when the
pump is either operating or not operating.

Electromechanical RLL
Control Drawing Pump Motor Control
M1 M1
A
Control
Relay
Solenoid
Valve
Contacts
for coils
Thermal
Overload
Coil OLA2
A1
Off on
Off on
Electronic Schematic
Pump motor
control switch
Valve
control
switch
Relay
Pump
motor
Solenoid
valve
A1
A2
A
Process Tank
Valve
(on-off)
Pump
Later, this electro-
mechanical ladder
logic will be
converted to PLC
RLL.

PLC/PAC Hardware
 System components (modular style)
 Rack or backplane or rail
 Power Supply
 Processor or Central Processing Unit (CPU)
 Input/Output (I/O) interface
 Communication connections
 Specialty interface connections
 Programmer (usually a computer)

System Components
Ethernet/IP
DeviceNet
scanner
ControlNet
scanner
Smart I/O
interface
Remote
racks
ASCII I/O
interface
SERCOS
interface
Special
Communications
Modules
And scanners
PLC special-
purpose
modules
PLC
output
modules
PLC
processor
PLC
input
modules
PLC
power
supply
Network
connections
Programmable
Logic
Controller
Information Layer
Control Layer
Ethernet
ProfiNet
EtherNet/IP
Rehg & Sartori, 2007, p.11

Electrostatic Discharge
Protection
 Before continuing:
 Many of the electronic components found in
PLC/PAC modules and PLC/PAC circuits are
sensitive to ESD (Electrostatic Discharge).
 It is not uncommon for our bodies to be elevated
to thousands, if not ten’s of thousands of volts.
This is why, when you touch something metal, you
get a shock.
 It takes approximately 5,000 volts to arc
across 1/16th inch in dry air.
 Electronic components must be protected from this discharge of voltage from our
bodies therefore:
 Ground yourself before handling electronic components or assemblies. Wear a
grounded wrist strap that provides a path to bleed off static charges.
 Be careful not to touch the backplane connectors, the surface of PCBs
(printed circuit boards) or connector pins of the PLC/PAC system.
 Create a static free work area by covering your workbench and floor area with
a non-metallic conductive surface that is grounded.
 If this is not possible, handle PLC/PAC components by the plastic or metal
frames or by the edges of the PCBs.

MODULAR PLC/PAC SYSTEMS
Rack (Backplane)

Rack or Backplane
(Modular System)
Module(s)
slides into
the rack or
snaps onto a
rail
Rack or
backplane
or rail
Data backplane
Processor
For SLC series
must be in slot 0
Graphic from: Rehg, James A. and Sartori, Glenn J. (2009). Programmable Logic Controllers (2nd ed.). Upper Saddle River, New Jersey, Pearson Prentice Hall

Graphic from: Rehg, James A. and Sartori, Glenn J. (2009). Programmable Logic Controllers (2nd ed.). Upper Saddle River, New Jersey, Pearson Prentice Hall
Processor
Rack or Backplane
(Modular System)

Rail System
Modules mount
on to a rail

Rail System
Connectors join the
Modules together
A module will attach to the rail
and into the connector that is
part of the previous module

PLC/PAC HARDWARE COMPONENT POWER SUPPLY
Power Supply

PLC/PAC Power Supply
 The PLC/PAC power supply
 Is integrated into the circuitry of the fixed
I/O units.
 Is either attached to a rack or slid into one
of the slots of a rack depending upon the
manufacturer of the PLC/PAC.
 This power supply supplies DC power
to the rack and to the modules that
are plugged into the rack.
 In large systems this power supply will
not supply power to the field devices.
A separate power source is used to
supply power to the field devices.
 In small and micro systems this power
supply can be used to supply power to
the field devices. Care should be
taken when doing this as to not
exceed the current rating of the DC
power supply.
ControlLogix Power Supply
Siemens Power Supply

THE BRAINS OF THE SYSTEM
Processor (CPU)

PLC/PAC Processor
 The processor, also referred to as a Central
Processing Unit (CPU) is the “brains” of the PLC/PAC.
 It handles all logical operations, math computations,
communications with all other hardware, manages
memory, coordinates communications between all
hardware, etc.
 When using a modular system, some manufacturers
specify a location (slot) in the rack that the processor
must be located and that there can be only on
processor in the rack.
 The AB SLC500 series specifies that the processor
module must be in slot 0 and there can be only one
processor.
 Most new systems allow the processor to be placed in
any rack location (slot) and there can be multiple
processors in the same rack.

Programming Device
 PLCs/PACs are
programmed using
either a standard
desktop or laptop
computer, or a hand
held programmer.
(Handhelds are
rarely, if ever, used
today)
 Both will also allow
for monitoring a
running program and
troubleshooting.

THE I/O INTERFACE
Input/Output (I/O)

Input/Output Interface –
Fixed I/O
 Fixed I/O – Pico/Micro/Mini
systems
 Typical of small PLCs/PACs.
 Comes in one package with
no removable units or
modules.
 The processor and I/O are
packaged together as one
unit.
 Lower in cost but lacks
flexibility.
http://www.automation.siemens.com/MCMS/PROGRAMMABLE-LOGIC-
CONTROLLER/EN/LOGIC-MODULE-LOGO/Pages/Default.aspx
Retrieved: August 16, 2011

I/O Configuration - Modular
 Modular I/O
 Consists of compartments
called slots. Separate modules
can be plugged into the slots.
 This feature greatly increases
the options and the units
flexibility. Any module intended
for use for a particular system
can be plugged into a slot. The
choice of modules can be
mixed and matched for the
application being controlled.
 Unlike earlier PLCs, many
vendors allow the processor to
be placed into any slot and/or
allows the process to be
remote. Many also allow for
more than one processor to be
in the same rack, backplane or
rail.
Each module
is placed in a
Slot or on a rail

I/O Interface
 The I/O interface provides the link between the PLC/PAC
and the field devices. The term field device is used to
indicate that the device is not a part of the PLC/PAC
hardware.
 The I/O interface also acts as a signal conditioner to
change the many different types of I/O voltages to a
0Vdc to 5Vdc level used by the PLC/PAC.
 Many different types of I/O modules are available to
match the large variety of I/O devices available and the
number of I/O devices required for an application.
 DC I/O modules can be either
 Current Sourcing (current flows out of the module) or
 Current Sinking (current flows into the module).
There will be much more on Sink/Source later

I/O Section
 Input and output (I/O) modules
is the hardware component
that allows the PLC/PAC to
sense and control actions and
processes.
 The I/O section consists of:
 A Rack, Backplane or Rail
 I/O modules
 An I/O module is installed in a
location called a slot.
 Racks, Backplanes or Rails
are available in different sizes.
They are sized by the number
of slots available.
 Slots are numbered starting
with the number zero, with slot
zero being the first module on
the left, not including the power
supply.
 In the AB SLC500 series
modular PLC, slot zero is
reserved for the processor
(CPU) module. No I/O can be
placed in this slot.
 In the AB ControlLogix series
any module can be placed in
any slot.

Input Interface
 Input devices such as switches, pushbuttons
and sensors are wired to the input interface. The
input interface consists of input modules.
 The input module converts the signals from the
input field devices to a signal that the PLC
processor can understand.
 LED status indicators are provided on input
modules that can be used to aid the
maintenance technician in troubleshooting input
malfunctions. There will be one indicator for
each input device that is wired to the input
module. A lit indicator means that the device
wired to that input point is ON or CLOSED.

Input Interface
 Instructor led demonstration of
input indicator operation using
the PLC lab training panels
and the panel I/O diagram
provided.
 Instruct the students as to how
to switch the I/O cabling
between the Siemens and
ControlLogix.
 Turn on the training panel and
exercise the input devices
while watching the LED
indicators on the input
modules of the PLC.

Output Interface
 Output devices such as relay coils, motor starter coils,
pilot lamps and solenoids are wired to the output
interface. The output interface consists of output modules.
 The output module receives signals from the PLC/PAC
processor (CPU) to turn on or off switches in the module.
These switches make or break the appropriate voltage
level required to turn ON or OFF the output field devices.
 Switches in an output module are usually a solid state device such
as an SCR, Triac, Transistor, FET, etc. Small relays are used in
some output modules, but this is the exception, not the rule and
depends upon the application.
 LED status indicators are provided on output modules to
aid the maintenance technician in troubleshooting output
device malfunctions. There will be one indicator for each
output device that is wired to the output module. A lit
indicator means that the device wired to that output point
is ON or ENERGIZED. (More about the accuracy of the
output indicators later in the course).

PLC Types
 PLC/PACs are grouped into three
operational classifications:
 Rack or address-based systems
 Tag-based systems
 Soft PLCs or PC-based systems

Rack / Address-based
Systems (Brief introduction)
 In a rack or address-based system the address, or
memory location, of the inputs and outputs in the
PLC are determined by the type of module and the
slot number in the rack where the module is located.
 As an example:
 If there is an input module in slot 1 of the rack and an input
field device wired to input 4 of that input module, the
address of the input device would be:
 There will be much more on addressing later in this
course.
AB SLC 500 ControlLogix Siemens
Automation
Direct
I:1/4 (short form)
I1:1.0/4 (long form)
tagName
<Local:1:I.Data.4>
tagName
1.4
X4

Tag Based System
(Brief Introduction)
 Many PLC/PAC’s are tag based instead of address
based.
 A tag is a friendly name for a memory location. It is the
same as a variable that is declared in high-level
programming languages such as Visual Basic .NET, C++
or Java. PLC/PAC manufacturers use the term tag
instead of variable.
 I/O are assigned tag names at design time. Later, the tag
is assigned to an I/O module and specific terminal
number (bit number).
 The tag is the only reference used when the program is
developed.
 The data type of a tag must be declared at design time.
 Much more on tags and data types later in the course.

Discrete I/O Modules
 Discrete, sometimes referred to as digital I/O
modules are used to connect devices that have two
states, ON or OFF.
 The module I/O is powered by a user supplied
power supply that is external to the PLC/PAC and
can be of different magnitudes depending upon the
application.
 Typical I/O ratings (The most typical shown in red):
Inputs
• 12VAC or (DC Sink/Source)
• 5VDC (TTL level)
Outputs
• 12-48VAC
• 5VDC (TTL level)
• 24VDC (Sink/Source)
• Relay (Any voltage within spec)

Discrete Input Module
 A typical input module consists of two sections
 Power
 Logic
 The power section conditions the input signal and provides filtering,
debouncing and signal level detection.
 The logic section processes the signal and sends it to the processor.
 The power and logic sections are optically isolated from each other.
Level
Detection
Opto-Coupler Logic
LED Status
Indicator
Input Device
Power Supply
To Processor
Via Backplane
Power Section Logic SectionElectical Isolation

Input Interface
Terminal
Block 1
0
1
2
3
4
5
6
7
Input Devices
L1
L1
L2
PLC
CPU
L2
Input Image
Register or Input Data table
consisting of binary word(s)
of 1’s and 0’s.
(Part of the logic circuit. More on
this later)
0
0
0
1
0
0
0
0
Input Module
LED Status Indicators. Lit
means device closed or on

Input Modules
 Input modules perform four tasks in the
PLC/PAC control system:
 Sense when a signal is received from a
sensor on the machine or process.
 Convert the input signal to the correct voltage
level for the particular PLC/PAC.
 Isolate the PLC/PAC from fluctuations in the
input signal’s voltage or current.
 Send a signal to the processor indicating
which sensor originated the signal.

Types of Discrete Output
Modules
 Discrete (Digital) output modules are
used to control devices that have two
states; ON or OFF.
 Most AC modules use Triacs as the output
switch. Triacs can only be used for switching
AC.
 DC modules can use Transistors or Field
Effect Transistors (FETs) as output switches.
 Relay outputs can be used to switch AC or
DC. The disadvantage to relays is that they
have slower switching times than solid state
devices.
Triac
Transistor
Relay

Discrete Output Module
 A typical output module consists of two sections
 Logic
 Power
 The logic section is used to determine the output status.
 The power section is an electronic switch used to control the output
field device.
 The power and logic sections are optically isolated from each other.
Electronic or
Relay Switch
Opto-Coupler
From Processor
Via Backplane
Output Device
Power Supply
Logic Section Power SectionElectical Isolation
Logic
LED Status
Indicator

Output Module
Terminal
Block1
0
1
2
3
4
5
6
7
PLC
CPU
L1
Output Devices
L2
L2
L1
0
0
0
1
0
0
0
0
Output Module
Output Image
Register or Output Data table
consisting of binary word(s)
of 1’s and 0’s.
(Part of the logic circuit. More on
this later)
L2 L2
LED Status Indicators. Lit means
device energized or on

Output Modules
 Individual output point currents are limited by the electronic
switch in the output module.
 Triac for AC
 Transistor, field effect transistor (FET) or equiv. for DC
 Relay for AC or DC
 Output current is usually limited, 1A to 2A per output point.
 The module will also have a maximum current rating for the
entire module. As an example:
 A module has 16-output points, each point is rated at 500mA.
 The maximum current rating of the entire module is rated at 4A.
 Therefore only 8-outputs can be on any given time:
 Outputs should be individually fused according to the
amount of current being drawn by the output.
 Some modules have blown fuse indicators.
outputperCurrentcurrentodmMaxONoutputsofNumber /..

Controlling High Current
Output Loads
 Use an interposing relay when the PLC/PAC needs to
control a load that has a higher current rating then the
maximum rating of a single output of the module.

DC Sink/Source I/O
Modules
 PLC/PAC modules use solid state devices and therefore
require current to be flowing in the proper direction.
 The terms Sink and Source are used to describe the
direction of current flow and are derived from conventional
current flow.
 Reminder:
 The current that is read on a current meter is electron flow. Electron flow
is from negative to positive.
 Conventional current is the current flow that solid state engineers and
physicists use to describe circuit operation. Conventional current flows
from positive to negative. It is sometimes referred to as “hole flow”.

DC Sink Input Connections
 A Sink module provides the I/O
power supply common (return)
path.
 In this diagram the input
module is of type Sink. A Sink
module has at least one
terminal wired to the DCV
Common (return).
 The input devices are then
wired as type Source.
 A Source device supplies the
voltage/current to the module.
 In this diagram the input device
is of type Source. At least one
terminal of a Source device is
wired to the positive DCV.
 Sink modules can only have
Source devices wired to them.
LS1
Input Module+DCV
+DCV
Common
Opto-
coupler
Conventional
Current Flow
Conventional
Current Flow

DC Source Input
Connections
 A Source module supplies the
voltage/current to the I/O field
device.
 In this diagram the input module
is of type Source. A Source
module has at least one
terminal wired to the positive
DCV. (Some source modules will also
have a DCV Common connection as
shown in this diagram).
 The input devices are then
wired as type Sink.
 A Sink device provides the DCV
Common (return).
 In this diagram the input device
is of type Sink. At least one
terminal of a sink device is wired
to the DCV Common (return).
 Source modules can only have
Sink devices wired to them.
LS1
Input Module
+DCV
+DCV
Common
Opto-
coupler
Conventional
Current Flow
Conventional
Current Flow
Conventional
Current Flow

DC Sink Output
Connections
 A Sink module provides the I/O
power supply common (return)
path.
 In this diagram the output
module is of type Sink. A Sink
module has a least one
terminal wired to the DCV
Common (return).
 The output field devices are
then wired as type Source.
 A Source device supplies the
voltage/current.
 In this diagram the output I/O
device is of type Source. At
least one terminal of a source
device is wired to the positive
DCV.
 Sink modules can only have
Source devices wired to them.
PL1
Output Module
+DCV
Common
+DCV
Conventional
Current Flow
Conventional
Current Flow

DC Source Output
Connections
 A Source module supplies the
voltage/current to an I/O field
device.
module is of type source. A
Source device has at least one
terminal wired to the positive
DCV. (Some Source modules will
also have a DCV Common connection
as shown in this diagram).
 The output devices are then
wired as type Sink.
device is of type Sink. At least
one terminal of a Sink device
is wired to the DCV Common
(return).
 Source modules can only have
Sink devices wired to them.
PL1
Output Module
+DCV
Common
+DCV
Conventional
Current Flow
Conventional
Current Flow
Conventional
Current Flow

Examples of Sink/Source
Input Module Wiring

Examples of Sink/Source
Output Module Wiring

Sink/Source I/O
Modules
 Classroom discussion questions:
 What is the most common polarity used for
input modules (Sink or Source) and why?
 What is the most common polarity used for
output modules (Sink or Source) and why?

Special I/O Modules
 PLC/PAC manufacturers and 3rd party vendors
have developed modules that serve to meet
several specialized needs. A sample of these
modules are:
• High-speed counter modules
• Thumbwheel module (rarely used)
• TTL module
• Encoder-Counter module
• Step-motor module
• BCD-output module (rarely used)
• Servo control module
• Communication modules
• DeviceNet
• ControlNet
• Ethernet/IP
•Modbus
•Etc.
• Basic or ASCII module
• Thermocouple module
• SERCOS
• Smart I/O

I/O Module Specifications
 PLC/PAC manufacturers provide electrical and
mechanical specifications for the safe use of I/O
modules. Some of these specifications are:
• Nominal input voltage
• On-State input voltage range
• Nominal current per input
• Ambient temperature range
• Input delay (response time)
• Nominal output voltage
• Output voltage range
• Maximum output current rating
per output and module
• Maximum surge current per
output
• Off-state leakage current per
output
• Electrical isolation
• Number of inputs and outputs
per card
• Backplane current draw
• Resolution
• Input impedance and
capacitance
• Common mode rejection ratio

I/O Specification Details
 Nominal Input Voltage
 Specifies the AC or DC magnitude and type of voltage signal
that the module will accept.
 On-State Input Voltage Rating
 Specifies the voltage at which an input module recognizes an
input device as being ON.
 Maintenance personnel need to know this value when
troubleshooting input device problems. When measuring the
amount of voltage at an input point of an input module it must be
known if the voltage being measured is enough to tell the
module that the input device is ON.
 Nominal Current per Input
 Specifies the nominal current that an input device must be
capable of supplying to operate the input modules input circuitry.

 Nominal Output Voltage
 Specifies the AC or DC magnitude and type of
voltage that can be controlled by an output module.
 Output Voltage Range
 Specifies the minimum and maximum output
operating voltages. As an example:
 An output rated a 120VAC might have an absolute working
range of 92VAC to 138VAC.
 An output rated at 24VDC might have an absolute working
range of 10VDC to 50VDC (typical).
 Off-State Leakage Current
 Specifies the maximum amount of leakage current
that is allowed to flow through a device when the
device is considered in its OFF state.

 Maximum Output Current Range per Output
Module
 Specifies the maximum current that a single output
point of an output module can safely supply and the
maximum current that the entire output module can
safely supply at the rated voltage. As an example:
 A 16-point output module has a rating of 500mA (0.5A) per
output.
 The output module total current capability is 6000mA (6.0A).
 Assume that 16-devices are wired to this module, each
requiring 500mA of current.
 This means that only 12-output devices can be on at any one
time or the total capability of the output module will be
exceeded.
OutputperCurrentCurrentTotalModuleONOutputsofNumber /

 Number of Inputs and Outputs per Module
 Specifies the number of input and/or output field devices that can
be connected to a module.
 Some modules provide more than one common terminal and/or
more than one voltage terminal. This allows the connection of
devices that use different voltage ranges on the same module.
 Backplane Current Draw
 Specifies the amount of current a module requires from the
backplane.
 The sum of the backplane current draw from all the modules that
are plugged into a rack should not exceed the maximum rating of
the rack power supply.
 Some modules have a backplane current draw value specified
for +5VDC and +24VDC. In this case all +5VDC currents must
be summed and all +24VDC currents must be summed and
checked against the ratings of the rack power supply.

Selecting Input Modules
Example
 Example 1-1 Selecting Input Modules
 An application has the following input field devices
that must be interfaced to the SLC500 PLC. For each
combination of field devices select the appropriate
combination of input modules. Although figure 1-12 in
your textbook shows the SLC500 discrete I/O
modules, use the appropriate file from the CD that
came with your textbook or use the Internet.
 I/O devices
 (17) 120VAC inputs
 (5) 5VDC input devices are Sinking, which require a
Sourcing module
 (3) 24VDC inputs devices are Sinking, which require a
Sourcing module.
The SLC500 Discrete I/O PDF

Example 1-1 Solution
 Use:
 (1) 1746-IA16 AC module with 16-inputs
 (1) 1746-IA4 AC module with 4-inputs
 (1) 1746-IG16 5V TTL Sourcing module with
16-I/O points
 (1) 1746-IV8 or 1746-IV16 Sourcing module
with 8-I/O points
There can be other combinations of modules
that will also work.

Selecting Output Modules
Example
 Example 1-2 Selecting Output Modules
 An application has the following output field devices
to be interfaced to an SLC500 PLC. For each
combination of field devices select the appropriate
combination of output modules
 Output devices:
 (10) 90VAC output devices that require isolation.
 (21) 12 to 20VDC current sinking output field
devices
 (28) 28VDC pneumatic valves
 (10) 24VDC control signals used to control a CNC
machine that has sourcing inputs
 (3) 120VAC motors with a continuous current rating <1.0A
and starting surge current of <10.0A

Example 1-2 Solution
 Use:
 (2) 1746-OX8 8-relay output
 (1) 1746-OB32 32-Sourcing DC output
 (1) 1746-OV16 16-Sinking DC output
 (1) 1746-OAP12 High current AC output
There can be other combinations of modules
that will also work.

Ladder Logic Programming
 Relay Ladder Logic (RLL) has been the
preferred programming language for PLCs since
their inception. (See the history of PLCs in
appendix C of your textbook.
 Converting the tank control logic introduced
earlier shows that the same I/O field devices are
required, only now they are wired to the
PLC/PAC I/O interface. Input field devices are
wired to input modules and output field devices
are wired to output modules.
 The RLL is then created in the PLC/PAC
memory.

Re-examine This Process
Control Drawing Pump Motor Control
M1 M1
A
Control
Relay
Solenoid
Valve
Contacts
for coils
Thermal
Overload
Coil OLA2
A1
Off on
Off on
Electronic Schematic
Pump motor
control switch
Valve
control
switch
Relay
Pump
motor
Solenoid
valve
A1
A2
A
Process Tank
Valve
(on-off)
Pump
Let’s convert this to
PLC/PAC RLL.

PLC/PAC Solution to Tank
Control
Input field
devices are wired
to the PLC input
module.
Output field
devices are wired
to the PLC input
module.
User supplied
power supply.
Usually the same
supply.
PLC ladder logic. The
mechanical control
relay is not needed
because a virtual relay
can be used.
--| |-- is a symbol used
to represent any NO
discrete input device.
--( )-- is a symbol used
to represent any
discrete output device.
Pseudo addressing is shown

Alternate Solution to Tank
Control
In this solution the virtual
relay is not used. The
valve solenoid is
controlled directly by the
Valve control switch. In
the electromechanical
solution S1 (I1) is
controlling two output
devices therefore a DPDT
switch was needed. In the
PLC solution a SPST
switch can be used and
programmed twice.
Pseudo addressing is shown

01 introduction to_plc-pac_rev01_fa16

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