Controls and Instrumentations ENGR 363
What are the major parts of a plc?
The CPU, I/O, power supply, and programming device
Architecture
Refers to PLC hardware or software
Open architecture
Allows the system to be connected easily to devices and programs made by other manufacturers
Closed architecture
A system whose design is proprietary making it difficult to connect to other systems
Fixed I/O
Small PLC that comes in one package with not separate removable units. The processor and I/O are packaged together
Modular I/O
A PLC that is divided by compartments into which separate modules can be plugged
Power supply
Provides DC power to all modules that plug into the rack
CPU or processor
A microprocessor for implementing the logic and controlling the communications among the modules
PLC scan cycle
Reads inputs -> executes program -> diagnostics & communication -> updates outputs
I/O (Input/Output) system
Forms the interface by which field devices are connected to the controller.
I/O (Input/Output) system
Forms the interface by which field devices are connected to the controller
Programming device
Used to enter the desired program into the memory of the processor
Personal computer (PC)
The most commonly used programming device
Program
A series of instructions that directs the PLC to execute actions
Relay ladder logic
The standard programming language, is based on electromagnetic relay control
Address
Where an input or output device is conncected
PC vs PLC
PLC is designed to operate in the industrial environment with wide ranges of ambient temperature and humidity
Human machine interface (HMI)
Software that allows the user to monitor and control the process
Programmable Automation Controllers (PACs)
Combine PLC ruggedness with PC
functionality
Nano
Smallest size with less than 15 I/O
Micro
15-128 I/O
Medium
128-512 I/O
Large
Over 512 I/O
Single ended application
One PLC controlling one process
Multitask application
One PLC controlling several processes
Control management application
One PLC controlling several others
Memory
Part of a PLC controller that stores data, instructions, and the control program
Memory size
Expressed in K values
1K =
1024
Amount of memory required depends on
• Number of I/O points
used
• Size of control
program
• Data-collecting
requirements
• Supervisory functions
required
• Future expansion
Input interface modules
accept signals from the
machine or process devices and convert them into
signals that can be used by the controller
Output interface modules
convert controller signals
into external signals used to control the machine or
process
chassis
hardware assembly that houses I/O modules,
processor modules, and power supplies
logical rack
an addressable unit consisting of 128
input points and 128 output points
remote rack
A rack that is located away from the processor module
address
a label or number that indicates where
a certain piece of information is located in a PLC's
memory
Rack/slot-based addressing schemes
used with
Allen-Bradley PLC-5 and SLC 500 controllers
Tag-based addressing
used with Allen-Bradley
ControlLogix controllers
Combination I/O
modules
can
have both input
and output
connections in
the same
physical module
points
refers to the number of inputs or outputs
available
discrete I/O
interface module
connects
field input
devices of the
ON/OFF nature
discrete I/O module is powered by
some field supplied voltage source
Backplane power
Used to power the electronics that
reside on the I/O module circuit board
optical isolator
used to provide electrical isolation
between the power and logic circuitry
Interposing relay
used
to control a larger load
or motor starter
Output modules can be purchased with
Transistor,triac, or relay output
If the module is a current-sourcing module then the
input or output device must be a
current-sinking device
If the module is a current-sinking module,
then the input or output device must be a
current-sourcing device
The two basic types of analog input modules are
voltage sensing and current sensing
Bipolar input modules
Accept signals that swing
between a negative and positive value
Unipolar input modules
Accept an input signal
that varies in the positive direction only
Resolution refers to
the smallest change in input
signal value that can be sensed
The main element of the analog
output module is an
digital-to-analog (D/A) converter
analog output module
converts digital signal into
a proportional voltage or current to control an
analog field device
High-speed counter modules
used to count
pulses from sensors, encoders, and switches
that operate at very high speeds
TTL module
allows the