Download Programmable Logic Controllers - Goodheart

Chapter
2
PLC Selection, Components,
and Communication
Objectives
• Identify important questions to determine
which PLC device to purchase.
• Distinguish between Allen-Bradley fixed and
modular SLC 500 processors.
• Select modular Allen-Bradley SLC 500
series processor and chassis.
• Select input or output modules for AllenBradley SLC 500 PLCs.
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Permission granted to reproduce for educational use only.
Objectives
• Recognize how to connect sensors and
switches to PLC ports.
• Describe the different types of PLC memory.
• Describe the method to connect PLC devices
to programming modules.
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Permission granted to reproduce for educational use only.
PLC Selection
• Factors:
– Manufacturer’s support.
– Serviceability.
– Flexibility.
– Expandability.
– Programming software.
– Training.
– Documentation.
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Permission granted to reproduce for educational use only.
Allen-Bradley SLC 500
Components
• For Allen-Bradley SLC 500 PLC, select
either:
– Fixed PLC.
– Modular PLC.
• In a single housing, a fixed PLC has
– the processor (CPU).
– power supply.
– I/O modules.
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Permission granted to reproduce for educational use only.
Allen-Bradley SLC 500
Components (Cont.)
• Modular Allen-Bradley SLC 500 PLCs:
– Chassis.
– Power supply.
– Processor.
– I/O modules.
• Allen-Bradley refers to the central
processing unit (CPU) as processor.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Chassis
• In an Allen-Bradley
SLC 500 modular
PLC, provides
electrical current to:
– Processor.
– Input module.
– Output modules.
(Used with permission of Rockwell Automation, Inc.)
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Chassis (Cont.)
• Four chassis sizes for the modular SLC 500
series processors:
–
–
–
–
Four-slot.
Seven-slot.
Ten-slot.
Thirteen-slot.
• Number of slots can be expanded to a
maximum of 30 by interconnecting up to
three chassis using cables.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Chassis (Cont.)
• When installing the PLC components
in a chassis:
– Processor must be placed in slot zero.
– Input and output modules can be in any
other slot.
– Slot zero in the modular PLC is reserved
for the processor.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Chassis (Cont.)
• A complete unit in an Allen-Bradley
SLC 500 fixed PLC includes:
– Power supply.
– Processor.
– I/O modules.
• An I/0 module is referred to as Module
Zero.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Chassis (Cont.)
• I/O port types and voltage or current
rating cannot be altered.
• However, a two-slot chassis can be
added so two more I/O modules can
be installed.
– Fixed PLC power supply provides power.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Power Supply
• Provides power to the chassis
backplane, and thus to the processor,
modules, and peripherals attached to
the chassis.
• Must be able to provide ample current
to these components.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Processor
• Allen-Bradley SLC
500 modular
processors:
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Permission granted to reproduce for educational use only.
I/O Modules
• Allen-Bradley SLC 500 series
modules:
– Digital (discrete) I/O modules.
– Analog (variable) I/O modules.
– Specialty I/O modules.
– Distributed or networking I/O modules.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
I/O Modules (Cont.)
• Each input or output module has
several ports, or terminals.
• For example, input module one may
have sixteen ports, and output module
two may have eight ports.
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Permission granted to reproduce for educational use only.
I/O Modules (Cont.)
• Ports on the modules:
– Specified when they are specifically used
in a ladder logic diagram.
– Addressed by their module and port
number.
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Permission granted to reproduce for educational use only.
I/O Modules (Cont.)
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Permission granted to reproduce for educational use only.
Wiring Connection Diagram
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Permission granted to reproduce for educational use only.
Wiring
• PLC I/O ports:
– Typically rated for low amperage.
– Ordinarily connected to I/O devices
through American Wire Gauge size
fourteen (AWG 14).
• AWG 14 wire has more than sufficient
ampere rating for carrying current for
the PLC I/O ports.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Wiring (Cont.)
• In most applications, the output devices
require larger voltage and current
ratings.
• A magnetic relay should be used to
connect larger voltage and current
equipment to the output device.
• PLC output port provides voltage and
current to the input coil of the magnetic
relay.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Sourcing and Sinking
• The way current flows through the
input or output module DC circuit.
• DC I/O modules labeled as sourcing or
sinking.
• Sourcing device always attaches to a
sinking I/O module.
• Sinking device always attaches to a
sourcing I/O module.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Sourcing
• Sourcing occurs
when the current
flows out of the
PLC port.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Sinking
• Sinking occurs
when the current
flows into the PLC
port.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
PLC Memory Expansion and
Usage
• User memory:
– Holds the ladder logic program.
– About 75% of the total memory.
• System memory:
– Stores information needed to execute the
user program.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
PLC Memory Expansion and
Usage (Cont.)
• Input status: holds the status of discrete input ports.
• Output status: holds the status of discrete output
ports.
• Timers’ status: holds the preset values and the
accumulated values of the timers.
• Counters’ status: holds the preset values and the
accumulated values of the counters.
• Math and logic instructions’ status: used for
instructions such as addition, subtraction,
multiplication, division, sequencer, shift registers, and
comparison.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
PLC Memory Expansion and
Usage (Cont.)
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Permission granted to reproduce for educational use only.
PLC Communication
Systems
• PLCs communicate with:
– Industrial input and output devices.
– Peripheral control components.
•
•
•
•
•
PCs.
Printers.
Display panels.
Peripheral interfaces.
Programming devices.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
PLC Communication
Systems (Cont.)
• Allen-Bradley SLC 500 PLCs:
– Channel 0 is used to attach peripheral
devices.
– Channel 1 is used to communicate with
other PLCs and PCs.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
PLC Communication
Systems (Cont.)
• All of the Allen-Bradley SLC 500 PLCs have
a Channel 1, but only the SLC 5/03, 5/04,
and 5/05 also have a Channel 0.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
PLC Communication
Systems (Cont.)
• Network communications:
– DH-485.
– DH+.
– Ethernet.
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Permission granted to reproduce for educational use only.
PLC Communication
Systems (Cont.)
• DF1:
– Provides serial communications through
an RS-232 interface.
– Can be used for remote communications
through a modem or radio modem,
monitoring PLC status, and programming.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
PLC Communication
Systems (Cont.)
• Two types of DF1 communications:
– Full-duplex.
– Half-duplex.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
PLC Communication
Systems (Cont.)
• Full-duplex:
– DF1 allows for two-way communications
between two devices.
• Half-duplex:
– DF1 allows for communications in one
direction at a time in a network of up to
255 devices.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
ASCII
• Allows for communications with other
devices that use the ASCII protocol.
– Serial printers.
– Bar code readers.
• Takes place over the RS-232 interface
or connector.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Serial Communication
• Serial communication data:
– Transmitted one bit at a time through one
transmission medium.
– Can be framed prior to transmission.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Baud (Bd) Rate
• Includes framing bits.
• A PLC communicates with its peripheral
devices at different baud rates.
– A PLC may operate at a rate of 19,200 Bd
with the CPU and 2400 Bd with a printer.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Framing
• Process of indicating the beginning
and ending of a transmission.
• Ensures each byte of data is the
correct transmission of data.
• Device receiving the information is
alerted when data transmission starts
and ends.
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Permission granted to reproduce for educational use only.
Framing (Cont.)
• Framing bits include:
– Start bit.
– Stop bit.
– Parity bit.
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Permission granted to reproduce for educational use only.
Parity
• Parity bit:
– Turned on or off to make transmitted data
to have either an odd number of ones
(i.e., odd parity) or an even number of
ones (i.e., even parity).
• Parity bit check:
– Used to find transmission errors.
• Errors come in the form of lost or changed
bits.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Parity (Cont.)
• Even parity check.
– Parity bit turns on or off to ensure the
number of high bits in one byte (8 bits) of
transmitted data is even.
• Odd parity check.
– Parity bit turns on or off to ensure the
number of high bits in one byte of
transmitted data is odd.
• No parity check.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
DTE and DCE
• Data terminal equipment (DTE):
– Initiate communication by sending messages.
• Device that initiates communication is also referred to
as a master or initiator.
• Computers or master PLCs.
• Data communication equipment (DCE):
– Respond to the messages transmitted by the
DTE.
• Device that responds to transmitted messages can
also be referred to as a slave or responder.
• Modems or slave PLCs.
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Permission granted to reproduce for educational use only.
DTE and DCE (Cont.)
• Single master/multiple slave network:
– Network with one initiator and several
responders.
• Multi-master network:
– Network with more than one initiator.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Channeling Systems
• Three ways to
channel
communication:
– Simplex.
– Half duplex.
– Full duplex.
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Permission granted to reproduce for educational use only.
Channeling Systems (Cont.)
• Simplex channeling:
– When the DTE either transmits data to or
receives data from the DCE.
– Will not do both.
– Data flows only in one direction.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Channeling Systems (Cont.)
• Half duplex channeling:
– DTE transmits data to and receives data from
the DCE.
– Transmission and reception of data cannot occur
simultaneously.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Channeling Systems (Cont.)
• Full duplex channeling:
– DTE transmits data to and receives data from
the DCE through two separate transmission
lines.
– Data flow to and from DTE and DCE is
simultaneous.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Programming Module (PM)
• Three different types of PLC
programming modules:
– Hand-held.
– Dedicated.
– Personal computer.
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Permission granted to reproduce for educational use only.
Hand-held Programmer
• Typically used for
programming smaller
PLC devices.
• After programming the
PLC, these temporary
programming modules
are disconnected from
the system.
(Used with permission of Rockwell Automation, Inc.)
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Dedicated Programmer
Module
• Used for larger PLC
systems.
• Permanently left in
the PLC circuit.
• Maintenance
personnel can use
it to modify or
troubleshoot the
PLC programs.
(Used with permission of Rockwell Automation, Inc.)
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Personal Computer (PC) or
Laptop Computer
• Can be used to
program a PLC.
• A device that
bridges
communication
between two
different interfaces
must be used
between the PC or
laptop and the PLC.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Communication Settings
• To prevent unauthorized personnel
from running or altering a PLC
program, most PLCs have different
hardware and software communication
settings.
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Permission granted to reproduce for educational use only.
Common Communication
Settings
– Off: System cannot run or be programmed.
– Offline program: PLC’s program can be
modified while the system is not running.
– Disable: Turns all the PLC outputs off or
sets them to the inoperable state.
– Run: System can run, but its program
cannot be altered.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Common Communication
Settings (Cont.)
– Online Monitor/Run: Turns on to display
the PLC program while the PLC is running.
– Forced: Allows system to run and allows
the program to be modified while the
system is running.
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Permission granted to reproduce for educational use only.
Forced Mode
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Permission granted to reproduce for educational use only.