plc selection, components, and communication
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter
2PLC Selection, Components,
and Communication
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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 Allen-Bradley SLC 500 PLCs.
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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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PLC Selection
• Factors:– Manufacturer’s support.– Serviceability.– Flexibility.– Expandability.– Programming software.– Training.– Documentation.
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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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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.
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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.)
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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.
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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.
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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.
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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.
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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.
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Processor
• Allen-Bradley SLC 500 modular processors:
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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.
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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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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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I/O Modules (Cont.)
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Wiring Connection Diagram
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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.
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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.
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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.
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Sourcing
• Sourcing occurs when the current flows out of the PLC port.
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Sinking
• Sinking occurs when the current flows into the PLC port.
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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.
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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.
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PLC Memory Expansion and Usage (Cont.)
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PLC Communication Systems
• PLCs communicate with:– Industrial input and output devices.– Peripheral control components.
• PCs.• Printers.• Display panels.• Peripheral interfaces.• Programming devices.
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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.
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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.
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PLC Communication Systems (Cont.)
• Network communications:– DH-485.– DH+.– Ethernet.
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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.
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PLC Communication Systems (Cont.)
• Two types of DF1 communications:– Full-duplex.– Half-duplex.
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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.
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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.
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Serial Communication
• Serial communication data:– Transmitted one bit at a time through one
transmission medium.– Can be framed prior to transmission.
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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.
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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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Framing (Cont.)
• Framing bits include:– Start bit.– Stop bit.– Parity bit.
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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.
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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.
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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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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.
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Channeling Systems
• Three ways to channel communication: – Simplex.– Half duplex.– Full duplex.
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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.
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Channeling Systems (Cont.)
• Half duplex channeling:– DTE transmits data to and receives data from
the DCE. – Transmission and reception of data cannot occur
simultaneously.
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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.
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Programming Module (PM)
• Three different types of PLC programming modules:– Hand-held.– Dedicated.– Personal computer.
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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.)
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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.)
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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.
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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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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.
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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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Forced Mode