PLC Selection, Components, and Communication

Chapter

2PLC Selection, Components,

and Communication

Permission granted to reproduce for educational use only.© Goodheart-Willcox Co., Inc.

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.


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.


PLC Selection

• Factors:– Manufacturer’s support.– Serviceability.– Flexibility.– Expandability.– Programming software.– Training.– Documentation.


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.


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.


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.)


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.


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.


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.


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.


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.


Processor

• Allen-Bradley SLC 500 modular processors:


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.


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.


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.


I/O Modules (Cont.)


Wiring Connection Diagram


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.


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.


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.


Sourcing

• Sourcing occurs when the current flows out of the PLC port.


Sinking

• Sinking occurs when the current flows into the PLC port.


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.


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.


PLC Memory Expansion and Usage (Cont.)


PLC Communication Systems

• PLCs communicate with:– Industrial input and output devices.– Peripheral control components.

• PCs.• Printers.• Display panels.• Peripheral interfaces.• Programming devices.


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.



• 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.



• Network communications:– DH-485.– DH+.– Ethernet.



• 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.



• Two types of DF1 communications:– Full-duplex.– Half-duplex.



• 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.


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.


Serial Communication

• Serial communication data:– Transmitted one bit at a time through one

transmission medium.– Can be framed prior to transmission.


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.


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.


Framing (Cont.)

• Framing bits include:– Start bit.– Stop bit.– Parity bit.


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.


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.


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.


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.


Channeling Systems

• Three ways to channel communication: – Simplex.– Half duplex.– Full duplex.


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.



• Half duplex channeling:– DTE transmits data to and receives data from

the DCE. – Transmission and reception of data cannot occur

simultaneously.



• 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.


Programming Module (PM)

• Three different types of PLC programming modules:– Hand-held.– Dedicated.– Personal computer.


Hand-held Programmer

• Typically used for programming smaller PLC devices.

• After programming the PLC, these temporary programming modules are disconnected from the system.



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.



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.


Communication Settings

• To prevent unauthorized personnel from running or altering a PLC program, most PLCs have different hardware and software communication settings.


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.


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.


Forced Mode

PLC Selection, Components, and Communication

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