2. PLC Selection, Components, and Communication2
PLC Selection, Components, and Communication

3. 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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4. 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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5. PLC Selection Factors: Manufacturer’s support. Serviceability.
Flexibility.
Expandability.
Programming software.
Training.
Documentation.
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6. 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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7. 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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8. 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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9. 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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10. 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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11. 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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12. 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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13. 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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14. Processor Allen-Bradley SLC 500 modular processors:
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15. 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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16. 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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17. 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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18. I/O Modules (Cont.)
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19. Wiring Connection Diagram
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20. 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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21. 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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22. 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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23. Sourcing Sourcing occurs when the current flows out of the PLC port.
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24. Sinking Sinking occurs when the current flows into the PLC port.
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25. 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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26. 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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27. PLC Memory Expansion and Usage (Cont.)
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28. 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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29. 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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30. 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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31. PLC Communication Systems (Cont.)
Network communications:
DH-485.
DH+.
Ethernet.
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32. 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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33. PLC Communication Systems (Cont.)
Two types of DF1 communications:
Full-duplex.
Half-duplex.
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34. 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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35. 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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36. 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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37. 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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38. 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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39. Framing (Cont.) Framing bits include: Start bit. Stop bit. Parity bit.
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40. Parity Parity bit: Parity bit check:
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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41. Parity (Cont.) Even parity check. Odd parity check. No 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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42. 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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43. 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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44. Channeling Systems Three ways to channel communication: Simplex.
Half duplex.
Full duplex.
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45. 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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46. 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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47. 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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48. Programming Module (PM)
Three different types of PLC programming modules:
Hand-held.
Dedicated.
Personal computer.
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49. 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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50. 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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51. 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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52. 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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53. 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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54. 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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55. Forced Mode
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56. Glossary
Baud (Bd) rate: A unit for measuring the speed of serial communication transmission. Baud rate measures the bits per second (bps) that are sent or received.
Chassis: A rack that serves as an electrical backplane for a PLC processor and I/O modules.
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57. Glossary
Data communication equipment (DCE): Respond to the messages transmitted by the DTE.
Data terminal equipment (DTE): Initiate communication by sending messages.
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58. Glossary
Full duplex: Channeling system that uses two lines. Data can be transmitted on one line and received on the other simultaneously.
Half duplex: Channeling system in which the data is transmitted and received on one line, but not simultaneously.
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59. Glossary
Programming module (PM): A peripheral device used to transfer ladder diagrams to the PLC.
Serial communication: Communication where data is transmitted one bit at a time through one transmission medium.
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60. Glossary
Simplex: A channeling system that uses a single line and communication is unidirectional.
Sinking: Connecting a device to a PLC I/O module so that current flows into the PLC port.
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61. Glossary
Sourcing: Connecting a device to a PLC I/O module so that current flows out of the PLC port.
System memory: Memory that stores information required for user program execution.
User memory: Memory that holds the PLC ladder logic diagram.
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