PLC Selection, Components, and Communication

2. 2 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.

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.

5. PLC Selection • Factors: • Manufacturer’s support. • Serviceability. • Flexibility. • Expandability. • Programming software. • Training. • Documentation.

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.

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.

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.)

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.

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.

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.

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.

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.

14. Processor • Allen-Bradley SLC 500 modular processors:

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.

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.

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.

18. I/O Modules (Cont.)

19. Wiring Connection Diagram

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.

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.

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.

23. Sourcing • Sourcingoccurs when the current flows out of the PLC port.

24. Sinking • Sinkingoccurs when the current flows into the PLC port.

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.

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.

27. PLC Memory Expansion and Usage (Cont.)

28. PLC Communication Systems • PLCs communicate with: • Industrial input and output devices. • Peripheral control components. • PCs. • Printers. • Display panels. • Peripheral interfaces. • Programming devices.

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.

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.

31. PLC Communication Systems (Cont.) • Network communications: • DH-485. • DH+. • Ethernet.

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.

33. PLC Communication Systems (Cont.) • Two types of DF1 communications: • Full-duplex. • Half-duplex.

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.

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.

36. Serial Communication • Serial communication data: • Transmitted one bit at a time through one transmission medium. • Can be framed prior to transmission.

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.

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.

39. Framing (Cont.) • Framing bits include: • Start bit. • Stop bit. • Parity bit.

40. 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.

41. 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.

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.

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.

44. Three ways to channel communication: Simplex. Half duplex. Full duplex. Channeling Systems

45. Channeling Systems (Cont.) • Simplexchanneling: • When the DTE either transmits data to or receives data from the DCE. • Will not do both. • Data flows only in one direction.

46. Channeling Systems (Cont.) • Half duplexchanneling: • DTE transmits data to and receives data from the DCE. • Transmission and reception of data cannot occur simultaneously.

47. Channeling Systems (Cont.) • Full duplexchanneling: • 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.

48. Programming Module (PM) • Three different types of PLC programming modules: • Hand-held. • Dedicated. • Personal computer.

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.)

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.)