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PLC Processor: Function, Operating Modes, and Watchdog Timer

This chapter discusses the function of the PLC processor, its operating modes, and the importance of the watchdog timer. It also covers the processor's scan cycle and communication capabilities.

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PLC Processor: Function, Operating Modes, and Watchdog Timer

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  1. Chapter 10 PLC Processors

  2. Objectives • Describe the function of the processor. • Describe processor operating modes. • Explain the function of the watchdog timer.

  3. The PLC Processor • The processor is a digital computer that reads inputs, solves the ladder logic, and updates the output field devices. • Processors are available with differing capabilities within a specific PLC family. • Processors have different operating modes.

  4. Processor Operating Cycle • Processor is designed to perform specific duties in a specific sequence. • This operating cycle is called the processor’s scan or sweep. • In run mode, the operating sequence is continuous. • Operating cycle consists of numerous steps.

  5. Input Scan • During the input scan, the processor scans each input module for the ON or OFF states of each input point. • Input status information is stored in the input status file.

  6. Program Scan (1 of 2) • Processor scans ladder program starting at rung zero at the left power rail, working left to right, evaluating one instruction at a time until the output instruction is reached. • The output instruction will be either true or false.

  7. Program Scan (2 of 2) • A logical 1 or 0 will be placed in the output status table address referenced by the particular output instruction. • With a rung completed, the processor goes on to the next rung, sequentially until the last rung of the program has been executed.

  8. Output Scan • The output scan is where the processor writes the ON or OFF status, one word at a time, to the associated output module. • Output module updates each field device’s status.

  9. Service Communications • After all output modules have been updated, the processor services communications. • Updates hand-held or personal computer monitor screens • Sends communications to other devices on the network

  10. Reset Watch-Dog Timer • Last part of scan is to reset the watch-dog timer. • Watch-dog timer ensures current scan is complete. • When watch-dog timer is reset, the scan starts over.

  11. Serial Communications • The simplest method to communicate between a PLC processor and a computer is serial communications. • Connect proper serial cable between computer’s COM port and processor’s RS-232 communications port. • Not all processors have RS-232 communications port.

  12. Selecting Proper RS-232 Interface Cable • Most serial communication problems between computer and PLC are due to incorrect cable. • Two types of serial cables • Straight-through cable • Null-modem cable

  13. Straight-through Cabling Between Computer and Peripheral Equipment

  14. Straight-through Serial Cable • In straight-through serial cable, all wires go through the cable without crossing. • Wires are identified as pin numbers. • Pin 2 goes straight through to pin 2. • Pin 3 goes straight through to pin 3. • Used when there is an intermediate communication device between two computers.

  15. Directly Connecting Computers with a Straight-through Cable • Both computers sending information out on pin 2 • Computers expecting information on pin 3 • Communication will not be successful.

  16. Connecting Two Computers with a Straight-through Cable

  17. Null-modem Cable Wired so Pin 2 Crosses to Pin 3 • Pin 2 physically crosses to pin 3 in the cable. • Information sent from the computer crosses from pin 2 to pin 3 so it can be received by other computer device. • Communication is successful.

  18. Null-Modem Cable Crosses Pin 2 to Pin 3

  19. PLC Processors Have Multiple Operating Modes • Processor has two modes of operation. • Program mode • Some variation of run mode

  20. Program Mode • In program mode, the processor is accepting new instructions, either as a new program or as changes (edits) to an existing program.

  21. Run Mode • When the creating or editing of a program is complete, the processor is put into run mode to run or execute the program. • In run mode, the processor executes the operating cycle called the scan or sweep.

  22. Remote Run Mode • Some processors have key switches on the processor module to change operating modes. • Changing the operating mode to run or program using a key switch is called local mode. • Changing operating mode from a personal or industrial computer through a communications cable is remote mode.

  23. Remote Program Mode • If the processor is put into program mode from a remote programming device, this is called remote program mode. • The remote computer device can perform any programming operations remotely. • When completed, the processor can be returned to remote run mode to execute PLC program.

  24. Test Mode • Test mode is used to test a program during installation, maintenance, or troubleshooting. • Test mode reads inputs and solves ladder program, but does not allow field devices to be energized.

  25. Processor Communications • The physical connection between a processor and the outside world is called a communication port.

  26. SLC 500 Processor Communication Capabilities (1 of 2) • MicroLogix 1000 • DH-485 and Ethernet • 5/01 processor • DH-485 • 5/02 processor • DH-485

  27. SLC 500 Processor Communication Capabilities (2 of 2) • 5/03 processor • Serial and DH-485 • 5/04 processor • Serial and DH Plus • 5/05 processor • Serial and Ethernet

  28. SLC 5/03 and 5/04 Communication Ports Image courtesy of Allen-Bradley, a Rockwell Automation business

  29. Processor Capabilities • A processor’s capabilities are determined by • Memory size • Instruction set • I/O supported • Communication ports

  30. SLC 5/01 Processor Specifications Image courtesy of Allen-Bradley, a Rockwell Automation business

  31. SLC 5/02 Processor Specifications Image courtesy of Allen-Bradley, a Rockwell Automation business

  32. SLC 5/02 Processor Image courtesy of Allen-Bradley, a Rockwell Automation business

  33. SLC 5/03 Processor Image courtesy of Allen-Bradley, a Rockwell Automation business

  34. SLC 5/04 Specifications Image courtesy of Allen-Bradley, a Rockwell Automation business

  35. SLC 5/04 Processor Image courtesy of Allen-Bradley, a Rockwell Automation business

  36. SLC 5/05 Processor Specifications Image courtesy of Allen-Bradley, a Rockwell Automation business

  37. SLC 5/05 Modular Processor Image courtesy of Allen-Bradley, a Rockwell Automation business

  38. General Electric Series 90-30 • Series 90-30 processors come in two configurations. • Processor embedded in baseplate • Modular processor

  39. Model CPU 311 and Model CPU 313 Image courtesy of GE Fanuc Automation

  40. GE Series 90-30 Embedded Processors Image courtesy of GE Fanuc Automation

  41. Five-slot Modular CPU Baseplate Image courtesy of GE Fanuc Automation

  42. Modular CPU 351and CPU 363 Image courtesy of GE Fanuc Automation

  43. Series 90-30 Ten-Slot Modular CPU Baseplate Image courtesy of GE Fanuc Automation

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