chapter 10 plc processors

Chapter 10

PLC Processors

Objectives

• Describe the function of the processor.

• Describe processor operating modes.

• Explain the function of the watchdog timer.

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.

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.

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.

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.

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.

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.

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

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.

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.

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

Straight-through Cabling Between Computer and Peripheral Equipment

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.

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.

Connecting Two Computers with a Straight-through Cable

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.

Null-Modem Cable Crosses Pin 2 to Pin 3

PLC Processors Have Multiple Operating Modes

• Processor has two modes of operation.– Program mode– Some variation of run mode

Program Mode

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

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.

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.

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.

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.

Processor Communications

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

SLC 500 Processor Communication Capabilities (1 of 2)

• MicroLogix 1000 – DH-485 and Ethernet

• 5/01 processor– DH-485

• 5/02 processor– DH-485

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

SLC 5/03 and 5/04 Communication Ports

Image courtesy of Allen-Bradley, a Rockwell Automation business

Processor Capabilities

• A processor’s capabilities are determined by– Memory size– Instruction set– I/O supported– Communication ports

SLC 5/01 Processor Specifications


SLC 5/02 Processor


SLC 5/03 Processor


SLC 5/04 Specifications


SLC 5/04 Processor


SLC 5/05 Modular Processor


General Electric Series 90-30

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

Model CPU 311 and Model CPU 313

Image courtesy of GE Fanuc Automation

GE Series 90-30 Embedded Processors


Five-slot Modular CPU Baseplate


Modular CPU 351and CPU 363


Series 90-30 Ten-Slot Modular CPU Baseplate


chapter 10 plc processors

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