Copyright © 2002 Delmar Thomson Learning 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 CycleProcessor is designed to perform

specific duties in a specific sequence.

This operating cycle is called the processors 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

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 (cont’d.)

A logical 1 or 0 will be placed in the output status table address referenced by the particular output instruction.

With rung completed, 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 handheld 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 CommunicationsSimplest 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 cableNull-modem cable

Straight Through Cabling between Computer and Peripheral Equipment

Straight Through Serial Cable

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 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

Null-Modem Cable Wired so Pin 2 Crosses to Pin 3

Pin 2 physically crosses to pin 3 in cable.

Information sent from 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 modeSome 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 processor module to change operating modes.

Changing operating mode to run or program using 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 processor 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, 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

MicroLogix 1000 DH-485 and Ethernet

5/01 processorDH-485

5/02 processorDH-485

SLC 500 Processor Communication Capabilities

(cont’d.)

5/03 ProcessorSerial and DH-485

5/04 ProcessorSerial and DH Plus

5/05 ProcessorSerial 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 byMemory sizeInstruction setI/O supportedCommunication 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 baseplateModular 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

Copyright © 2002 Delmar Thomson Learning Chapter 10 PLC Processors.

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