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