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 RS232 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
Image courtesy of Allen-Bradley, a Rockwell Automation business
SLC 5/02 Processor Specifications
Image courtesy of Allen-Bradley, a Rockwell Automation business
SLC 5/02 Processor
Image courtesy of Allen-Bradley, a Rockwell Automation business
SLC 5/03 Processor
Image courtesy of Allen-Bradley, a Rockwell Automation business
SLC 5/04 Specifications
Image courtesy of Allen-Bradley, a Rockwell Automation business
SLC 5/04 Processor
Image courtesy of Allen-Bradley, a Rockwell Automation business
SLC 5/05 Processor Specifications
Image courtesy of Allen-Bradley, a Rockwell Automation business
SLC 5/05 Modular Processor
Image courtesy of Allen-Bradley, a Rockwell Automation business
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
Image courtesy of GE Fanuc Automation
Five-slot Modular CPU Baseplate
Image courtesy of GE Fanuc Automation
Modular CPU 351and CPU 363
Image courtesy of GE Fanuc Automation
Series 90-30 Ten-Slot
Modular CPU Baseplate
Image courtesy of GE Fanuc Automation