What is
plc
Programmable
Logic Controller is a Binary computer used for industrial
automation These controllers entire production line . and machine function .
manufacturing process or any activity that requires high reliability control
and ease of programming and process ...
Programmable
Logic Controllers.
Control
Systems and Application Programs
–
Downloading and perform user (step logic) programs.
– interface
with devices.
• I/O
modules.
• Other PLCs
on a network.
– Holding
presentation data such as.
• Status
information .
Operating
systems and application Programs .
A PLC maintain
its control system, user Programs, and some data in persistent
(nonvolatile)
memory.
– While the
PLC is off or is even detached from the power supply (given a battery backup).
• A PLC can
then restart running a user program alternately power is reinstall.
– The PLC may
be programmed to require some operator action before resuming user program implementation
(for safety purposes) .
• A PLC
perform an boot up step when placed in run mode, then repeatedly perform a scan
cycle sequence.
scan cycle consists of three steps
– An input
scan
– A user program
scan
– An output
scan
• The total
time for one total program scan is a function of processor speed, I/O modules
used, and length of user program
• Typically,
hundreds of complete scans can take place in 1 second .
Input
scan
• throughout
the input scan, data is taken from all input
modules in
the system and placed into an area of PLC memory mention to as the input image
area
User program scan
• when the program scan , data in the input image area is applied to
the user program, the user .
program is perform and the output image area is updated.
Output scan
• in between output scan, data
is taken from the output image area and sent to all output modules in the
system.
PLC
scanning considerations
• In Between the input scan, input terminals are read and the input
image area is updated accordingly.
• In Between the program scan
– data in the input image area is register to the user program
– the program is perform (instructions carried out in sequence)
– the output image area is updated proper.
• in between the output scan, data associated with the output image
area is transferred to output terminals.
• It is main to understand that the input,program, and output scans are
separate, independent functions.
– Any changes in the status of input devices in between the program or
output scan are not recognized until the next input scan.
•additionally , data changes in the output table are not transferred to
the output
terminals during the input and program scans.
• The transfer affecting the output devices takes place only in between
the output scan.
• First, all Programmable Logic Controller (PLC), take a specific amount of time to scan their
operational programs completely
• Typically, the program scanning takes place left to right covering
each rung and from the top to bottom rungs, in order.
• Usually, the complete steps scan time is a few milliseconds.
•one more scanning consideration
involves proper operational sequencing of events
• An output might not go on instantly in sequence as it would in a
relay logic system
• In a relay logic system, an event takes place anywhere in the ladder
control system results in immediate action
• In a PLC ladder control diagram, however, no effect occurring until the rung is scanned
• Program (step logic) scanning
typically takes place left to right across each rung and from the top to bottom
rungs, in order
– Known as rung scanning, it is the rule used, for example, by
Allen-Bradley
• Another method, used by Modicon/Schneider Automation is known as
column scanning
– Here the processor "looks" at the first connection at the
top left corner and reads the first column from top to bottom.
– It next reads the second column from top to bottom, and so on
• Either method, rung or column, is appropriate.
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