D V P - E S 2 / E X 2 / S S 2 / S A2 / S X 2 / S E & T P O p e r a t i o n M a n u a l - P r o g r a m m i n g
1.1 PLC Scan Method
PLC utilizes a standard scan method when evaluating user program.
Scanning process:
Scan input status
Read the physical input status and store the data in internal
memory.
Evaluate user program
Evaluate the user program with data stored in internal memory.
Program scanning starts from up to down and left to right until
reaching the end of the program.
Refresh the outputs
Write the evaluated data to the physical outputs
X0
Y0
Y0
M0
Input X
Input terminal
Store to memory
Input signal memory
De
vi
ce
Me
m
o
ry
Read X0 status from memory
Write Y0 state into
Read Y0 state from memory
Write M0 state into
Output
Program
Input signal
Output
Output Y
Output terminal
Output latched memory
Input signal:
PLC reads the ON/OFF status of each input and
stores the status into memory before evaluating
the user program.
Once the external input status is stored into
internal memory, any change at the external
inputs will not be updated until next scan cycle
starts.
Program:
PLC executes instructions in user program from
top to down and left to right then stores the
evaluated data into internal memory. Some of this
memory is latched.
Output:
When END command is reached the program
evaluation is complete. The output memory is
transferred to the external physical outputs.
Scan time
The duration of the full scan cycle (read, evaluate, write) is called “scan time.” With more I/O or
longer program, scan time becomes longer.
Read
scan time
PLC measures its own scan time and stores the value (0.1ms) in register
D1010, minimum scan time in register D1011, and maximum scan time in
register D1012.
Measure
scan time
Scan time can also be measured by toggling an output every scan and then
measuring the pulse width on the output being toggled.
Calculate
scan time
Scan time can be calculated by adding the known time required for each
instruction in the user program. For scan time information of individual
instruction please refer to Ch3 in this manual.
Scan time exception
PLC can process certain items faster than the scan time. Some of these items interrupts and halt
the scan time to process the interrupt subroutine program. A direct I/O refresh instruction REF
allows the PLC to access I/O immediately during user program evaluation instead of waiting until
the next scan cycle.
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