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EL C Pro g rammi ng Manu al
F o r m o r e i nf o r m a t i o n v i sit :
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MN05003003E
1-2
1 ELC
Concepts
1.1 ELC Scan Method
ELC utilizes a standard scan method when evaluating the 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 top to bottom 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:
The ELC 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:
The ELC executes instructions in the 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 the “scan time.” With more I/O or
a longer program, the scan time becomes longer.
Read
scan time
The ELC measures its own scan time and stores the value (0.1ms) in register
D1010, the minimum scan time in register D1011, and the 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 for each instruction,
please refer to Ch3 in this manual.
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