DL06 Micro PLC User Manual, 3rd Edition, Rev. E
3-20
Chapter 3: CPU Specifications and Operation
1
2
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4
5
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7
8
9
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12
13
14
A
B
C
D
YES
Power up
Initialize hardware
Check I/O module
config. and verify
Initialize various memory
based on retentive
configuration
Update input
Read input data from
Specialty and Remote I/O
Service peripheral
PGM
Mode?
RUN
Execute ladder program
Update output
Write output data to
Specialty and Remote I/O
Do diagnostics
OK
NO
NO
Fatal error
Force CPU into
PGM mode
OK?
Report the error, set flag,
register, turn on LED
YES
CPU Bus Communication
Update Clock / Calendar
PID Equations (DL250)
CPU Scan Time Considerations
The scan time covers all the cyclical tasks that are performed
by the operating system. You can use
Direct
SOFT or
the Handheld Programmer to display the minimum,
maximum, and current scan times that have occurred
since the previous Program Mode to Run Mode transition.
This information can be very important when evaluating
the performance of a system. As we’ve shown previously
there are several segments that make up the scan cycle.
Each of these segments requires a certain amount of time to
complete. Of all the segments, the following are the most
important:
• Input Update
• Peripheral Service
• Program Execution
• Output Update
• Timed Interrupt Execution
The one you have the most control over is the amount of
time it takes to execute the application program. This is
because different instructions take different amounts of
time to execute. So, if you think you need a faster scan,
then you can try to choose faster instructions.
Your choice of I/O type and peripheral devices can also
affect the scan time. However, these things are usually
dictated by the application.
The following paragraphs provide some general information
on how much time some of the segments can require.
Reading Inputs
The time required during each scan to read the input status
of built-in inputs is 52.6 µs. Don’t confuse this with the
I/O response time that was discussed earlier.
Writing Outputs
The time required to write the output status of built-in
outputs is 41.1 µS. Don’t confuse this with the I/O
response time that was discussed earlier.
