CPU Specifications
and Operation
4–18
CPU Specifications and Operation
CPU Scan Time Considerations
The scan time covers all the cyclical tasks
that are performed by the operating
system. You can use DirectSOFT

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.
S Input Update
S Peripheral Service
S Program Execution
S Output Update
S Timed Interrupt Execution
The only one you really 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.
YES
Power up
Initialize hardware
Initialize various memory
based on retentive
configuration
Update input
Service peripheral
PGM
Mode?
RUN
Execute program
Update output
Do diagnostics
OK
NO
NO
Fatal error
Force CPU into
PGM mode
OK?
Report error, set flag
register, turn on LED
YES
Update Special Relays
The time required during each scan to read the input status is 40 mS. Don’t confuse
this with the I/O response time that was discussed earlier.
The time required to write the output status is 629 mS. Don’t confuse this with the I/O
response time that was discussed earlier.
Reading Inputs
Writing Outputs
To Next Page
Loading...
Need help?
General
