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 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 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 modules and system
configuration, such as expansion or remote
I/O, can also affect the scan time; however,
these things are usually dictated by the
application.
For example, if you have a need to count
pulses at high rates of speed, then you’ll
probably have to use a High-Speed Counter
module. Also, if you have I/O points that
need to be located several hundred feet
from the CPU, then you need remote I/O
because it’s much faster and cheaper to
install a single remote I/O cable than it is to
run all those signal wires for each individual
I/O point. The following paragraphs
provide some general information on how
much time some of the segments can
require.
DL205 User Manual, 4th Edition, Rev. B
Chapter 3: CPU Specifications and Operations
1
2
3
4
5
6
7
8
9
10
11
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 (DL250-1/DL260)
3–29
