CPU Specifications
and Operation
4–15
CPU Specifications and Operation
Once the application program has solved the instruction logic and constructed the
output image register, the CPU writes the contents of the output image register to the
corresponding output points. Remember, the CPU also made sure that any forcing
operation changes were stored in the output image register, so the forced points get
updated with the status specified earlier.
During this part of the scan, the CPU performs all system diagnostics and other tasks
such as calculating the scan time and resetting the watchdog timer. There are many
different error conditions that are automatically detected and reported by the DL05
PLCs. Appendix B contains a listing of the various error codes.
Probably one of the more important things that occurs during this segment is the
scan time calculation and watchdog timer control. The DL05 CPU has a “watchdog”
timer that stores the maximum time allowed for the CPU to complete the solve
application segment of the scan cycle. If this time is exceeded the CPU will enter the
Program Mode and turn off all outputs. The default value set from the factory is 200
ms. An error is automatically reported. For example, the Handheld Programmer
would display the following message “E003 S/W TIMEOUT” when the scan overrun
occurs.
You can use AUX 53 to view the minimum, maximum, and current scan time. Use
AUX 55 to increase or decrease the watchdog timer value.
I/O Response Time
I/O response time is the amount of time required for the control system to sense a
change in an input point and update a corresponding output point. In the majority of
applications, the CPU performs this task in such a short period of time that you may
never have to concern yourself with the aspects of system timing. However, some
applications do require extremely fast update times. In these cases, you may need to
know how to to determine the amount of time spent during the various segments of
operation.
There are four things that can affect the I/O response time.
S The point in the scan cycle when the field input changes states
S Input Off to On delay time
S CPU scan time
S Output Off to On delay time
The next paragraphs show how these items interact to affect the response time.
The I/O response time is shortest when the input changes just before the Read
Inputs portion of the execution cycle. In this case the input status is read, the
application program is solved, and the output point gets updated. The following
diagram shows an example of the timing for this situation.
Write Outputs
Diagnostics
Is Timing Important
for Your
Application?
Normal Minimum
I/O Response
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