AutomationDirect DL105, Руководство пользователя

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CPU
S
pecifications
and
Operation
4--14
CPU Specifications and Operation
DL105 PLC User Manual, 3rd Edition
The CPU evaluates each instruction in the
application program during this segment
of the scan cycle. The instructions define
the relationship between the input
conditions and the desired output
response. The CPU uses the output
image register area to store the status of
the desired action for the outputs. Output
image register locations are designated
with a Y followed by a memory location.
The actual outputs are updated during the
write outputs segment of the scan cycle.
There are immediate output instructions
available that will update the output points
immediately instead of waiting until the
write output segment. A complete list of
the Immediate instructions is provided in
Chapter 5.
The internal control relays (C), the stages
(S), and the variable memory (V) are also
updated in this segment.
Read Inputs
Solve the Application Program
Write Outputs
Diagnostics
Update Special Relays
Service Peripherals
Normal Run mode scan
You may recall that you can force various types of points in the system. (This was
discussed earlier in this chapter.) If any I/O points or memory data have been forced,
the output image register also contains this information.
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 DL105
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 DL105 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.
Solve Application
Program
Write Outputs
Diagnostics