Update System Control (Sc) Relays And System Data (Sd) Registers; Solve Application Program; Write Outputs; Diagnostics - AutomationDirect C0-USER-M Manual

Chapter 2: Specifications
WARNING: Only authorized personnel fully familiar with all aspects of the application should make
changes to the program. Make sure you thoroughly consider the impact of any changes to minimize the
risk of personal injury or damage to equipment.

Update System Control (SC) Relays and System Data (SD) Registers

The CLICK PLC units have system memory locations that hold this information. This portion
of the execution cycle ensures these locations get updated on every scan. Also, there are several
different system control relays, such as diagnostic relays, etc., that are also updated during this
segment.

Solve Application Program

The CLICK PLC 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 CLICK PLC 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.
The internal control relays (C) and the data registers (DS, DD, DF and DH) are also updated
in this segment.
You may recall that you can force various types of points in the system, discussed earlier in this
chapter. If any I/O points or memory data have been forced, the output image register also
contains this information.

Write Outputs

Once the application program has solved the instruction logic and constructed the output image
register, the CLICK PLC writes the contents of the output image register to the corresponding
output points. Remember, the PLC also ensured that any forcing operation changes were
stored in the output image register, so the forced points get updated with the status specified
earlier.

Diagnostics

During this part of the scan, the PLC 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 CLICK PLC. Chapter 6:
Troubleshooting contains a listing of the various error codes with a description of the possible
causes.
Probably one of the more important things that occurs during this segment is the scan time
calculation and watchdog timer control. The CLICK PLC has a watchdog timer that stores the
maximum time allowed for the PLC to complete the solve application part of the scan cycle. If
this time is exceeded, the PLC will enter the Stop mode and turn off all outputs. An error is
automatically reported. The default value of the watchdog timer is 200ms and can be adjusted
between 5–10,000 ms. Refer to the online help available from the CLICK Programming
Software, C0-PGMSW, for additional information in regards to the Watchdog Timer.
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CLICK PLC Hardware User Manual, 6th Edition, Rev. F – C0-USER-M