What does the terms Initial value (declaration view), Initial value (load memory) and Actual value mean in data blocks when a controller is loaded / restarted?
When a plant is commissioned or in operation, certain values in data blocks may be adapted in line with the process. For example, the controller parameters or other parameters (e.g. runtimes, limits, ...) are generally changed during commissioning. If these changes are only made online, the current values in the data blocks may be overwritten by the originally configured values when the program is reloaded, the hardware configuration loaded or when the controller is cold-restarted. In order to understand how data blocks behave after the controller is loaded or started, it makes sense to become familiar with the memory concept and restart characteristics of the AS.
The following graphic will give you a general view of the storage concept:
- Read-back: The actual values of the data blocks in the memory are transferred into the offline ES data blocks.
- Load program: The current offline - code and data blocks are loaded onto the CPU's load memory.
- Cold start: You transfer sequence-relevant sections of the code and data blocks from the load memory to the main memory.
On restart the program is edited at the point at which it was stopped. The actual values are retained. When warm starting, the program begins again in the main memory. The blocks with a programmed startup behavior are reset.
You can find additional information about the memory concept for the AS:
- in the PCS 7 Online Help under "Help for STEP 7 > Appendix > S7 CPU memory areas > Load memory and main memory"
- in the PCS 7 Online Help under "Help for STEP 7 > Load > Load from the PG into the PLC > Load and main memory in the CPU"
You can find information about the restart characteristics (warm restart, cold restart, hot restart)
- in the PCS 7 Online Help under "Configuration - Engineering System > Implementing the PCS 7 Configuration > Configuring the Hardware > Configuring the SIMATIC 400 Station (CPU, CPs, Central I/O) > Setting the CPU Properties".
The table below describes the meaning of the terms Initial value (Declaration view), Initial value (Load memory) and Actual value of data blocks.
|1||Initial value (Declaration view)|
The initial value is only of significance to the "offline" configuration of a data block. If a new data block is created offline or if a new variable is created in a data block in the course of the offline configuration, the initial value is applied to the actual value.
|2||Initial value (Load memory)|
When a data block is loaded into the controller, it is saved in the controller's load memory. The data block's actual values from the offline configuration are copied to the load memory. During loading into the controller, the runtime-relevant data blocks continue to be loaded into the main memory. When the controller is restarted cold, the runtime-relevant data from the load memory is also loaded into the main memory, overwriting the existing data blocks with the values in the loading memory. The values in the load memory are also referred to as initial values. The initial values in the loading memory are overwritten or changed when the AS program is loaded from the ES. The current values in the main memory are generally changed during commissioning or in process mode. These changes in value are not automatically applied to the initial values in the load memory.
The term "actual value" is used in the offline and online configuration: