S7-300 storage concept:
Load memory:
Load all the blocks to their full length including the description of parameter interfaces into the CPU load memory. This is the part that also files the format definitions for the individual data of a data block for example.
The load memory with the S7-300 CPUs consists - depending on the CPU - of an internal memory, an external memory on a slot card or a combination of the two. With CPUs that have an external memory the load memory can be expanded. With CPUs with MMC the load memory is contained on the MMC and the size is the exact size of the MMC.
Note:
The executable parts of all the blocks in the load memory may not exceed the size of the RAM. Otherwise you have to do a general reset of the CPU because the RAM is not big enough.
Main memory:
The main memory only files data relevant to program execution. The main memory is integrated in the CPU and is not expandable. It has a battery powered buffer. If a CPU is used with an MMC, there is no battery for the buffer because the main CPU memory is basically remanent.
Note:
With the CPU 317 the whole user memory is no longer remanent. In this case only 256 Kbytes of 512 Kbytes of user memory are remanent. So here only max. 256 Kbytes of remanent data blocks can be loaded. The rest of the main memory can only be used for code blocks, not remanent DBs and SDBs.
System memory:
The system memory contains the storage areas for:
Process map of input and output signals (PAE, PAA)
Marker (M)
Time functions (T)
Counter functions (Z)
Local data stack (L)
Non-volatile memory:
The non-volatile memory (NV-RAM) is used as buffer space for markers, times, counters and data blocks and again does not have a buffer battery. Which areas are to be buffered is specified when setting the parameters for the CPU. One case where the non-volatile memory is used is "power off" without buffer. For S7-300 CPUs with Micro Memory Card the remanent memory is provided on the MMC and on the CPU and is available all the time. No parameters need be or can set with HW Configuration. The content of the remanent memory is retained even with "power OFF" and restart.
Expanding the memory with a memory card:
When a memory card is slotted in, the operating system orders an overall rest (STOP - LED flashes slowly). The reset is performed by holding the operating mode switch in the "MRES" position. Thereafter the executable program elements are transferred from the memory card to the main memory.
Important:
The memory card must remain slotted as long as the program is being edited. When using CPUs with MMC, the memory cannot be expanded. If more space is required, then you have to use a bigger MMC.
Reaction of the CPU after switching power ON
The CPU knows whether power ON is a buffered or unbuffered input.
If the power ON is buffered, the program section contained in the main memory is used to go on working.
If power ON is unbuffered, the executable parts are transferred from the load memory to the user memory. In this case remanent data is used for the remanent data areas; all other data areas are given the default value. Where CPUs have an MCC, no distinction is made between buffered and unbuffered power ON, because they don't have a battery. All DBs keep their actual values - the remanent markers, counters and timers keep their values. The non-remanent user data is initialized with "0".
Note:
With the CPU 317 this only applies to the remanent DBs; the non-remanent DBs are initialized again with their default values from the load memory.
Special feature of buffering with accumulator:
The accumulator only buffers the date and time. So when an accumulator is used, the CPU behaves as is it were in unbuffered mode. For CPUs with MMC, an accumulator is not needed and cannot be used for time buffering.
Important:
The storage concept of CPU 318-2DP corresponds to that of the 400 CPUs. You will find this under Entry ID 7302549
Or will I have to think up my own one?
