CPUs Memory Concept

Dear Friends

I have some questions related to memory concept of CPU:

1- Why the “internal load memory” is necessary? I mean, isn’t it possible to load the program and data from programming device to the external memory (MMC) and copy it directly from MMC to work memory?

2- All series of CPU (300, 400) have the “internal load memory” or not?

3- For serie-400 CPUs, there are two types of memory cards, RAM Mem. Cards and FEPROM. What’s the difference between them? Which cases the first one should be used and which cases the second one?

4- As I’ve seen in the specification of CPU 417-4H, the amount of internal load memory is 256 Kbyte.

Is it enough to keep both the program and user data? In the other hand, the minimum size of external load memory is 1MByte. Why the external load memory should be 5 times bigger than internal load memory?!

5- External load memory is just a Buck-Up for Internal load memory, or it can be work as extension of internal load memory?

6- In CPUs series 300 or 400, Is it possible to plug out the memory card during the operation of the system or not?

Thank you so much for your patience

Sincerely

Mazdak

The internal load memory is the basic load memory of the CPU. If this load memory is to small you can expand it with external load memory, for S7-300 with a Siemens MMC, for S7-400 with a RAM or FEPROM module.

The new generation of S7-300 CPU's have no internal load memory, so you should always use a Siemens MMC. The older S7-300 CPU had internal load memory which was expandable with a FLASH EPROM or RAM module.

In case of the S7-400 CPU's they have internal load memory which is expandable with a RAM module or with a FLASH EPROM.
The RAM modules are volatile so they should be backed-up with a battery, the FEPROM's not.
Depending on your application (for S7-400) you can use the internal load memory without external load memory (by RAM or FEPROM). In case of a big application which requires a lot of S7-programming the internal load memory will not be big enough and so you will need to expand the load memory with a RAM or FEPROM.

Except the fact that RAM modules are volatile and the FEPROM's not, is there one more difference in the use of these modules. If you use a FEPROM you will have to execute a COPY RAM TO ROM after each download to the CPU. If not the changes are not stored on the FEPROM and after a power-off / power-on they will be lost.

For both series (S7-300/400) and for all the memory cards: Do not plug them out in the operation. You should only plug them out with the System powered offf

Dear kjulienh thank you for your explanation ;

Now why Siemens is releasing two types of memories (RAM and FEPROM) if the are the same.

I meant that : the volatility is one disadvantage for RAM,then why Siemens is releasing it and why some times we are using RAM not FEPROM ,actually i was searching about that and i think it is a matter of downloading in RUN-mode, but i can not understand that because i have CPUs315 and as you now these CPUs have no integrated RAM(only external MMC-FEPROM ) ,so when you give download your program is going to the MMC and the PLC should ask to change to stop but some times it was downloading without stop means in RUN-mode. 

For both series (S7-300/400) and for all the memory cards: Do not plug them out in the operation. You should only plug them out with the System powered offf 

Hi...

it means if card is to be "plugged out" then firstly we should put CPU to "STOP mode " and then plug the card out and card will run again as normal and will retain its all previous values and load memory???