S7 - will bigger MMC increase No. of DB's available

Johnny T

Member
Johnny T
Join Date
Jul 2003
Location
Fife, Scotland
Posts
642
Hi

I'm in the middle of a job on an S7-300 series PLC that uses a LOT of datablocks (about 200 each one around 500 words). The PLC is full and won't let me load the rest. I only have a 128Kb Micro Memory Card.

Somewhere in the back of my mind I have it that if I buy a bigger MMC then I'll be fine as DB's are stored in 'offline' memory and only called into memory when needed.

Is this the case? I don't want to spend money on a bigger MMC if its the processor thats stopping the extra DB's being loaded.

Thanks

JT ;-)
 
Then the limitation is the CPU. It has 64kB memory.

There is the option to mark a datablock as "unlinked"
From the online help:
Data blocks with the "Unlinked" property are only stored in the load memory. The block occupies no space in the working memory and is not included in the program. These data blocks cannot be accessed with MC7 commands. Depending on the CPU, the contents of such a DB can only be transferred to the working memory by using SFC 20 BLKMOV or SFC 83 READ_DBL.
Click to expand...
Be warned that it is not recommendable to write to the MMC's cyclically.

If you cannot go this route, then you have to do something drastic like upgrading to a 315-2DP or higher. The 200 x 500 words equals approximately 100 kB.
 
Johnny T said:
...a LOT of datablocks (about 200 each one around 500 words). The PLC is full and won't let me load the rest. I only have a 128Kb Micro Memory Card.
Click to expand...
Hi JT,

I can not confirm you assumption, but try to find out just many DB's you can download in an 313-2DP with an empty 128kB memory card. This PLC only has 32kB, so if you can store more than 32kB of DB's, it means you must be right about where the offline DB's are stored. In that case, your problem should be solved by installing a bigger MCC.
 
Oops, sorry Jesper, missed your post... but are you sure about memory size? Maybe I need an update, but the 313-2DP's I've seen only have 32kB.

BTW, not trying to be the wiseguy, but 200x500WORDS should equal to 200kBYTES.
 
JesperMP said:
Then the limitation is the CPU. It has 64kB memory.

There is the option to mark a datablock as "unlinked"
From the online help:

Be warned that it is not recommendable to write to the MMC's cyclically.

If you cannot go this route, then you have to do something drastic like upgrading to a 315-2DP or higher. The 200 x 500 words equals approximately 100 kB.
Click to expand...

Jesper

These datablocks are for storing offline 'programs' or 'recipes'. They are only required when the operator selects that particular program. I then use SFC20 to 'copy' in the five pertinent data blocks of that program to the working datablocks of the program. Does this mean then that if I make all the offline program DB's unlinked they should all fit in?

Cheers

JT ;-)
 
Sparkz

Thanks for your suggestion. Will try it over the weekend when I'm back in work.

Cheers

JT ;-)

PS. Hope you're right and I don't need to upgrade the PLC...
 
sparkz said:
Oops, sorry Jesper, missed your post... but are you sure about memory size? Maybe I need an update, but the 313-2DP's I've seen only have 32kB.
Click to expand...
The latest version in my HW catalog now have 64kB.

sparkz said:
BTW, not trying to be the wiseguy, but 200x500WORDS should equal to 200kBYTES.
Click to expand...
You are indeed wiser than me.

Johnny T said:
These datablocks are for storing offline 'programs' or 'recipes'. They are only required when the operator selects that particular program. I then use SFC20 to 'copy' in the five pertinent data blocks of that program to the working datablocks of the program. Does this mean then that if I make all the offline program DB's unlinked they should all fit in?
Click to expand...
If you have a big enough MMC card, then this sounds like it could work.
 
Well, just to tie up the loose ends in this topic I tried to add those DB's to a larger MMC at weekend and it didn't work. The only way I could do it was to 'unlink' the DB's as Jesper suggested and then use SFC 83 and SFC 84 to copy them into, and out of, active datablocks that weren't 'unlinked'.

This solution worked a treat...

Many thanks for all your help

JT ;-)
 
All the CPU's have a threefold memory wall:

1. How big a Block can be (16K on 300's)
2. How many block (CPU to CPU)
3. How much load memory (total block mem)

For recipes I would suggest you store them in the HMI. It's easy and you have "lots" of mem available via CF. The CPU would thus only store working recipes.

Nick
 
Nick,

Thanks for the info on maximum DB size etc.

Normally I would store the recipes in the HMI but I couldn't on this job. They needed to be in the PLC. Its fine because the blocks containing the info are 'unlinked' so the PLC is chugging along quite happily and just drawing them in as and when it needs them...

Cheers

JT ;-)
 

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