Siemens S7-1500 Max Number of Structures in Datablock

[X-tech]

Hi Looking for some advice.

I have a project that was developed for an S7-300. In that project I have a number of datablocks which only contain structures. Each structure is 16 BOOLs with the idea that you could access the "Structure" if you wanted word access or the individual structure elements if you want Boolean access. Works great with an S7-300. Some of these datablocks have upwards of 800 structures configured.

My issue is that I want to migrate this project to an S7-1500 and TIA Portal. With an S7-1500, I am led to believe that the maximum number of structures you can have in a data block is 252? Without having to modify the structure of my DBs and logic that calls them, can I have more than 252 structures some how? Am I missing a trick? Strange that I could do this OK on an S7-300 but not a 1500? Also not sure why its 252, seems a stranger value.

Any help much appreciated.

 

[mk42]

I've never heard of this limitation. Can you find a source?


Also, are they actual STRUCT structures or are they user defined data types (the same structure used over and over)? In 1500/TIA structs are technically possible, but user defined types are strongly preferred.

 

[X-tech]

Hi, thanks for the reply

It mentions it on the TIA Portal help pages and I also get the error message "The maximum number of structures within the block interface has been exceeded." when I try compile the DB (with structure 253 onwards showing in fault)

My data block elements are definitely declared as type "STRUCT".


TIA Portal help Says (On help page "basic information on STRUCT")

A maximum of 16K components can be defined per hierarchy level for each structure description (for example, a DB or a PLC data type (UDT)). These components can also be an ARRAY or a STRUCT data type. When a structured component is hereby derived from an explicitly declared PLC data type, there is no restriction. For each hierarchy level, 16K components of PLC data type (UDT)/SDT are supported. However, a maximum of 252 "anonymous structures" may exist within a block.


What is an anonymous structure?????

 

[X-tech]

I might get it. So sounds like if I was declare my 16 Bools as a new PLC Data Type and then change my data block variables of type "Struct" to the new PLC datatype I shouldn't have this restriction? However if I were to change my variables from Struct to the new PLC Data Type I will loose all my symbol names and comments for the individual BOOLs which make up the struct.

 

[tdoa]

You could use words and then use the .%X0 to .%X15 alias to access the bit values.
Or use the AT keyword.

 

[mk42]

Are the names of the bools different every time? Are there some common patterns?
If the names are completely unique, maybe you could use the comments instead of the tagname to document what it represents.



Big picture, what do you use these for? Are you trying to set up alarm words or similar? If you explain the bigger problem you're trying to solve, there might be a better way to do it; the 1500s generally want to do things symbolically compared to the absolute addressing of the 300s, and it changes a lot of best practices.

 

[JERA]

The limits for DB size in the different PLC models are:

PLC DB optimized DB no optimized
1511 1 Mb 64 kb
1513 1,5 Mb 64 kb
1515 3 Mb 64 kb
1516 5 Mb 64 kb
1517 8 Mb 64 kb
1518 10 Mb 64 kb

 

[mk42]

The limits for DB size in the different PLC models are:

PLC DB optimized DB no optimized
1511 1 Mb 64 kb
1513 1,5 Mb 64 kb
1515 3 Mb 64 kb
1516 5 Mb 64 kb
1517 8 Mb 64 kb
1518 10 Mb 64 kb

to be clear, I'm pretty sure the optimized limit is the same amount as the max data work memory of the PLC (I think that lines up with what you had above?) and the old S7-300 limit for unoptimized on all PLCs.