Siemens Step5 Symbol Confusion

Hi! I am new to Step5 programming. I am trying to understand the program in a line. I am confused because everywhere DW 20 is used, regardless of the actual DB last called, the same symbol shows up. How do I know which data block actually has the word described by that symbol? Thanks! Dave Duback

Hi

All DW after a declarated DB is in this DB, if an other DB is declarated, the followings DW are from the second DB from.

This is a fast response, hope someone can explain it better.

Regards

Yes, I understand the DW is from the last DB called. But how come the symbol for DW 20 never changes regardless of the last DB called? This is confusing as DW 20 is definitely used for different purposes in every DB...

Thanks!

Dave Duback

Could be that DW20 represents a value that is the same no matter which DB is used, but why they did not use a constant is beyond me unless the parameter needs to be changed by an HMI for instance.

Dave,

in STEP5 the assignment list is only a list with words, which replace the actual addresses in the program. In teh list there's only one symbol defined for each address. DW20 is on address with only one symbol attached to it. There is a maximum of 256 DW20's possible, but since there is only one symbol available for the address DW20, they all get the same symbol. There's no way around it.

Now, with STEP7 it's a totally different story. You can define a symbol for every item in a datablock. If you give the datablock a symbolic name, you can address the item within it by a symbolic name made up of two parts (or sometimes even more): the name of the block and the name of the item within. So suppose you define DB10 with symbolic name DataBlock_10 and in DB10 you give DBW20 the name TestValue, then you can address DBW20 of DB10 as DataBlock_10.TestValue. Furthermore: in S5 you almways have to open the DB and then address the DW in it, in S7 you can do that too, but you can also address the DW without opening the DB. In that case you address DBx.DBWy.

Why isn't this possible in STEP5? Well, that's simply a question of technology. S5 exist since the late seventies, and therefor uses the technology of that age. S7 has been started in the second halve of the nineties and uses technology that's a lot more advanced.

Kind regards,

That clears it up

Many thanks! I will stop using the symbols for the most part now, it will be easier to understand the program.

Hello,

I will stop using the symbols for the most part now, it will be easier to understand the program.

Click to expand...

Why? Just don't you symbols for DW's unless you only make use of 1 DB.
Indirect adressing is a very handy feature, it also avoids typing mistakes. I have used it all the time and still do.

Rudi