Siemens S7 PLC communications with Automod - datablocks higher than 255

[Chipley]

Hello,

I am a PLC n000b and working on my first emulation project utilizing Automod as the emulation tool and a Siemens Simatic S7-400 PLC . I will have several questions going forward, but first, can anyone verify if this statement is true:

"The Fetch/Write protocol does not support the reading of datablocks with numbers greater than 255 - this is defined by the Siemens."

The I/O list that I am working with utilizes datablocks higher than 255, this may be an error in the I/O list creation, so I am looking to find out whether the above statement is true or false, which came from a counterpart.

Thanks,
~Chipley

P.S. - I'm also trying to figure out still how to ask the right questions about PLCs, so my apologies if this post is too vague.

 

[Bratt]

Hello,

"The Fetch/Write protocol does not support the reading of datablocks with numbers greater than 255 - this is defined by the Siemens."

This is correct it has to do with S5 compability issues i think.

 

[Chipley]

Hey Bratt,

Thanks for the reply. Interesting, so...hmm...

I have a small test program with a db1000, 1 boolean toggling 1/0 every two seconds. I have Wireshark installed to monitor my network packets which is not seeing any error msgs coming from the PLC, and I have developed a small HMI program using Iconics that reads the db1000 and shows the db1000 bit toggling every 2 seconds. However, Automod does not read the db1000. Like I mentioned, I'm a PLC n000b, and so is my counterpart who developed test PLC program.

What would the expected error msg be from a db1000?

Also, could you elaborate on what the S5 compatibility issues might be?

Thanks again,
~Chipley

"Just tryin' to remove myself from teh PLC n000b status"

 

[L D[AR2P#0.0]]

Why not use a DB in the range 1..255 ?

 

[Chipley]

Heya LD,

Will do, most definitely, but I'm trying to understand why you can only use db 1 - 255 and have solid explanation of this concept, because some of my counterparts have developed an in-depth program using many db's higher than 255, and I'm trying to discern whether it will work or not, and if it won't work, I'd like to have an explanation of why. If there are resources that explain this, I'd love have a read, but so far I haven't been able to any documentation on this.

Back to searching, any info would be most appreciated,
~Chipley

 

[cjd1965]

I have no idea of why there is a limit of 255 but it sounds like an 8 bit boundary in the good old days 11111111=255 so maybe certain software designed to interface to a whole range of stuff (including older) wont like a value of above 255

 

[L D[AR2P#0.0]]

How is Automod connected to the 400 plc ?

 

[Chipley]

LD,

Automod connects to PLC via a customized .dll that some collegues created.

cjd and Bratt,

I'm getting conflicting info on the db 1 - 255, my collgues in Eurpoe say that the db's have a limit of 255, and you guys are reinforcing that fact, but my other engineer collegues here in that states say that the db is pretty much unlimited to account for expanding systems. Would it make a difference if I mentioned that we are using a Siemens Simatic S7-400 PLC with two CP 433-1 communication boards and a CPU 416-2?

Totally confused about what's what,
~Chipley

 

[Bratt]

S5 gurus correct me if im wrong but dident the S5's have a limit of 255 db's. I think that the Fetch/Write protocol was devoloped in the S5 days. The S7 has many other protocols you can use to communicate that suports dbnumbers higher than 255. The fetch/write protocol is acutally on of the few protocols that Siemens has released detail information on. So the limit is in the protocol not in the Plc. The plc can have much higher DBnumbers.

 

[Bratt]

Found a manual look at chapter C Linking to Other Systems with
FETCH/WRITE

DBNR DB, from which the source data are taken or to which the dest data aretransferred

permitted range 1...255

 

[Chipley]

Hey all,

Thanks for all the replies. Bratt, that manual helped big time, thanks for that...

Now how do i mark this post as resolved...?

: )

Chipley