Siemens S7-300 CP to OPC, large String with Limited backplane 187kb

Kasperv79

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Join Date
Mar 2012
Location
Copenhagen
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Hi Guys.

I hav an general question about data traffic between CPU and CP on K-Bus backplane.

We got an issue where a vendor has an S7-300 cpu with CP connected to our SCADA via OPC (Keepware). the problem is that the 300 serie only have187kb/s on backplane which limit the use of external CP cards.

So our problem is the huge amount of strings. My question is if it does any different for the data from CPU to CP when requesting string value which is updated "LEN" value compared to full lengt. string.

E.g.

string_1[256]= "12345" where byte 1/Len is "5"
string_2[256] "12345" where byte 1/Len is "256"


Does this have any impack on the amount of data transfered from CPU to CP (backplane)


And no we dont have the option to ask the vendor to use the internal CP since its occupied by many servedrives which need the bandwith :(


/kasper
 
Last edited:
the two first bytes for Step 7 strings store the first the max. string size (always <= 254) and the second the actual size of string, 5 in your case, and followed by the character string in UTF-8 encoding
Then the byte that really matters is the second because theoretically the drive has to read this byte first and then read only the bytes used, but if this byte is wrongly set to 256(254) instead of 5, then it will read more bytes than necessary.
 
Last edited:
the two first bytes for Step 7 strings store the first the max. string size (always <= 254) and the second the actual size of string, 5 in your case.
Then the byte that really matters is the second because theoretically the drive has to read this byte first and then read only the bytes used, but if the byte is wrongly set to 256(254) instead of 5, then it will read more bytes than necessary.

Sure, that was what i ment by Byte 1 = 5.

sorry, my fault, I ment maximum lengt 254. it was only for state the wurst scenario.


So you says that, it does have an impack on data communication if Byte 1 is not reduced to actual lenght?

I on the same "teori" but need someone to back me up in this before I'll go to vendor and ask's them to update their code :)
 
The impact of transmitting 5 or 254 bytes is difficult to quantify, but it will be probably very little, since using TCP protocol when such small packets are transmitted the most part of time is used in the server-client negotiation and in some delays than in the transmission of data itself.

For this small packets the use of UDP instead TCP is far more efficient.
 
The impact of transmitting 5 or 254 bytes is difficult to quantify, but it will be probably very little, since using TCP protocol when such small packets are transmitted the most part of time is used in the server-client negotiation and in some delays than in the transmission of data itself.

For this small packets the use of UDP instead TCP is far more efficient.


Thanks for your quick reply! it was helpfull.
 

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