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Entry type: Application example Entry ID: 6184284, Entry date: 07/26/2001
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creating a checksum over values in a data block

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QUESTION:
How can I create a checksum over the values in a data block?

ANSWER:
In the sample program attached, the function FC1 is used to create a checksum over the values stored in a data block (e.g. DB1). In order to obtain the checksum, in the function you must specify the data block "DB_NO", the initial address "START_ADD" and the length "LENGTH".

Parameters of the function FC1:

Parameter

Declaration

Data type

Description

DB_NO

INPUT

BLOCK_DB

Data block over which the checksum is to be created.

START_ADD

INPUT

INT

Initial address at which the checksum is to start being created

LENGTH

INPUT

INT

Specifies the length in bytes over how many elements the checksum is to be created.

CHECKSUM

OUTPUT

BYTE

Output of the checksum.

Using the function FC1, all the values in the data block that are covered by the parameters "START_ADD" and "LENGTH" are linked consecutively via an XOR function. The result is presented as checksum at the output "CHECKSUM".

The example contains a STEP 7 V5 project with the described function FC1 and a sample call from the organization block OB1. The data block DB1 is also included. In this example a checksum is created over the first seven values of the data block DB1. You can adapt the function as you wish to suit your requirements.

check_sum1.exe

Copy the "check_sum1.exe" file into a separate directory and then start it with a double-click. The S7 V5 project is now unpacked automatically with all the associated subdirectories. You can then open and process the unpacked project with the SIMATIC Manager.

Search terms:
CRC, Value change



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