Process interfacing via an automation system (PLC, PC)
10.4 Control with FB79
SIMATIC MV420 / SIMATIC MV440
Operating Instructions, 04/2013, A5E02371045-06
275
10.4.2 Sample program for data exchange when code length <= 27 bytes
Below, you will find a sample program for data exchange when the code length is a
maximum of 27 bytes.
Note
The sample program can only run on the following S7-CPUs:
• On a CPU 318 and the S7-400-CPUs as of firmware version V3.0.0
• On S7-300 CPUs with MMC
On all other S7-300 and S7-400 CPUs, you must use SFCs 14 and 15 for all access to the
two user data interfaces.
Explanation of the program
● Only the pure result string with data ID B#16#81 is output.
● The I/O controller does not check how many bytes the reader has returned but always
reads 27 bytes. The I/O controller does not monitor anything.
● The start of the "Send" user data interface is at address 4 of the process image input.
The start of the "Receive" user data interface is at address 4 of the process image of the
outputs.
● To ensure the consistency over the entire area of 32 bytes, there must be no update of
the process image during the program shown here.
● The user data is stored in data block DB17 starting at byte 4.
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