Siemens Simatic S7-300 CP 341 User Manual page 54

Basic Principles of Serial Data Transmission
2.5 Data Transmission with the RK 512 Computer Connection
Sending data
The SEND request is executed in the following sequence:
● Active partner
Sends a SEND message frame. This contains the message frame header and data.
● Passive partner
Receives the message frame, checks the message frame header and the data, transfers
the data to the CPU, and then acknowledges this with a response message frame.
● Active partner
Receives the response message frame.
Sends user data.
If the volume of user data exceeds 128 bytes, the active partner sends a continuation
SEND message frame.
● Passive partner
Receives the continuation SEND message frame, checks the message frame header and
the data, transfers the data to the CPU, and then acknowledges this with a continuation
response message frame.
Note
If the CPU does not receive the SEND message frame error-free or if an error has
occurred in the message frame header, the communication partner enters an error
number in the 4th byte of the response message frame. This does not apply when
protocol errors occur.
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CP 341 Point-to-Point Communication, Installation and Parameter Assignment
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Manual, 09/2008, A5E02191071-01