Data Transmission With The Rk 512 Computer Connection - Siemens Simatic S7-300 CP 341 User Manual

Basic Principles of Serial Data Transmission

2.5 Data Transmission with the RK 512 Computer Connection

2.5 Data Transmission with the RK 512 Computer Connection
The RK 512 computer link controls data transmission in the case of a point-to-point
connection between the CP 341 and a communication partner.
Unlike the 3964(R) procedure, the RK 512 computer link includes not only the physical layer
(layer 1) and the data-link layer (layer 2), but also the transport layer (layer 4). The RK 512
computer link also offers higher data integrity and better addressing options.
Response message frame
The RK 512 computer link answers every command message frame it receives correctly with
a response message frame to the CPU (transport layer). This allows the sender to check
whether its data has arrived error-free at the CPU or whether the the requested data is
available on the CPU.
Command message frame
Command message frames are either SEND or FETCH message frames.
Refer to section "Communication via Function Blocks (Page 127)" for information on how to
initiate a SEND or FETCH message frame.
SEND message frame
In the case of a SEND message frame, the CP 341 sends a command message frame with
user data, and the communication partner replies with a response message frame without
user data.
FETCH message frame
In the case of a FETCH message frame, the CP 341 sends a command message frame
without user data, and the communication partner replies with a response message frame
with user data.
Continuation message frame
If the volume of data exceeds 128 bytes, SEND and FETCH message frames are
automatically accompanied by continuation message frames.
Message frame header
With RK 512, each message frame begins with a message frame header. It can contain
message frame IDs, information on the data destination and source and an error number.
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CP 341 Point-to-Point Communication, Installation and Parameter Assignment
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Manual, 09/2008, A5E02191071-01