Siemens Simatic S7-300 CP 341 User Manual page 37

Performance limits with 3964R
● Further processing of the send/receive data by the PLC program in the communication
partner is not guaranteed. You can only ensure this by using a programmable
acknowledgment mechanism.
● The block check of the 3964R procedure (EXOR operation) cannot detect missing zeros
(as a whole character) because a zero in the EXOR operation does not affect the result of
the calculation.
Although the loss of an entire character (this character has to be a zero!) is highly
unlikely, it could possibly occur under very bad transmission conditions.
You can protect a transmission against such errors by sending the length of the data
message frame along with the data itself, and having the length checked in the
communication partner.
● Such transmission errors are ruled out when the RK 512 computer link is used for data
transmission, because here (unlike the 3964(R) procedure) further processing (e.g.,
storage in the destination data block) is acknowledged via response message frames and
the send data length is recorded in the message frame header. This enables the RK 512
to achieve a higher Hamming distance (of 4) than the 3964R.
Transmission Integrity with RK 512
Very high data integrity through use of RK 512:
● The Hamming distance with the RK 512 and 3964R is 4. This is a measure of the integrity
of a data transmission.
● Using the RK 512 computer link guarantees high transmission integrity on the data line
(because the RK 512 uses the 3964R procedure for data transport).
● Further processing in the communication partner is ensured (because the RK 512
interpreter checks the additional length specification in the header and, after storing the
data in the destination data area of the communication partner, generates a message
frame acknowledging the success or failure of the data transport).
● The RK 512 computer link independently guarantees the correct use of the 3964R
procedure and the analysis/addition of the length specification as well as the generation
of the response message frames. There is no user handling! All you need to do is
evaluate the positive/negative final acknowledgment.
Performance limits with RK 512
● Using the RK 512 computer link provides maximum data integrity! You can improve this
still further by, for example, using other block check mechanisms (such as CRC checks).
CP 341 Point-to-Point Communication, Installation and Parameter Assignment
Manual, 09/2008, A5E02191071-01
www.gkbpx.com
Basic Principles of Serial Data Transmission
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2.3 Transmission integrity
37