Siemens Simatic S7-400

Classifying the Supplied Protocols
The CP 440 can use the following protocols:
• ASCII driver
• 3964(R) Procedure
The 󹪝gure below illustrates how these protocols of the CP 440 󹪝t into the ISO reference model:
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Figure 2-3 Position of the supplied protocols of the CP 440 in the reference model
Transmission Integrity with the ASCII Driver
Data Integrity When Using the ASCII Driver:
• When data is transmitted via the ASCII driver, there are no data integrity precautions other
than the use of a parity bit (can also be canceled, depending on how the character frame is
set). This means that, although this type of data transport has a very e󹪟cient throughput
rate, security is not guaranteed.
• Using the parity bit ensures that the inversion of a bit in a character to be transmitted can be
recognized. If two or more bits of a character are inverted, this error can no longer be
detected.
• To increase transmission integrity, a checksum and length speci󹪝cation for a message frame
can be employed. These measures must be implemented by the user.
• A further increase in data integrity can be achieved by means of acknowledgment message
frames in response to send or receive message frames. This is the case with high-level
protocols for data communication (see ISO 7-layer reference model).
Transmission integrity with 3964(R)
Enhanced Data Integrity with the 3964(R) Procedure:
• The Hamming distance with the 3964(R) is 3. This measures the integrity of data
transmission.
• The 3964(R) procedure ensures high transmission integrity on the data line. This high
integrity is achieved by means of a 󹪝xed message-frame set-up and clear-down as well as the
use of a block check character (BCC).
Basic Principles of Serial Data Transmission
2.3 Transmission integrity
Point-to-point connection CP 440 Installation and Parameter Assignment
22 Equipment Manual, 03/2022, A5E00057742-AE