Siemens SIMATIC CP 440 Manual page 27

Transmission Integrity with the ASCII Driver
Data Integrity When Using the ASCII Driver:
When data is transmitted by means of the ASCII driver, there are no measures
to ensure data integrity other than the use of a parity bit (which can also be
canceled, depending on how the character frame is set). This means that,
although data transmission with the ASCII driver is very efficient in terms of the
throughput rate, the integrity of the data is not checked.
Using the parity bit ensures that the inversion of a bit in a character to be
transmitted can be detected. 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 specification 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
Enhanced Data Integrity with the 3964R Procedure:
The Hamming distance with the 3964R is 3. This measures the integrity of data
transmission.
The 3964R procedure ensures high transmission integrity on the data line. This
high integrity is achieved by means of a fixed message-frame setup and
cleardown as well as the use of a block check character (BCC).
Two different procedures for data transmission can be used, either with or without
a block check character:
Data transmission without a block check character: 3964
Data transmission with block check character: 3964R
In this manual, the designation 3964(R
to both data transmission procedures.
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Basic Principles of Serial Data Transmission
is used when descriptions and notes refer
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