Siemens SIMATIC CP 340 User Manual page 31

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
● Using the parity bit ensures that the inversion of a bit in a character to be transmitted can
● To increase transmission integrity, a checksum and length specification for a message
● A further increase in data integrity can be achieved by means of acknowledgment
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
● The 3964(R) procedure ensures high transmission integrity on the data line. This high
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 a block check character: 3964R
In this manual, the designation 3964(R) is used when descriptions and notes refer to both
data transmission procedures.
Performance Limits with 3964(R)
● Further processing of the send/receive data by the PLC program in the communication
● The block check of the 3964R procedure (EXOR operation) cannot detect missing zeros
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 along with the data itself, and having the length checked at the other end.
PtP coupling and configuration of CP 340
Manual, 04/2011, A5E00369892-03
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 efficient
throughput rate, security is not guaranteed.
be recognized. If two or more bits of a character are inverted, this error can no longer be
detected.
frame can be employed. These measures must be implemented by the user.
message frames in response to send or receive message frames. This is also the case
with high-level protocols for data communication (see ISO 7-layer reference model).
transmission.
integrity is achieved by means of a fixed message-frame set-up and clear-down as well
as the use of a block check character (BCC).
partner is not guaranteed. You can only ensure this by using a programmable
acknowledgment mechanism.
(as a whole character) because a zero in the EXOR operation does not affect the result of
the calculation.
Basic Principles of Serial Data Transmission
2.3 Transmission integrity
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