Siemens SIMATIC CP 440 Manual page 28

Basic Principles of Serial Data Transmission
Performance Limits with 3964(R)
Performance Limits of the 3964(R) Procedure:
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 3964(R) procedure (EXOR logic operation) cannot
detect missing zeros (as a whole character) because a zero in the EXOR logic
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 along with the data itself, and having the length checked at the
other end.
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Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02