Siemens SIMATIC CP 440 Manual page 50

Basic Principles of Serial Data Transmission
Procedure Errors
The procedure detects both errors which are caused by the communication partner
and errors caused by faults on the line.
In both cases, the procedure makes repeated attempts to send/receive the data
block correctly. If this is not possible within the maximum number of transmission
attempts set (or if a new error status occurs), the procedure aborts the send or
receive operation. It reports the error number of the first detected error and returns
to idle mode. These error messages are displayed in the STATUS output of the
FB.
If the system program regularly reports an error number at the STATUS output of
the FB for send and receive repetitions, this indicates occasional trouble affecting
the data traffic. The large number of transmission attempts compensates for this,
however. In this case you are advised to check the transmission connection for
possible sources of interference, because frequent repetitions reduce the
user-data rate and integrity of the transmission. The disturbance could also be
caused, however, by a malfunction on the part of the communication partner.
If BREAK occurs on the receive line (receive line interrupted), an error is displayed
at the STATUS output of the FB. No repeat is started. The BREAK status is
automatically reset as soon as the connection is restored on the line.
For every detected transmission error (lost character, frame or parity error), a
standard number is reported, regardless of whether the error was detected during
sending or receiving of a data block. The error is only reported, however, following
unsuccessful repetitions.
Startup of the 3964(R) Procedure
The figure below illustrates the startup of the 3964(R).
Figure 2-17
2-32
Power-up after restart of the CPU
or voltage recovery
Evaluate parameter
assignment
Initialize interface
Send NAK
Flow Diagram of the Startup of the 3964(R) Procedure
Point-to-point connection CP 440 Installation and Parameter Assignment
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