Siemens Simatic S7-400 - Page 47

Initialization Con󹪞ict
The 󹪝gure below illustrates the transmission sequence during an initialization con󹪞ict.
STX
STX
DLE
STX
DLE
DLE
ETX
BCC
DLE
¡
¡
¡
¡
Communication partner
(higher priority)
1st byte
2nd byte
1st data byte
2nd data byte
nth byte
nth data byte
2nd setup attempt
Start code (02H)
Start code (02H)
Pos. acknowledgment (10H)
Start code (02H)
Pos. acknowledgment (10H)
End code (10H)
End code (03H)
3964(R) only
Pos. acknowledgment (10H)
Connection
setup
Connection
setup
User data
Connection
cleardown
CP 440
(low priority)
Figure 2-20 Data Tra󹪟c During an Initialization Con󹪞ict
If a device responds to the communication partner's send request (code STX) within the
acknowledgment delay time by sending the code STX instead of the acknowledgment DLE or
NAK, an initialization con󹪞ict occurs. Both devices want to execute a send request. The device
with the lower priority withdraws its send request and responds with the code DLE. The device
with the higher priority sends its data in the manner described above. Once the connection has
been released, the lower-priority device can execute its send request.
To be able to resolve initialization con󹪞icts you must assign parameters with di󹪜erent priorities
for the communication partners.
Procedure Errors
The procedure recognizes 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 process. It reports the error number of
the 󹪝rst recognized 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 a󹪜ecting the data tra󹪟c. The large
number of transmission attempts compensates for this, however. In this case you are advised to
check the transmission link for possible sources of interference, because frequent repetitions
Basic Principles of Serial Data Transmission
2.5 Data Transmission with the 3964(R) Procedure
Point-to-point connection CP 440 Installation and Parameter Assignment
Equipment Manual, 03/2022, A5E00057742-AE 47