Siemens Simatic S7-400

2.5.3 Sending Data with 3964(R)
Transmission Sequence
The 󹪝gure below illustrates the transmission sequence when data is sent with the 3964(R)
procedure.
CP 440
STX
DLE
DLE
ETX
BCC
DLE
¡
¡
¡
¡
Communication
partner
1st byte
2nd byte
1st data byte
2nd data byte
nth bytenth data byte
Start code (02H)
Pos. acknowledgment (10H)
End code (10H)
End code (03H)
3964(R) only
Pos. acknowledgment (10H)
Connection
setup
User
data
Connection
cleardown
Figure 2-14 Data tra󹪟c when sending with the 3964(R) procedure
Establishing a Send Connection
To establish the connection, the 3964(R) procedure sends the control code STX. If the
communication partner responds with the DLE code before the acknowledgment delay time
expires, the procedure switches to send mode.
If the communication partner answers with NAK or with any other control code (except for DLE
or STX), or the acknowledgment delay time expires without a response, the procedure repeats
the connection setup. After the de󹪝ned number of unsuccessful connection attempts, the
procedure aborts the connection setup and sends the NAK code to the communication partner.
The CP 440 reports the error to the SEND_440 function block (STATUS output parameter).
Sending Data
If a connection is successfully established, the user data contained in the output bu󹪜er of the CP
440 is sent to the communication partner with the selected transmission parameters. The
partner monitors the times between incoming characters. The interval between two characters
must not exceed the character delay time.
If the communication partner sends the NAK control code during an active send operation, the
procedure aborts its transmission of the block and tries again as described above, beginning
with connection setup. If a di󹪜erent code is sent, the procedure 󹪝rst waits for the character delay
time to expire and then sends the NAK code to change the mode of the communication partner
to idle. Then the procedure starts to send the data again with the connection setup STX.
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
38 Equipment Manual, 03/2022, A5E00057742-AE