Siemens SIMATIC CP 440 Manual page 44

Basic Principles of Serial Data Transmission
Sending Data with 3964(R)
The figure below illustrates the transmission sequence when data is sent with the
3964(R) procedure.
CP 440
Start code (02H)
Pos. acknowledgment (10H)
1st data byte
2nd data byte
nth data byte
End code (10H)
End code (03H)
3964(R) only
Pos. acknowledgment (10H)
Figure 2-13
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 (ADT) 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 parameterized
number of unsuccessful setup 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).
2-26
Data Traffic when Sending with the 3964(R) Procedure
Point-to-point connection CP 440 Installation and Parameter Assignment
Communication partner
STX
Connection
DLE
1st byte
2nd byte
nth byte
DLE
ETX
Connection
BCC
DLE
setup
User
data
cleardown
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