Siemens SIMATIC CP 440 Manual page 46

Basic Principles of Serial Data Transmission
Receiving Data with 3964(R)
The figure below illustrates the transmission sequence when data is received with
the 3964R procedure.
Communication
partner
Connection
setup
User
data
Connection
cleardown
Figure 2-14
Note
As soon as it is ready, the 3964(R) procedure sends a single NAK to the
communication partner to set the latter to idle.
Establishing a Receive Connection
In idle mode, when there is no send job to be processed, the procedure waits for
the communication partner to establish the connection.
If no empty receive buffer is available during a connection setup with STX, a wait
time of 400 ms is started. If there is still no empty receive buffer after this time has
elapsed, the CP 440 reports the error (error message at the STATUS output of the
FB). The procedure sends an NAK and returns to idle mode. Otherwise, the
procedure sends a DLE and receives the data.
If the procedure receives any control code except for STX or NAK in idle mode, it
waits for the character delay time to expire, then sends the code NAK. The CP 440
reports the error to the RECV_440 function block (STATUS output parameter).
2-28
STX
DLE
1st byte
2nd byte
nth byte
DLE
ETX
BCC
DLE
Data Traffic When Receiving with the 3964(R) Procedure
Point-to-point connection CP 440 Installation and Parameter Assignment
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)
A5E00057742-02