Siemens SIMATIC CP 440 Manual page 52

Basic Principles of Serial Data Transmission
Receiving with the 3964(R) Procedure (Part 1)
The figure below illustrates receiving with the 3964(R) procedure.
T = 400 ms
NAKTIM
W = transmission attempt counter
Immediate return to initial state at line break (BREAK)
Figure 2-19
2-34
Characters except
STX, NAK
G
Note
NAK
4
Send DLE
4
Flow Diagram of Receiving with the 3964(R) Procedure (Part 1)
Point-to-point connection CP 440 Installation and Parameter Assignment
Intention to send
2
Receive
STX
Repetition
expected
1
W +1
Start
T
NAKTIM
Wait
T > T
NAKTIM
Buffer free Buffer not free
Send NAK
G
5
Initialization
conflict, low
priority
A5E00057742-02