Siemens SIMATIC CP 440 Manual page 39

Data Flow Control/Handshaking
Handshaking controls the data flow between two communication partners.
Handshaking ensures that data is not lost in transmissions between devices that
work at different speeds. Software handshaking is supported with XON/XOFF in
the CP 440.
Data flow control is implemented as follows on the CP 440:
As soon as the CP 440 is switched by parameterization to the flow control
operating mode, it sends the XON character.
When the parameterized number of message frames is reached, or
alternatively 50 characters before the receive buffer overflows (size of the
receive buffer: 2000 bytes), the CP 440 sends the XOFF character. If the
communication partner continues to send data regardless of this, the receive
buffer overflows and an error message is generated. The data received in the
last message frame is discarded.
As soon as a message frame is fetched by the S7 CPU and the receive buffer
is ready to receive, the CP 440 sends the XON character.
If the CP 440 receives the XOF character, the CP 440 interrupts transmission.
If an XON character is not received before a parameterized time has elapsed,
the transmission is aborted and an appropriate error message (0708H) is
generated at the STATUS output of the function blocks.
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Basic Principles of Serial Data Transmission
2-21