Siemens Simatic S7-400

The receive bu󹪜er on the CP 440 is a ring bu󹪜er:
• If several message frames are entered in the receive bu󹪜er of the CP 440, the following
applies: it is always the oldest one that is sent from the CP 440 to the CPU.
• If you only ever want to send the most recent message frame to the CPU, you must set the
value "1" for the number of bu󹪜ered message frames and deactivate the overwrite protection.
Note
If the constant reading out of the receive data in the user program is interrupted for a while,
you may 󹪝nd that when the receive data is requested again, the CPU 󹪝rst receives old message
frames from the CP 440 before it receives the most recent one. The old message frames are
those on their way when transmission between the CP 440 and the CPU was interrupted, or
which had already been received by the FB.
Data Flow Control/Handshaking
Handshaking controls the data 󹪞ow between two communication partners. Handshaking
ensures that data is not lost in transmissions between devices that work at di󹪜erent speeds.
Software handshaking is supported with XON/XOFF in the CP 440.
Data 󹪞ow control is implemented as follows on the CP 440:
• As soon as the CP 440 is switched by the con󹪝guration to the 󹪞ow control operating mode,
it sends the XON character.
• When the con󹪝gured number of message frames is reached, or alternatively 50 characters
before the receive bu󹪜er over󹪞ows (size of the receive bu󹪜er: 2000 bytes), the CP 440 sends
the XOFF character. If the communication partner continues to send data regardless of this,
the receive bu󹪜er over󹪞ows 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 bu󹪜er 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 con󹪝gured time has elapsed, the transmission is aborted
and an appropriate error message (0708H) is generated at the STATUS output of the function
blocks.
2.4.4 Topologies Between the Communication Partners
Application options
The CP 440 can be used in di󹪜erent topologies in the RS422 and RS485 operating modes.
Distinctions are drawn between connections with:
• Two nodes (point-to-point) and
• Several nodes (multipoint)
Basic Principles of Serial Data Transmission
2.4 Data Transmission with the ASCII Driver
Point-to-point connection CP 440 Installation and Parameter Assignment
32 Equipment Manual, 03/2022, A5E00057742-AE