Basic Principles of Serial Data Transmission
2.4 Data Transmission with the ASCII Driver
Point-to-point connection CP 440 Installation and Parameter Assignment
Manual, 09/2007, A5E00057742-03
39
Receive Buffer on CP 440
The CP 440 receive buffer accommodates 2000 bytes. During the parameter assignment,
you can specify whether overwriting of data in the receive buffer should be prevented. You
can also specify the value range (1 to 10) for the number of buffered receive message
frames or use the complete receive buffer.
You can delete the CP receive buffer at startup. The setting can be made either by using the
parameter assignment interface or by calling the RES_RCV function block.
The receive buffer on the CP 440 is a ring buffer:
●
If several message frames are entered in the receive buffer 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 buffered 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 find that when the receive data is requested again, the CPU first 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.
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 the configuration to the flow control operating
mode, it sends the XON character.
●
When the configured 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 configured time has elapsed, the transmission is
aborted and an appropriate error message (0708H) is generated at the STATUS output of
the function blocks.
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