Siemens Simatic S7-300 CP 341 User Manual page 74

Basic Principles of Serial Data Transmission
2.6 Data Transmission with the ASCII Driver
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.
There are essentially two types of handshaking:
● Software handshaking (e.g., XON/XOFF)
● Hardware handshaking (e.g., RTS/CTS)
Data flow control is implemented as follows on the CP 341:
● As soon as the CP 341 is assigned for operation in a mode with flow control, it sends the
XON character or sets the RTS line to ON.
● If the assigned number of message frames or 50 characters are reached before the
receive buffer overflows (size of the receive buffer: 4096 bytes), the CP 341 sends the
XOFF character or sets the RTS line to OFF. If the communication partner ignores this
state and continues transmission, an error message is generated if the receive buffer
overflows. The data received in the last message frame will be discarded.
● As soon as a message frame is fetched by the S7 CPU and the receive buffer is ready to
receive, the CP 341 sends the XON character or sets the RTS line to ON.
● The CP 341 interrupts the send operation when it receives the XOFF character or the
CTS control signal of the communication partner is set to OFF. If an XON character is not
received or the partner does not set CTS to ON within an assigned time period, the send
operation is canceled and a corresponding error message (0708H) is generated at the
STATUS output of the function blocks.
Note
When RTS/CTS data flow control is assigned, you must fully wire the interface signals in
the plug connection. When RTS/CTS data flow control is assigned, RTS control by
means of the FC V24_SET function is not possible.
Reading/controlling via FC V24_STAT and FC V24_SET
The FC V24_STAT function can be used to determine the status of each RS 232C
accompanying signal. The FC V24_SET function can be used to control the DTR and RTS
output signals.
Switchover times for RS485 module in half-duplex mode
The maximum switchover time between sending and receiving is 1 ms.
This value is applicable to modules as of MLFB number 6ES7341–1_H01–0AE0.
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CP 341 Point-to-Point Communication, Installation and Parameter Assignment
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Manual, 09/2008, A5E02191071-01