Basic Principles of Serial Data Transmission
2.5 Data transfer using the ASCII driver
PtP coupling and configuration of CP 340
Manual, 04/2011, A5E00369892-03
49
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 on the CP 340 is implemented as follows:
● As soon as the CP 340 is configured for operation in a mode with flow control, it sends
the XON character or sets the RTS line to ON.
● When the programmed number of telegrams or 50 characters are reached before the
receive buffer overflows (size of the receive buffer: 1,024 bytes), the CP 340 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 telegram will be discarded.
● As soon as a telegram is fetched by the S7 CPU and the receive buffer is ready to
receive, the CP 340 sends the XON character or sets the RTS line to ON.
● The CP 340 interrupts transmission if it receives the XOFF character or when control
signal CTS is set to OFF. If neither an XON character is received nor CTS is set to ON
once a configured time has elapsed, transmission is aborted and an appropriate error
message (0708H) is generated at the STATUS output of the function blocks.
Note
When RTS/CTS data flow control is configured, you must fully wire the interface signals
used in the plug connection (see Appendix "Connecting Cables (Page 165)"). |When
RTS/CTS data flow control is
configured, RTS control by means of the V24_SET FC is
not possible.
Tasks of the V24_STAT/SET FC
The V24_STAT function can be used to determine the status of each RS 232C
accompanying signal. The V24_SET function can be used to control the DTR and RTS
output signals.
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