Siemens Simatic S7-300 CP 341 User Manual page 72

Basic Principles of Serial Data Transmission
2.6 Data Transmission with the ASCII Driver
Automatic control of accompanying signals
Automatic control of the RS 232C accompanying signals is implemented on the CP 341 as
follows:
● As soon as the CP 341 is assigned for operation in a mode with automatic control of the
RS 232C accompanying signals, it sets the RTS line to OFF and the DTR line to ON
(CP 341 ready for operation).
This prevents sending and receiving of message frames until the DTR line is set to ON.
No data is received at the RS 232C interface as long as DTR = OFF. Any send requests
will be cancelled with a corresponding error message.
● When a send job is queued, the module sets RTS=ON, and triggers the configured data
output wait time. After the data output time has elapsed and CTS = ON, the data is sent
via the RS 232C interface.
● If during sending the CTS line is not set to ON within the data output wait time or if CTS
changes to OFF during the send operation, the send request is canceled and a
corresponding error message is generated.
● Once the data has been sent and the assigned Clear RTS time has elapsed, the RTS line
is set to OFF. There is no waiting time for the transition from CTS to OFF.
● Data can be received via the RS 232C interface when DSR=ON. If the receive buffer of
the CP 341 is close to overflow, the CP 341 will not respond.
● If DSR changes from ON to OFF, an active send request as well as the receipt of data will
be canceled with an error message. The message "DSR = OFF (automatic use of V24
signals)" is entered in the diagnostic buffer of the CP 341 use of V24 signals").
Note
Automatic control of the RS 232C accompanying signals is only possible in half-duplex
mode. When automatic control of the RS 232C accompanying signals is assigned,
neither RTS/CTS data flow control nor RTS and DTR control by means of the
FC V24_SET function are possible.
Note
The "Clear RTS time" must be set in the parameter assignment interface so that the
communication partner can receive the last characters of the message frame in their
entirety before RTS, and thus the send request, is canceled. The "data output wait time"
must be set so that the communication partner can be ready to receive before the time
elapses.
72
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CP 341 Point-to-Point Communication, Installation and Parameter Assignment
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Manual, 09/2008, A5E02191071-01