Siemens Simatic S7-300 CPU 31xC and CPU 31x S7-300 - Manual (Page 53)

Communication
3.2 Communication services
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05
3-25
Establishing a connection for communication
FB 65 "TCON" establishes communication between the CPU and a communication partner.
You can establish up to eight connections. The CPU automatically monitors and holds the
active connection.
Communication partner A must initiate the connection. When the connection of
communication partner A is active, it transmits a request to connect to communication
partner B. Communication partner B waits until it receives the request for a passive
connection.
In your connection configuration, you define which communication partner activates the
connection, and which communication partners respond to the request with a passive
connection.
Both communication partners must have established their connection in order to be able to
exchange data.
Data exchange
Bidirectional data exchange is enabled after you established communication, that is, data
can be transmitted and received in parallel. FBs available for data exchange:
Name of the FB Description
FB 63 "TSEND" Transmit data
FB 64 "TRCV" Receive data
You can transmit and receive up to 1460 bytes of user data.
Disconnecting
FB 66 "TDISCON" disconnects the CPU from a communication partner.
Communication interruptions
Events causing interruptions of communication:
•
You program the cancellation of connections at FB 66 "TDISCON."
• The CPU goes from RUN to STOP.
• At POWER OFF / POWER ON
Reference
For detailed information on the blocks described earlier, refer to the STEP 7 Online Help.