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Und in die S7 200 Handbücher..
Creating User-Defined Protocols with Freeport Mode
Freeport mode allows your program to control the communications port of the S7-200 CPU. You
can use Freeport mode to implement user-defined communications protocols to communicate with
many types of intelligent devices. Freeport mode supports both ASCII and binary protocols.
To enable Freeport mode, you use special memory bytes SMB30 (for Port 0) and SMB130 (for
Port 1). Your program uses the following to control the operation of the communications port:
Transmit instruction (XMT) and the transmit interrupt: The Transmit instruction allows the
S7-200 to transmit up to 255 characters from the COM port. The transmit interrupt notifies
your program in the S7-200 when the transmission has been completed.
Receive character interrupt: The receive character interrupt notifies the user program that a
character has been received on the COM port. Your program can then act on that character,
based on the protocol being implemented.
Receive instruction (RCV): The Receive instruction receives the entire message from the
COM port and then generates an interrupt for your program when the message has been
completely received. You use the SM memory of the S7-200 to configure the Receive
instruction for starting and stopping the receiving of messages, based on defined
conditions. The Receive instruction allows your program to start or stop a message based
on specific characters or time intervals. Most protocols can be implemented with the
Freeport mode is active only when the S7-200 is in RUN mode. Setting the S7-200 to STOP mode
halts all Freeport communications, and the communications port then reverts to the PPI protocol
with the settings which were configured in the system block of the S7-200."