Siemens SIMATIC NET CP 243-1 IT Technical Manual page 92

Configuration
Free connections
A free connection can only be used when your S7-300 or S7-400 system is to be
the server. Free connections do not have to be configured in STEP 7. Standard
S7-300 and S7-400 systems are designed so that they can communicate via free
connections.
If you want to take advantage of the free connection routine on you S7-300 or S7-
400 system, the client side must still be configured for each of these connections.
STEP 7 Micro/WIN 32 does not distinguish between the use of such connections
and communication via connections which were configured on the S7-300 or S7-
400 system. Just remember that, with free connections, the first byte of the TSAP
with which such connections run on S7-300 and S7-400 systems must always
have the value 0x03. The second byte of the TSAP is derived similar to the above
described procedure from the rack and the slot number of the CPU used on your
S7-300 or S7-400 system. This means that, with a free connection, the communi-
cation access point on the S7-300 or S7-400 system is always located on the CPU
and not on the Ethernet communications processor.
Note
S7-200 systems do not support free connections. This means that, with an S7-200
system, you will always have to configure each connection no matter whether your
system is being used as a client or a server.
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