Siemens SIMATIC NET S7-300 Manual page 202

7 Programming FCs (Functions) and FBs for S7 Ethernet CPs
FC6 AG_RECV / FC60 AG_LRECV - continued
Notice
Please note the following special feature on TCP connections:
With the older S7-CPs for S7-300, you must use FC AG_LRECV on TCP
connections!
With the latest S7-CPs for S7-300, you must also use FC AG_RECV for TCP
connections.
How the Block Works
The following diagram illustrates the normal sequence of data acceptance
triggered by an AG_RECV in the user program.
Each AG_RECV job in the user program is acknowledged by the Ethernet CP with
an entry in the output parameters NDR, ERROR and STATUS.
The way in which the FC functions depends on the CP type you are using.
S Case a: Sequence with older CP types
With older CP types, the transfer of the entire data area regardless of its length
is started after the first block call by the CP.
S Case b: Sequence with newer CP types (S7-300 only!)
With the newer CP types, optimized data transfer on the SEND/RECEIVE
interface is available. In particular with longer data records, this allows a much
higher data throughput on the interface between the CPU and CP.
A -202
S7-CPs for Industrial Ethernet Configuring and Commissioning
Release 01/2007
C79000-G8976-C182-07