Siemens SIMATIC S7 System Manual page 218

Program instructions
7.3 Communication
The OUC instructions maintain information about the connections so that your program does
not need to permanently allocate V memory for the OUC Tables. The data in the tables must
be kept constant while the OUC instruction is active.
The OUC instructions require additional communication background time when they are
processing/active/busy and also when they are just maintaining the connection to the other
device. The amount of communication background time required depends on the number of
OUC instructions that are active/busy, how often the OUC instructions are executed, and the
number of connections that are currently open. You should adjust the communication
background time to a higher value if the communication performance is slow. Refer to
"Configuring communication" (Page 135) for further information.
All of the OUC instructions use a table to store the parameters for the instructions. The
content of the tables for each instruction is described below.
The S7-200 SMART CPU uses the input table parameters to determine the instance of the
OUC instructions. The table parameters must be kept the same during an operation so that
the S7-200 SMART CPU knows that the particular instruction (instance) is the same as
during the previous scans.
Note
Siemens also offers the Open User Communication (OUC) library instructions for your
convenience. The OUC Library instructions build the tables for you based upon the inputs to
the library instructions. The Library instructions also retrieve the response information from
the tables and provide this information on the outputs of the library instructions. Refer to
"Open user communication library" (Page 498) for further information.
Table 7- 8
Inputs/Outputs
TABLE
Error conditions that set ENO = 0:
● 0006 (indirect address)
● If the function returns an error and sets the E bit of table status byte (see the figure
below)
218
Valid operands for the OUC instructions
Data Type Operands
BYTE IB, QB, VB, MB, SMB, SB, *VD, *LD, *AC
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART