Siemens SIMATIC S7 System Manual page 192

Program instructions
7.3 Communication
The CPU utilizes a single connection for all GET/PUT instructions that are directed to the
same IP address. For example, if there are three GET instructions enabled at the same time
when the remote IP address is 192.168.2.10, then the GET instructions execute sequentially
on one Ethernet connection to IP address 192.168.2.10.
If you try to create a ninth connection (a ninth IP address), the CPU searches through the
connections to find the connection that has been inactive for the longest period of time. The
CPU disconnects this connection and then creates a connection to the new IP address.
The GET and PUT instructions require additional communication background time (refer to
"Configuring communication" (Page 135)) 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 GET and PUT
instruction that are active/busy, how often the GET and PUT 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.
Table 7- 2
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)
192
Valid operands for the GET and PUT 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