Connections - Siemens SIMATIC S7 System Manual

Communication
8.4 Ethernet
ISO-on-TCP
ISO-on-TCP is a protocol add-on using RFC 1006. The main advantage of ISO-on-TCP is
that you have a clear data end sign in the data so that you know when the entire message
has been received. SPS7 protocol (Put/Get) utilizes ISO-on-TCP protocol. ISO-on-TCP only
uses port 102 and utilizes TSAPs (Transport Services Access Point) to route the message to
the proper receiver instead of a port as in TCP.
The ISO-on-TCP protocol delineates each message received. For example, a client sends
three messages to a server using the ISO-on-TCP protocol. Even if the server waits for all
messages to accumulate before checking for a received message, the server will get each
message as it was sent and see three distinct messages. This is the difference between
TCP protocol and the ISO-on-TCP protocol.
8.4.7.2

Connections

The S7-200 SMART CPU has two OUC instructions to perform connection management:
● The TCON instruction to establish an active connection (client) or open a passive
connection (server)
● The TDCON instruction to force a disconnection (for example, close a connection). A
RUN-to-STOP transition forces the closure of all open connections by the CPU.
The CPU supports two types of connections for OUC:
● Active: A connection established and maintained by the local CPU. The local CPU is
responsible for issuing the connection request to another device and maintaining the
connection so that the connection does not time out due to inactivity.
● Passive: A passive connection is one in which the local CPU opens a port and/or TSAP to
accept the connection request from another device.
The CPU supports eight active and eight passive connections.
The CPU creates a passive or active connection based upon the connection table passed to
the TCON instruction. UDP connections are always passive. TCP and ISO-on-TCP protocol
connections use a configuration parameter to determine the type of connection.
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System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART