Open User Communication; Protocols - Siemens SIMATIC S7 System Manual

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Open user communication

Open User Communication (OUC) provides a mechanism for your program to transmit and

receive messages over an Ethernet network. You can select the Ethernet protocol used as

the transport mechanism: UDP, TCP, or ISO-on-TCP

The CPU models CPU CR20s, CPU CR30s, CPU CR40s, and CPU CR60s have no

Ethernet port and do not support any functions related to the use of Ethernet

communications.

UDP (User Datagram Protocol)

User Datagram Protocol (UDP) uses a simple connectionless transmission model with a

minimum of protocol overhead. There is no handshake mechanism in UDP protocol so the

protocol is only as reliable as the underlying network. There is no guarantee of delivery,

ordering, or duplicate message protection. UDP does provide checksums for data integrity

and generally uses different port numbers to address different functions.

TCP (Transmission Control Protocol)

Transmission Control Protocol (TCP) is a core internet protocol. TCP provides a reliable,

ordered, and error-checked delivery of messages between applications running on hosts

communicating over an Ethernet network. TCP guarantees that all bytes received are

identical with the bytes sent, and in the original order. TCP protocol creates a connection

between an active device (the device that initiates the connection) and a passive device (the

device that accepts the connection). Once a connection is established, either device can

initiate a transfer of data.

TCP protocol is a "streaming" protocol. This means that there is no end sign included in a

message. All the messages that are received are considered to be part of a "stream" of data.

For example, the client device sends three messages of 20 bytes each to a server. The

server sees only a "stream" of 60 bytes received (assuming the server executes a receive

operation after the three messages are received.)

S7-200 SMART

System Manual, V2.3, 07/2017, A5E03822230-AF

Communication

8.4 Ethernet

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