Point-To-Point Link - Siemens SIMATIC ET 200AL System Manual

Distributed i/o system

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Point-to-point link

Functionality

A point-to-point connection for S7-1500, ET 200MP and ET 200SP is established via

communications modules (CMs) with serial interfaces (RS232, RS422 or RS485):

• S7-1500/ET 200MP:

– CM PtP RS232 BA

– CM PtP RS422/485 BA

– CM PtP RS232 HF

– CM PtP RS422/485 HF

The bidirectional data exchange via a point-to-point connection works between

communications modules or third-party systems or devices capable of communication. At

least 2 communication partners are required for communication ("point-to-point"). With

RS422 and RS485, more than two communications partners are possible.

Protocols for communication via a point-to-point connection

• Freeport protocol (also called ASCII protocol)

• Procedure 3964(R)

• Modbus protocol in RTU format (RTU: Remote Terminal Unit)

• USS protocol (universal serial interface protocol)

The protocols use different layers according to the ISO/OSI reference model:

• Freeport: Uses layer 1 (physical layer)

• 3964 (R), USS and Modbus: Use layer 1 and 2 (physical layer and data link layer; therefore

greater transmission reliability than with Freeport). USS and Modbus use additionally layer

Properties of the Freeport protocol

• The recipient recognizes the end of the data transfer by means of a selectable end

criterion (e.g. character delay time elapsed, receipt of end character, receipt of a fixed

amount of data).

• The sender cannot recognize whether the sent data arrived free of errors at the recipient.

Communication

Function Manual, 05/2021, A5E03735815-AJ

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