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

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
• ET 200SP:
– CM PtP
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
4.
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
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