General Introduction And Requirements - Siemens SIPROTEC 4 Manual

Redundant Modbus TCP

6.1 General Introduction and Requirements

6.1 General Introduction and Requirements
Redundant communication in a system using Modbus TCP can be achieved by providing the possibility to
establish more than 1 logical connection between 2 stations.
The station performing the connection establishment is referred to as the controlling station (station A) in the
subsequent description. The partner station is referred to as the controlled station (station B).
The following general rules apply to this clause concerning redundant connections:
•
The controlling and the controlled station must be able handling multiple (N) logical connections.
•
The N logical connections represent 1 redundancy group.
•
Only 1 logical connection is in the started state and is sending/receiving user data at the same time for 1
redundancy group.
•
The controlling station decides which of the N logicalconnections is in started state.
•
A redundancy group is based on only 1 process image (database/event buffer) which determines the
member of the current redundancy group.
For Modbus TCP, multi-clients are supported. The specific behavior is described in the following.
Connection
2 Modbus TCP masters can communicate with the device at the same time.
If the first master communicates with the device via default port 502/504, the other master can only communi-
cate via another default port 504/502. The Port number can be assigned by setting.
The IP addresses of Modbus TCP masters are not limited. Any master with a suitable IP address and
assigned TCPPortNumber can be connected.
Master Client
The first connected Modbus TCP client will be recognized as the Modbus TCP Master client. The time
synchronization required by the Modbus TCP Master client will be accepted. The time-synchronization
request from the other client will be rejected.
If the first connected Modbus TCP client is disconnected, the second client will act as the Modbus TCP
Master client.
Data process
The data in monitoring direction can be sent to both clients. Commands in control direction from both clients
can be accepted properly.
56 SIPROTEC 4, Modbus TCP Profile, Manual
C53000-L1840-C582-1, Edition 07.2015