Modbus/TCP
What you will find in this section:
Modbus/TCP
In the case of Modbus/TCP, a special header (Modbus Application Header [MBAP]) is prefixed to the Modbus message. This Modbus/TCP message is then transmitted via an IP-based network in a TCP frame.
The EMpro acts as Modbus/TCP server and opens port 502 for communication with any Modbus/TCP clients. This port cannot be configured by the user.
Since TCP is a connection-oriented protocol, a client first has to establish a TCP connection with the EMpro to exchange data via Modbus/TCP.
The number of Modbus/TCP connections that can be open in parallel on each version of the EMpro product family is limited to four.
If there are already four open Modbus/TCP connections on the EMpro, additional connection queries on port 502 are rejected.
Additionally, to enable Modbus/TCP communication, the user has to make sure that this port is enabled in the network.
The MBAP contains a field that is declared as unit identifier in the specification. This unit identifier is comparable with the device address of Modbus/RTU. However, it is not relevant for the EMpro in the case of Modbus/TCP and is ignored by the device since through its IP address the device already has a unique address in a network . An exception applies if the device is operated in gateway mode (see Modbus gateway).
The Modbus TCP specification recommends using 255 as unit identifier for queries.
All EMpro versions support the Modbus TCP protocol.
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