Modbus TCP, local TCP/IP connection

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3. Modbus TCP, local TCP/IP connection

Note – Only works in Router mode.

TCP connection is established only locally between Modbus devices and the connected RipEX units. TCP protocol overhead is not transmitted over the Radio channel. Secured TCP/IP transfer is not necessary because in Router mode every packet in the Radio channel is acknowledged on every radio hop. A packet is therefore repeated directly in the part of the network where it is lost, not across the entire radio network as in TCP/IP. This improves latency and increases network throughput.

Modbus TCP local

Fig. 3.1: Modbus TCP local

Set your Modbus TCP Master to use a single IP to communicate with Modbus TCP Slaves (RipEX ethernet IP) and set TCP port to 502. Communication begins on port 502 from where it is redirected to other RipEX ports, corresponding to the individual RTU’s, based on negotiation with the Modbus TCP Master.

To set up RipEX connected to Modbus TCP Master:

  • Set Modbus TCP/RTU to On. Type the port number on which the connected Modbus TCP Master initiates communication, by default 502, into “My TCP Port” field.

  • Select how you want to translate Modbus addresses to RipEX IP addresses (using mask or table). Set the UDP interface to Terminal server (TS1-TS5). Set the same TS for remote RipEX’s too.


The maximum number of concurrent TCP/IP connections between a Modbus TCP device and RipEX is set to 10 due to limited computing capacity. (Note: The number of concurrently open TCP/IP connections can be increased using CLI if necessary.) Modbus TCP Master must be set to not open more than 10 TCP/IP connections at any given time.

To set up RipEX connected to Modbus TCP Slave:

  • Modbus TCP/RTU – Off

  • Terminal Servers – On

  • Set the Terminal Server (see RipEX Master settings) to TCP and set My Port to 502. Use the address of the connected Modbus Slave as the destination IP and fill in the destination port number which the connected Modbus Slave device scans for incoming communication.

  • Set Protocol to UNI and Mode of Connected device to Slave.