Moxa EDR-810-VPN-2GSFP [114/139] Modbus tcp policy

Industrial Secure Router User's Manual Firewall

8-10

Modbus TCP Policy

Modbus TCP is a Modbus protocol used for communications over TCP/IP networks, connecting over port 502 by

default. Some have experimented with using Modbus over UDP on IP networks, which removes the overheads

required for TCP. The following table shows the Modbus TCP frame format:

Modbus TCP Frame Format

Description Length Function

Transaction Identifier 2 bytes Synchronization between messages of server & client

Protocol Identifier 2 bytes The value is 0 for Modbus TCP protocol

Length Field 2 bytes Number of remaining following bytes in this frame

Unit Identifier 1 byte Slave Address (255 is used for device broadcast information)

Function code 1 byte Define message type

Data bytes n bytes Data block with additional information

Modbus Policy Setup

The Industrial Secure Router provides Modbus policy inspection of Modbus TCP packets, which allows users to

control Modbus TCP traffic based on the following parameters:

Add a Modbus TCP Filtering Rule

Check the “Enable” checkbox and input the correspondent Modbus TCP parameters in the page, and then click

“Add” to add it into the Modbus Filtering Table. Finally, click “Activate” to activate the configuration.

Delete a Modbus TCP Filtering Rule

Select the item in the Modbus Filtering Table, then, click “Delete” to delete the item.

Modify a Modbus TCP Filtering Rule

Select the item in the Modbus Filtering Table. Modify the attributes and click “Modify” to change the

configuration.

Activate Modbus TCP Filtering Table

After adding/deleting/modifying any Modbus TCP Filtering Rules, make sure to click “Activate” to activate the

item.