Ethernet Communications Modules, 3rd Edition, Rev. D
6-6
Chapter 6: H0/H2/H4-ECOM100 DHCP & HTML Configuration
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The Port Number and Unit ID are only used if the H0/H2/H4 -ECOM100 client is trying to
communicate with a Modbus TCP server device through a firewall or an Ethernet- to- serial
converter.
Port Number: The Modbus TCP specification states that all Modbus TCP devices respond
to (server) and send (client) on port 502. This will allow a Modbus TCP server device to
communicate with an H0/H2/H4- ECOM100 Modbus TCP client through a firewall.
Unit ID: The Unit ID may be required for the following applications:
• Modbus/TCP- to- Modbus/RTU converters: These devices convert Ethernet
(TCP) packets to serial (RTU) communications. The Unit ID field is used to
direct the TCP packet to the correct, serially connected RTU slave device.
• Some Modbus/TCP slave devices require this; It may be a holdover from an old serial
device driver that was converted to Ethernet. The slave device usually requires the Unit
ID to be set to “255”, but in some instances the slave requires the Unit ID to be set to
“0”. The Modicon specification states to use Unit ID set to “255”. If communications
are unsuccessful, it is recommended to try Unit ID of “0”. It is possible you may need
to get the actual Unit ID setting from the slave device documentation or manufacturer.
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