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Modbus to Ethernet Bridge
12
1.2 Mapping Modbus and IP Addresses
The Bridge maps messages between Modbus and IP addresses according to the type
of device you have configured at its Modbus port.
1.2.1
Mapping for a Modbus Master
When you configure a Modbus Master device at the Bridge’s serial port, you can
assign up to eight entries into an internal mapping table that is maintained in the
Bridge’s memory. You enter your intended mapping into the table during your
configuration of the Bridge.
Each table entry maps a single Modbus address, or a range of addresses, to a
destination IP address.
When the Bridge receives a message from the Master, it searches the mapping table
for an entry matching the Modbus address in the message. If one is found, the Bridge
sends the message to the IP address for that entry. If a matching entry is not found,
the Bridge returns an exception response to the Master application.
Note that the original Modbus address is retained in the message transmitted to the IP
destination. If the remote IP node is another Bridge, the message’s Modbus address
can be used to identify a Slave device at that Bridge’s serial port.
1.2.2
Mapping for a Modbus Slave
When you configure a Modbus Slave device or network with multiple Slaves at the
Bridge’s serial port, you have two options for routing messages to a Slave destination.
You assign your choice during your configuration of the Bridge.
Your options are: Message address routing; or, Fixed address routing.
You can specify that the Bridge must route each message to the Slave device that is
identified in the Modbus address field of the message. This allows you to address any
Slave device in a network of up to 247 devices at the Bridge’s Modbus port.
You can specify that the Bridge must route all messages to one fixed Slave address
that you define in the Bridge’s configuration. With this option the Modbus address field
is ignored, and each message is routed to that fixed Slave address only. This limits
addressing to a single device at the port.