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Configuring the Bridge
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3.10 Option 5: Unit ID to IP Address Mapping Table
If you have specified the Bridge’s serial port Attached Device parameter as a Modbus
Master (see Section 3.7), you will need to map the Slave addresses received in
messages from that Master to their intended IP address destinations. The Bridge
maps the one–byte Modbus Slave address to an IP address for delivery on Ethernet.
3.10.1
How the Address Mapping Works
The Bridge contains an address mapping table with eight entries. Each entry maps a
Modbus Slave address (range 1 ... 247 decimal) to a standard IP address
(XXX.XXX.XXX.XXX).
When the Bridge receives a message from the Master at its serial port, the Bridge
searches the table for a match between the message’s Slave address and a Slave
address entry in the mapping table. If a match is found, the Bridge forwards the
message to the IP address specified in that location in the table. If a match is not
found, a message timeout will occur. If a match is found, but there is not an active
TCP connection at the IP address, a message timeout will occur. The Master
application must provide its own method of handling the message timeout.
The Bridge begins its search at table location 1, and continues in the sequence
1, 2 ... 8. It stops at the first location which matches the Slave address, and sends the
message to the corresponding IP address. Thus any duplicate entry, if one exists in
the table, will be ignored.
If you have configured the serial port for a Modbus Master, you must use the mapping
table to associate the ranges of Slave addresses in your Master application to the IP
destination addresses for those Slaves.
3.10.2
Example: Address Mapping
If you have configured the serial port for a Modbus Master, and have entered your
mapping, it will appear on the Bridge’s initial Configuration Menu. Here is an example.
5) Unit ID –> IP Address Table:
001–001: 192.168.001.020 002–002: 192.168.001.021
003–003: 192.168.001.022 010–010: 192.168.001.024
011–247: 192.168.001.025
In this example, the Bridge maps Modbus Slave address 1 to IP address 192.168.1.20.
It maps Slave addresses 2, 3, 10 as shown in the table. It maps any Slave address in
the range 11 ... 247 to IP address 192.168.1.25.