ModMultiSim help v3.06 - 3.2. Simulation: Edit Slave Settings
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This is where you add or remove a Modbus slave, or modify settings specific to the slave but common to all of a slave's registers. These settings are the mapping of addresses and the configuration of 32-64 bit values.
To navigate to these settings, type Ctrl-W to bring up the Edit Settings... dialog then with the Simulation tab selected, click on the appropriate subpanel title under Edit Slave Settings..