CI853 as slave in 800xA Version 6 (the text error so i attached file)
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To implement the Slave functionality on CI853 You can use the access variable editor to map the defined modbus Slave addresses in the slave to the application variable. The communication is controlled by the master, which will need to connect, write and read variables.
On the hardware part You need to remember that a channel may be defined in one way: either as a Master or a Slave (You can't have master and slave on one channel). Moreover AC800M slave does not support multidrop mode, only point-to-point.
To get more info see manual 3BSE035982-600 AC 800M 6.0 Communication Protocols.
Hope this helps,
Source: 3BSE035982-600 AC 800M 6.0 Communication Protocols