IBM Modbus Plus - Manual (Page 17)

Mappings
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4.4 Local Addresses
When a mapping is created which allows a PLC to access data within the
X-Link, a local address dialog box must be completed. The information
required to complete a local address definition is the class of data to access
(Coils, Input Status, Input Registers or Holding Registers) and the offset into
the data table. If the access is specifying write, Input Status and Input
Registers are not available as options.
The data type maps to register numbers as:
Data Type Register Numbers
COILS 0xxxxx
INPUT STATUS 1xxxxx
INPUT REGISTERS 3xxxxx
HOLDING REGISTERS 4xxxxx
where 'xxxxx' represents a decimal number from 1 to 65535.
4.5 External Addresses
Mappings, which require the X-Link to originate commands, require the user
to complete an external address dialog box. The external address requires the
specification of a route to a remote node as well as a data type and offset.
To route directly to another PLC on the Modbus Plus, simply specify the
node number in decimal. If you are routing to a node through any routes, the
route is specified as up to five node numbers with periods between them.
To route to an X-Link or SA85 card, the route is specified as node followed
by data channel. Data channels are from 1 to 8.