Allen-Bradley MicroLogix 1400 Instruction Manual page 614

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612 Protocol Configuration
Publication 1766-RM001A-EN-P - October 2008
The most significant digit of the address is considered a prefix, and does
not get entered into the Modbus Data Address field when configuring the
message instruction.
When the message is sent, the address is decremented by 1 and converted
into a 4-character hex number to be transmitted via the network (with a
range of 0-FFFFh); the slave increments the address by 1, and selects the
appropriate memory group based on the Modbus function.
Modbus protocol may not be consistently implemented in the field. The
NOTE
Modbus specification calls for the addressing range to start at 1; however,
some devices start addressing at 0.
The Modbus Data Address in the Message Setup Screen may need to be
incremented by one to properly access a Modbus slave's memory,
depending on that slave's implementation of memory addressing.
Modbus RTU Slave
The coil and contact files can contain up to 4096 coils or contacts in each
register when the data table file is configured for a maximum size of 256
words. Each input register and holding register file can contain up to 256
registers when the data table file is configured for a maximum size of 256
words. With the "Expanded" box checked, the controllers can be
specifically configured to use up to six 256-word data table files for a total
of 1536 Modbus Holding registers.
A request to access a group of holding registers that span across two files
NOTE
is permitted. Note that the maximum number of registers in a command
does not allow for more than two files to be accessed during a single
Modbus command.
Modbus RTU Master Configuration
Select the Modbus RTU Master from the Channel Configuration menu as
shown below.