Publication 1763-RM001C-EN-P - October 2009
Protocol Configuration 525
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 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.
Modbus RTU Master Configuration
Select the Modbus RTU Master from the Channel Configuration menu as
shown below.
NOTE
Modbus protocol may not be consistently implemented
in the field. The 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.
NOTE
A request to access a group of holding registers that span
across two files 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.
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