Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 115)

Publication AG-UM008C-EN-P - February 2005
Configuring MicroLogix 1100/1200/1500 Controllers 3-47
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 up to six 256-word data
table files for a total of 1536 Modbus Holding registers.
TIP
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.
TIP
Modbus RTU Slave driver can be used with the
following controllers:
•
All MicroLogix 1100 controllers
•
All MicroLogix 1200 controllers
•
MicroLogix 1500 1764-LSP Series B and higher
•
All MicroLogix 1500 1764-LRP
TIP
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.