Rockwell Automation AADvance Controller Configuration Guide - Manual (Page 195)

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Document: 553633
ICSTT-RM405f-EN-P
Issue 07
9-9
Modbus Exception Responses
The AADvance controller uses the lack of a response from a slave device to detect an
error on a Modbus link; also in some cases the slave can return a code for a specific
exception.
The Modbus protocol allows for these errors by returning an error frame to the
master. The error frame consists of the original requested function code with the high
bit set and a data field consisting of the specific error code. The AADvance Modbus
master does not log these codes explicitly, but it does recognize them and treat them
as a general error to the device. The slave status variable is set to 'Slave Error' to allow
the application to respond to the exception.
AADvance Objects for Modbus Master
The AADvance controller implements its Modbus master functionality through one or
more Modbus master objects, which you create within the AADvance Workbench.
Each Modbus master object can connect to one Modbus TCP device; or to one or
more Modbus RTU devices.
Modbus master objects are allocated to 9110 processor modules; a Modbus master
object can use one of the two Ethernet ports or one of the two serial ports associated
with the processor module. If you need more than one Modbus master object, and the
controller has two or three processor modules, you can distribute your Modbus
master objects between the processor modules to make the best use of the ports.
This gives flexibility if for example the connections to your Modbus TCP devices are
through more than one network.
Failure of a processor module will cause the failure of the Modbus communications
allocated to that processor module. When a controller has more than one processor
module, you can distribute your Modbus master objects between two or three of the
processor modules. Do this within the AADvance Workbench when you create the
Modbus master objects. If a processor module fails, slave communications will continue
to operate on the channels associated with the remaining processor modules.
The AADvance Workbench represents each slave link by a slave object, one object for
each slave device. You can distribute slave objects between Modbus master objects as
desired, subject to a constraint of no more than one slave link for a master object
configured for Modbus TCP. Master/slave relationships are configured under the
master objects.
Modbus Master Capacities
Modbus master, slave and message configurations are subject to the limits shown in
the table.
Table 23: Modbus Master Capacities per processor module
Item Limit
Maximum number of masters 16
Maximum number of slaves 64