Reference MVI56-MCM ♦ ControlLogix Platform
Modbus Communication Module
ProSoft Technology, Inc. Page 147 of 159
July 24, 2008
_PassThru
This optional task is used to send pass-through data between the processor and
the Modbus devices connected to the MVI56-MCM module. Pass-Through
functionality for the module allows the user to read and write the same modbus
address when the module is setup as a Modbus Slave. If pass-through mode is
not chosen, then the attached Modbus Master device must read from one set of
Modbus register/bits and write to another set of Modbus register/bits. Pass-
Through mode takes a Modbus write command (FC5,6,15,and 16) and passes
that to the Control Logix processor. The pass-through ladder logic then parses
that information and updates the MCM.DATA.WriteData array with the new value
that has been written by the Modbus Master.
_ReadControl
This optional task is used to get status and event data from the Modbus devices
connected to the MVI56-MCM module. Special command blocks requested from
the module in the _WriteControl routine are processed and handled in this
routine. If command control, event command, or slave status blocks are not
going to be used in the application, then this rung and the _ReadControl and
_WriteControl ladder files may be removed.
_WriteControl
This optional task is used to send commands to the Modbus devices connected
to the MVI56-MCM module. Command Control, Event Command, and slave
status blocks are sent to the module in this task.
Copy the Controller Tags
The next step is to copy the Controller Tags from the sample program to your
existing program. The sample program includes the following tags in the MCM
structure.
MCM.ModDef configures the database in the module. The module uses this
database to store input and output data transferred between the processor
and the Modbus devices connected to the MVI56-MCM module.
MCM.Port1 and MCM.Port2 configure the module's serial ports for Modbus
communications. The sample program configures Port 1 as a Modbus
Master, and Port 2 as a Modbus Slave.
MCM.P1.Cmd and MCM.P2.Cmd configure the Modbus Master commands
for the module. This commands are active only if a port is configured as a
Modbus Master.
MCM.ReadData contains data read by the ControlLogix processor from the
MVI56-MCM module.
MCM.WriteData contains data read from the ControlLogix processor to the
module's internal database.
The remaining controller tags contain error and status information, and special
commands to execute.
1 In the Controller Organization pane in each instance of RSLogix 5000,
expand the Controller folder.
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