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MVI56-MCM
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ControlLogix Platform
Reference
Modbus Communication Module
Page 102 of 159
ProSoft Technology, Inc.
July 24, 2008
Step Description
3
After the module accepts the command, the data is immediately transferred to or from
the internal database in the module. If the command is a read command, the data is
read out of the database and a response message is built. If the command is a write
command, the data is written directly into the database and a response message is
built.
4
After the data processing has been completed in Step 2, the response is issued to the
originating master node.
5
Counters are available in the Status Block that permit the ladder logic program to
determine the level of activity of the Slave Driver.
Review Integrating the Sample Ladder Logic into an Existing Project (page 142)
for a complete list of the parameters that must be defined for a slave port.
An exception to this normal mode is when the pass-through mode is
implemented. In this mode, all write requests will be passed directly to the
processor and will not be placed in the database. This permits direct, remote
control of the processor without the intermediate database. This mode is
especially useful for Master devices that do not send both states of control. For
example, a SCADA system may only send an on command to a digital control
point and never send the clear state. The SCADA system expects the local logic
to reset the control bit. Pass-through must be used to simulate this mode. The
following illustration shows the data flow for a slave port with pass-through
enabled: