ProSoft Technology MVI71-MCM - Manual (Page 77)

Reference MVI71-MCM ♦ PLC Platform
Modbus Communication Module
ProSoft Technology, Inc. Page 77 of 111
December 28, 2007
Step Description
2 A Host device, such as a Modicon PLC or an HMI application, issues a read or write
command to the module's node address. The port driver qualifies the message before
accepting it into the module.
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 Configuring the Module (page 22) for a complete list of the parameters
that must be defined for a slave port.
Pass-Through
Each port can have the type parameter configured for pass-through mode where:
Swap Code Description
2 Pass-Through Formatted
3 Pass-Through Formatted with Byte Swap
Option 3 swaps all bytes in the data received from a Modbus Master command.
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 diagram shows
the dataflow for a slave port with pass-through enabled:
0
4999
40001
45000
Modbus
Addresses
Database
Addresses
ControlLogix
Controller Tags
Slave
Mode
Driver
Status
Configuration
Configuration
Data
Status
from Module
Register
Data
storage
Register
Data
Processor Memory MCM Module Backplane Interface
Configuration
Status
Read Functions
Write Functions