MVI56-MCM ♦ ControlLogix Platform Configuration as a Modbus Master
Modbus Communication Module
Page 34 of 159 ProSoft Technology, Inc.
July 24, 2008
Q. My slave shows addressing like 400,001 or 301,345?
For the 6 digit addressing, you will want to use the same function codes and
configuration as setup above, just the starting address has changed.
Below is a definition that will help with your DevAddress setup:
FC 1, 5, or 15 DevAddress = Modbus address in device - 0001
Example: Modbus address 0001 = DevAddress 0
Modbus address 1378 = DevAddress 1377
FC 2 DevAddress = Modbus address in device - 100001
Example: Modbus address 100001 = DevAddress 0
Modbus address 100345 = DevAddress 344
FC 3, 6, or 16 DevAddress = Modbus address in device - 400001
Example: Modbus address 400001 = DevAddress 0
Modbus address 400591 = DevAddress 590
FC 4 DevAddress = Modbus address in device - 300001
Example: Modbus address 300001 = DevAddress 0
Modbus address 304290 = DevAddress 4289
For example, our device listed above could show their addressing as follows.
To read the same parameter "Switch_Input_Status", you would still issue a
FC 4, and use a DevAddress of 512 decimal.
4.3 Master Command Samples
The following examples are going to guide you through the configuration of some
of the Modbus Master commands.
4.3.1 Read Holding Registers 4xxxx (Modbus Function Code 3)
The 4x holding registers are used for Analog Values such as Pressure,
Temperature, Current, and so on. These are 16 bit register values, but can also
be used for the storage of Floating Point data (see Floating Point Support in this
manual). These same Modbus addresses can be written to using a Modbus
Function Code 6 or 16.
Below is a sample command to read Modbus addresses 40001 to 40010 of node
1 on the Modbus network.
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