MVI69E-MBS ♦ CompactLogix™ Platform
MVI69E-MBS Backplane Data Exchange
Modbus Serial Enhanced Communication Module
User Manual
ProSoft Technology, Inc.
Page 75 of 159
Important: Take care when constructing each command in the list to ensure predictable operation of the
module. If two commands write to the same internal database address of the module, the results are invalid.
All commands containing invalid data are ignored by the module.
Master Command List
For a port to function in Master Mode, its Master Command List must be defined in
Prosoft Configuration Builder; refer to Modbus Port x Commands (page 44). This list
contains up to 310 individual entries, with each entry containing the information required
to construct a valid command. A valid command includes the following items:
Command enable mode: (0) disabled, (1) continuous or (2) conditional
Source or destination database address: The module database address where data
is written or read.
Count: The number of words or bits to be transferred
– up to 125 words for Function
Codes 3, 4, or 16, and up to 2000 bits for Function Codes 1, 2, or 15.
Note: 125 words is the maximum count allowed by the Modbus protocol. Some field devices may support
less than the full 125 words. Check with the device manufacturer for the maximum count supported by the
particular slave device.
Slave node address
Modbus Function Code: This is the type of command that is issued.
Source or destination address in the slave device
Command Error Codes
As the list is read in from the processor and as the commands are processed, an error
value is maintained in the module for each command. The definition for these command
error codes is listed in Communication Error Codes (page 110). You can view the
command error codes through the Ethernet diagnostics port; refer to Diagnostics and
Troubleshooting (page 66
). They can also be transferred from the module’s database to
the processor.
To transfer the Command Error List to the processor, set the
Command Error Offset
parameter in the port configuration to a module
database address that is in the module’s
Read Data area; refer to Additional Configuration Parameters as Master (page 42).
Note: The Command Error List must be placed in the Read Data area of the database, so it can be
transferred to the processor in the input image.