Reference
MVI56E-MCMR ♦ ControlLogix Platform
User Manual
Modbus Communication Module with Reduced Data Block
Page 162 of 223
ProSoft Technology, Inc.
Block Response from Module to Processor
Word Offset
Data Field(s)
Description
0
Number of
Commands to
report
This field contains the number of commands to report
in the response message. The value is always 100.
1
Start Index of First
Command
This field always contains a 0. The status of all 100
possible commands will be returned, starting with
Command Index 0, the first command in the
Command List
.
2 to 102
Command Error
List
Each word of this area contains the last execution
status value recorded for the command. The order of
status words is the same as the order of commands
in the
Command List
A status value of zero (0) in the
Command Error List
indicates either that the
corresponding command is not used or that it has been executed successfully.
Any non-zero value found in this list indicates that some type of error was
encountered while trying to execute the corresponding command.
There are many potential reasons why a command might fail. For more details,
see Standard Modbus Protocol Errors or Module Communication Error Codes.
Slave Status Blocks (9960, 9961)
The
Get Slave Status List
special function is available only when the module's
port is configured as a Modbus Master. Use block identification codes 9960 for
Port 1 or 9961 for Port 2 to request the current polling state of each Slave device
that could be polled by a Master port.
The results returned in this list have a one-to-one correspondence with the 248
possible Modbus Slave Device Address values, 0-247. Each word in the Slave
Status List corresponds to a single Modbus Slave Device Address.
This list is updated each time the Master driver attempts to poll a specific
Modbus Slave Device; but gives very little information regarding the success or
failure of that poll attempt. Therefore, this list is not very useful for determining
general communications health, for creating communication failure alarms, or for
troubleshooting network problems. A better tool for those needs is the
Command
Error List.
The
Get Slave Status List
message uses the "Get Attribute Single" service type
MSG instruction. The format of the data returned by the MSG is shown in the
following table.
