ProSoft Technology MVI56E-MCMR - Manual (Page 164)

Reference MVI56E-MCMR ♦ ControlLogix Platform
User Manual Modbus Communication Module with Reduced Data Block
Page 164 of 225 ProSoft Technology, Inc.
May 13, 2014
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.