ProSoft Technology MVI69-MCM - Manual (Page 113)

MVI69-MCM ♦ CompactLogix or MicroLogix Platform Reference
Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 113 of 167
March 22, 2011
Master Command List
In order to function in the Master Mode, you must define the module’s Master
Command List. This list contains up to 100 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

Slave Node Address

Command Type: Read or Write up to 125 words (16000 bits) per command

Database Source and Destination Register Address: The addresses where
data will be written or read.

Count: The number of words to be transferred - 1 to 125 on FC 3, 4, or 16.
Select the number of bits on FC 1, 2, 15.
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. This error list can be
transferred to the processor. The following tables describe the error codes
generated by the module.
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 your device manufacturer for the maximum count
supported by your particular slave.
Transferring the Command Error List to the Processor
You can transfer the command error list to the processor from the module
database. To place the table in the database, set the Command Error Pointer
parameter to the database location desired.
To transfer this table to the processor, make sure that the Command Error table
is in the database area covered by the Read Data.
Standard MODBUS Protocol Exception Code Errors
Code Description
1 Illegal Function
2 Illegal Data Address
3 Illegal Data Value
4 Failure in Associated Device
5 Acknowledge
6 Busy, Rejected Message
Module Communication Error Codes
Code Description
-1 CTS modem control line not set before transmit
-2 Timeout while transmitting message
-11 Timeout waiting for response after request
253 Incorrect slave address in response
254 Incorrect function code in response
255 Invalid CRC/LRC value in response