ProSoft Technology MVI46-MCM - Manual (Page 75)

MVI46-MCM ♦ SLC Platform Reference
Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 75 of 108
March 29, 2011
Refer to the Module Set Up section for a complete description of the parameters
required to define the virtual Modbus master port. Refer to Modbus Protocol
Specification (page 83) for a complete discussion of the structure and content of
each command. Care must be taken in constructing each command in the list for
predictable operation of the module. If two commands write to the same internal
database address of the module, the results may not be as desired. All
commands containing invalid data are ignored by the module.
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.