ProSoft Technology MVI69E-MBS - Manual (Page 79)

MVI69E-MBS ♦ CompactLogix Platform MVI69E-MBS Backplane Data Exchange
Communication Module User Manual
ProSoft Technology, Inc. Page 79 of 162
January 6, 2014
Step Description
1 Upon module boot-up, the Master driver obtains configuration data from the MBS
controller tags. The configuration data obtained includes port configuration and the
Master Command List.
Special Commands can be issued directly from the CompactLogix processor using Event
Commands and Command Control. These command values are used by the Master
driver to determine the types and order of commands to send to slaves on the network.
2 After configuration, the Master driver begins transmitting read and/or write commands to
slave nodes on the network. If the Master driver is writing data to a slave, the data for the
write command is obtained from the module’s internal database.
3 Once the specified slave has successfully processed the command, it returns a response
message to the Master driver for processing.
4 Data received from a slave in response to a read command is stored in the module’s
internal database.
5 Status is returned to the processor for each command in the Master Command List.
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 PCB (page 58). 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