ProSoft Technology MVI56E-MNET/MNETXT - Manual (Page 123)

MVI56E-MNET / MNETXT ♦ ControlLogix Platform Reference
Modbus TCP/IP Interface Module User Manual
ProSoft Technology, Inc. Page 123 of 181
April 23, 2014
Command Control Blocks (5001 to 5006)
Note: Command Control is not needed for normal Modbus command list polling operations and are
needed only occasionally for special circumstances.
During routine operation, the module continuously cycles through the user-
defined MNET Client 0 Command List (page 47), examining commands in the
order they are listed and sending enabled commands on the network. However,
the module also has a special command priority queue, which is an internal
buffer that holds commands from special function blocks until they can be sent
on the network.
When one or more commands appear in the command priority queue:
1 The routine polling process is temporarily interrupted.
2 The commands in the command priority queue are executed until the queue
is empty.
3 Then the module goes back to where it left off on the MNET Client 0
Command List and continues routine polling.
Like Event Command blocks, Command Control blocks place commands into the
module’s command priority queue. Unlike Event Commands blocks, which
contain all the values needed for one command, Command Control is only used
with commands already defined in the MNET Client 0 Command List.
Commands in the MNET Client 0 Command List may be either enabled for
routine polling or disabled and excluded from routine polling. A disabled
command has its Enable parameter set to NO (0) and is skipped during routine
polling. An enabled command has its Enable parameter set to YES (1) and is
sent during routine polling. However, Command Control allows any command in
the predefined MNET Client 0 Command List to be added to the command
priority queue, whether it is enabled for routine polling or not.
Command Control also gives you the option to use ladder logic to have
commands from the MNET Client 0 Command List executed at a higher priority
and out of routine order, if such an option might be required in special
circumstances.
A single Command Control block request can place up to six commands from the
MNET Client 0 Command List into the command priority queue.