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CTI 2572 Installation and Operation Guide
134
Uncoupled Mode Timing (CMD Mode =0)
Uncoupled Mode may be selected when close synchronization between the PLC and the module is
not required. In this mode, a single trigger can cause the module to execute a command many times.
Uncoupled mode is useful in applications where you want to continuously update the remote PLC.
Uncoupled Mode usually takes less PLC logic to implement than Coupled Mode. The following
figure illustrates the timing relationships between CMD TRIG and CMD BUSY for an Uncoupled
Mode command. The shaded area in the diagram indicates that it does not matter what the status of
the bit happens to be (don’t care condition).
At time A, the module has cleared
CMD BUSY, indicating that
Command processing is complete.
When CMD BUSY is low and
CMD TRIG is high the module
will assert CMD BUSY (Time B)
and begin command processing.
CMD BUSY will remain on
during the processing of the
Command Block. When the
processing is complete, the
module will lower CMD BUSY
(Time C). Note that in Coupled
Mode, the module would wait for
the PLC to lower the trigger
before clearing CMD BUSY.
At the next end of the PLC I/O
scan, the module will again
sample CMD TRIG. If CMD TRIG is high, the module will raise CMD BUSY and execute the same
command again, without re-reading the Command Block from the PLC. Note that the PLC logic is
not required to cycle CMD TRIG.
If continuous operation is desired, the PLC can simply leave the CMD TRIG on. The Module will
then repeat the command as fast as possible.
NOTE:
As long as the 2572 detects that CMD TRIG is high when CMD BUSY is lowered, it will execute the
same command again, It will not re-read the Command Block from the PLC. Therefore, if you wish
to change the command to the module, the PLC logic should clear CMD TRIG and ensure CMD
BUSY is cleared before changing the contents of the Command Block or the associated Command
Slot. Alternately, use Coupled Mode for these applications.
Figure 24. Uncoupled Mode Timing