Rockwell Automation GMLC Reference Manual - Manual (Page 356)

Publication GMLC-5.2 - November 1999
Watch Control 319
If the specified condition is not true when the Watch Control block is
executed, program flow branches to the bottom (false) node of the block.
When the selected watch position event occurs (regardless of what part of
the diagram is executing), the block connected to the top (true) node of
the Watch Control block is immediately executed.
You can activate multiple watch position events—one for each physical,
virtual, or imaginary axis—at any given time. However, only one event
action (either an input event action or a position event action) is allowed
per axis. Either a Watch Control block, with Enable Event/Action type
and Watch Position class selected, or a Watch Control block, with Arm
type and Watch Position class selected, can be used on a given axis, but
not both blocks. Configuring a second position type block on the same
axis supersedes the first block. Each position event is monitored
independently and can be checked using an On Watch block with Wait for
Position Event selected, a Show Axis Position block, or an On Watch
block with If Position Event selected. Watch position events do not
automatically reactivate when tripped. For repetitive conditions,
reactivate the watch position event with a new Watch Control block.
A Watch Control block, with Enable Event/Action type and Watch
Position class selected, is similar to a Watch Control block, with Arm type
and Watch Position class selected, followed by an On Watch block, with
Wait for Position Event selected, except that the event action occurs much
faster. In fact, all watch position event actions are designed to take effect
within two (2) servo update periods of the occurrence of the watch
position event. Furthermore, the action occurs automatically while
execution of the diagram continues, without the need for additional
blocks to monitor or wait for the event.
For example, the GML diagram below gears two axes when the axis
passes a certain position.