Rockwell Automation SA500 Drive Configuration and Programming - Manual (Page 90)

4-4 SA500 Drive Configuration and Programming
Step 4. SCAN_LOOP block/Enabling CCLK
This control block tells the UDC operating system how often to execute the
task based on the constant clock (CCLK) signal on the rack backplane. Note
that the CCLK signal must be enabled by a task in the rack before any UDC
tasks in the rack can b scanned beyond their SCAN_LOOP blocks. Note that
CCLK must be enabled again after a STOP ALL in the rack. CCLK is enabled
by setting the appropriate “CCLK enable” bit on certain modules in the rack,
such as the UDC module, CCLK must be enabled on one module only. If
CCLK is enabled on multiple modules in the rack, an overlap error will result
(error code 38).
The UDC task runs based on “ticks;” one tick is equal to one 500
µ
sec
(0.5 msec) CCLK interval. The value can range from 1 to 20 ticks.
The programmer must specify how often the task should run in the TICKS
parameter of the SCAN_LOOP block in the task itself. The TICKS value
represents the number of 500 will occur. In order to calculate this value, both
drive A and drive B tasks must be considered together because they execute
one immediately following he other (A, then B). See figure 4.1 for more
information.
When determining the value to enter, the programmer must consider how
long it will take both tasks to actually run, allow some time for processing
overhead, and use the resulting value to determine the TICKS value for the
SCAN_LOOP block in both the drive A and drive B tasks. The AutoMax
Control Block Language manual (J-3676) lists the execution times of the
Control Blocks.
For example, if the programmer assigns UDC task A a TICKS parameter of 8
(4 msec), then UDC task B must also have TICKS defined at 8, and both
tasks must be able to execute within an 8 tick window of time, or an overlap
error will result and all tasks in the rack will stop. If the tick rates do not
match, error code 956 will be reported for one or both tasks in the error log;
and all tasks in the rack will be stopped.
Note that, unlike Control Block tasks on AutoMax Processors, UDC tasks
cannot run on a hardware or software event basis. The EVENT parameter
cannot be specified in the SCAN_LOOP block in UDC tasks. This means that
there is no time-out for execution of the UDC tasks. If the UDC task is
scanned to the SCAN_LOOP block and CCLK is not on, the task will simply
wait without timing out.
Note that no other control blocks are permitted before the SCAN_LOOP
block. BASIC statements, however, are permitted before the SCAN_LOOP
block.
Step 5. Other Control Block and BASIC statements or functions
This portion of the task consists of the logic specifically required for the
application. This portion of the UDC task (after the SCAN_LOOP block) is the
only part of the task that executes after the initial scan of the task, after a
STOP ALL command and subsequent Run command, or after power is
cycled to the rack.