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Appendix B Frequently Asked Questions
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This is by Microsoft's design for these operating systems, however it is
possible to force the old operation, depending on which controls you need
to expose for use in CX-Supervisor. Contact your supplier to receive
details of the required Registry changes.
• How does Sleep() work?
The CX-Supervisor Sleep function is designed to create a short pause in
script execution. The parameter specifies the duration of the pause in
milliseconds. For example:
<statements A>
<statements B>
Sleep(100)
<statements C>
In this example 'statements B' will begin executing immediately when
'statements A' complete, though 'statements C' will not begin execution
until 100ms after 'statements B' have completed.
Note 1: In a well designed, truly event driven system use of the Sleep()
statement should never be required. Always consider if the statements
after the Sleep should be in their own script, executed when a Condition
occurs.
Note 2: The Granularity (or intervals) differs between Operating Systems.
In Windows NT (and 2000) expiration is checked every 10ms, so
'Sleep(100)' actually pauses for 100 to 109.99 milliseconds depending on
when it was started. For Windows 98 and ME the granularity is 55ms so
'Sleep(100)' actually pauses for 110 (2 times 55) to 164.99 milliseconds
(nearly 3 times 55). For this reason, Sleep statements can act differently
on different Operating Systems making the application OS dependant.
Note 3: Sleep should never be used as a delay for timing processes, for
the following reasons:
- The actual time delay depends on the OS
- There is always an error of 0 to 1 granularity, depending on when the
action is started.
- The frequency cannot be guaranteed as the OS may be busy, or
handling other processes.
Note 4: Because of Note 2 and Note 3 above, Sleep should not be used
for creating output or logic pulses e.g. setting a bit on, sleeping, then
setting the bit off. Instead, just use the PC to set the bit and allow the PLC
to clear the bit, either after a duration timed by the PLC real time clock or
when the triggered process is complete.
The requirement for Sleep is quite simple but the implementation is
actually very complex. In the example above, the application must
continue to run during the pause i.e. read PLC data, log data, check
alarms, animate graphics etc. For this reason, any script with a Sleep
statement must be executed in parallel with the system. This can result in
unexpected results when a Sleep is added, and previously functioning
script commands are now running in parallel. Redesign the script
considering parallel processing.
This can also result in unexpected behaviour when a Sleep is added to a
Project initialisation or Page initialisation script as some parts of the
system or objects may not be loaded at time of execution.