Sunday, February 20, 2011

The PLC Tasks Running by a PC





The equipment protection modules run in the PLC, and check the machine status against a table of hardware limits located in the dual port memory. These tasks run in the PLC because the D4-450 CPU can start scans on a timed basis, and generate exceptions if scan time take longer than their allotted time. The PC running a multi tasking operating system can not guarantee as precise scheduling as the PLC. Failure to complete a scan on time will be reported by status bits in the dual port memory. This status can also be reported by a report on exception feature of the Ethernet module, allowing instant notification to both the PC, and any other processors performing equipment protection table driven like the input or output configuration tasks, a generic software program can caused even in cases where equipment protection is not needed: an empty table implies no work for the task to perform.

Finally the PC is responsible for man or machine interface and configuration tasks. Monitoring machine status and updating hardware set points are performed by writing and reading values in the dual port memory. Equipment protection ranges are adjusted by updating the corresponding tables. Initial input or output configuration information is handled by querying the PostgreSQL database for channel vs dual port memory location mapping, then updating the appropriate tables in the PLC’s memory. Once the PLC’s are configured by the PC, equipment protection modules are no longer dependent on the PC for operation. This frees the PC for operator control, automation, and logging functions.

One of the major benefits of the linac control system upgrade will be automated logging of linac machine limits and parameters. The control system recently keeps historical records of orbits, ramp files, beam current, power supply diagnostic tests, and channel default values. Specialized analysis programs are used to determine gradual drifts in component performance.



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