Response Electronics CL6011B Specifikace stáhnout pdf (Strana 77)

3Ć57 Theory of Operation: Redundancy

Original Ć June 1990 CE4.2:CL6211

implemented at the DCU level. The redundant controllers perform simply

as slaves to the RDCU in a redundant switchover decision. The following

paragraph(s) briefly explain how this failure detection mechanism is

implemented.

The Redundant Data Concentrator Unit (RDCU) is designed to

periodically poll the ACTIVE controller for status information and

Operating Data, and at the same time periodically interrogate the

STANDBY controller for status information. Any failure critical to the

control or operator interface, detected at the ACTIVE RDCU, will cause a

switchover to occur. These conditions include:

H Primary controller response message timeout

H Arithmetic Processor error (Interactive controller only)

H Input/Output self-test error (includes calibration out of tolerance)

H Communications error rate excessive

Although multiple failures are not meant to be covered in all of their

potential combinations, there is a limited set of optimization switchover"

decisions that are incorporated into the RDCU decision mechanism. For

instance, if a communications timeout occurs on a primary controller

communications channel, the RDCU will switch even if the secondary

controller system is currently reporting a calibration error. The premise

here is that some control is better than no control at all.

Note however that a switchover would not occur if a communications

timeout occurs on the primary controller system, when another

communications failure is being reported on the secondary controller

system, because the RDCU cannot determine which would be a more

optimum" control system under the failure conditions reported. Also,

optimization cannot be performed when multiple communications fails on

a controller subsystem (example, two controller timeouts occurring on

the primary system, and only one on the secondary system), as there is

no voting mechanism to determine the highest availability of the control

system.

The RDCU will only make one switchover (or switchback) decision, and

then the automatic switch decision algorithm is disabled. Automatic

switch enable/disable status is reported on the RDCU diagnostic

displays. Disabling automatic switch will prevent any possibility that the

RDCU may cycle between redundant units when the system is

experiencing intermittent failures, such as power cycling problems, etc.

Once automatic switch is disabled, the operator must intervene to

correct the error condition, and then enable the automatic switch

decision algorithm. Any integrity error, including disabled automatic

switch, is reported as an integrity problem with the RDCU, requiring

operator attention.

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