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PP875 3BSE092977R1 ABB Control panel touch screen display panel 800

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S800 I/O units are connected to the two CPUs via the optical ModuleBus and two

TB840 cluster modems on each S800 I/O cluster (see Figure 60 on page 169). The

built-in electrical ModuleBus on the TP830 baseplate cannot be used for connecting

S800 I/O in a redundant system.

PM891 Redundancy

The Redundancy Link in PM891 consists of two physical links. These are the

RCU Data Link and the RCU Control Link.

The RCU Data Link is a fast communication channel used to transfer the data

required to keep the backup CPU synchronized with the primary CPU.

TK855 RCU Data Link Cable is used for the data link.

The RCU Control Link is used for role selection and CPU identity assignment

(UPPER/LOWER).

TK856 RCU Control Link Cable is used for the control link.

Fault Tolerance Principle

The principle of fault tolerance in the redundant processor units is based on

continuous updating of the backup unit to the same status as the primary unit. This

enables the backup unit to assume control without affecting surrounding systems in

a bumpless manner.

This principle involves dynamic division of the program execution into execution

units and the creation of rollback points at which the processor unit's status is

completely defined.

In this context, the processor unit's total status is defined as the processor unit's

internal status, that is, the contents of the processor registers, plus the contents of the

data memory.

The backup unit's status is updated each time the primary unit passes a rollback

point, enabling the backup unit to resume program execution from the last rollback

point passed, should the primary unit fail due to error.

In order to minimize the amount of information involved in the update, the backup

unit is updated only with the changes taking place since the latest rollback point.