Unique Redundancy Methodology

Introduction to Redundancy

While the SLS 1508 Logic Solver is rated suitable for use in SIL 3 applications

in simplex mode, it is installed in redundant pairs for increased process availability.

Redundant SLS 1508 Logic Solvers run in parallel at all times.Both read the inputs

from the I/O terminals, both execute the logic and both drive the outputs at the I/O terminals.

There is no concept of primary and backup or master and slave,which is unlike any other SIS.

The only difference between the two is that one communicates with both the engineering

and operator workstations and the dedicated safety network (SISNet);

this is the one with the Active light on the bezel.The other (Standby) is

communicating only on the SISNet.In the event that a failure is detected in

one of the SLS 1508 Logic Solvers, it automatically goes to a failed state. 

In this condition, all its output channels are de-energized;

this has no impact on the other SLS 1508 Logic Solver or the physical

outputs because the other SLS 1508 Logic Solver continuesto read inputs

execute logic and drive outputs. The transition from redundant to simplex

mode is therefore completely bumpless.

Multiple Domains: The simplest implementation of an SLS 1508 network is to have

one SIS Network ring and one SIS Network domain, that includes all of the

logic solvers in the SIS Network ring. In larger applications, the logic solvers

can be split into multiple SIS Network domains. Another approach is to have

multiple SIS Network rings, each on a SIS Network domain.