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.