Glossary 245
Trident Planning and Installation Guide
peer-to-peer Protocol that allow multiple systems on a proprietary network to exchange process
and safety information.
permanent fault Failure, fault or error in the system that is continuous and stable.
PHA Process hazard analysis.
process demand Occurrence of a process deviation that results in the SIS initiating a trip of the
process unit.
program Basic programming unit within a project. A set of instructions, commands, and/or
other directions. In TriStation 1131, programs are written in any of the available
languages, and are subsequently placed within a project’s hierarchy.
programmable
logic controller
“Black box” device which accepts analog or digital input signals, acts upon them
in a well-defined way, and produces appropriate output signals as a result.
proprietary
network
Network of controllers.
protocol Set of rules describing the format used for data exchange between two entities.
quad output
circuit
Provides fault-tolerant outputs. Each output is composed of four identical
switching elements in a “quad” arrangement.
reliability Probability that no failure of the system will occur in a given period of time.
scan time Period of the controller’s cycle of required control functions. Scan time is
composed of three elements:
• Input poll time (asynchronous with program execution)
• Time required to execute the program
• Output poll time
SIL Safety integrity level.
single module Digital Input Module which is optimized for safety-critical environments where
low cost is more important than maximum availability. On a single module, only
those portions of the signal path which are required to ensure safe operation are
triplicated. Special self-test circuitry detects all stuck-on and stuck-off fault
conditions in less than half a second.
SIS Safety instrumented system.
SRS Safety requirements specification.
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