Allen-Bradley 1794-IE8H Specifications download pdf (Page 18)

Publication 1794-RM001F-EN-P - September 2011

SIL Policy 1-13

SIL Compliance

Distribution and Weight

The programmable controller may conservatively be assumed to contribute

10% of the reliability burden. A SIL 2 system may need to incorporate multiple

inputs for critical sensors and input devices, as well as dual outputs connected

in series to dual actuators dependent on SIL assessments for the safety related

system.

Response Times

The response time of the system is defined as the amount of time it takes for a

change in an input condition to be recognized and processed by the

controller’s ladder logic program, and then to initiate the appropriate output

signal to an actuator. The system response time is the sum of the following:

• input hardware delays

• input filtering

• I/O and communication module RPI settings

• controller program scan times

• output module propagation delays

See Table 1.1 for associated module information.

Each of the times listed above is variably dependent on factors such as the type

of I/O module and instructions used in the ladder program. For examples of

how to perform these calculations, see publication 1756-RM001

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Allen-Bradley 1794-IE8H Specifications Page 18