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Rockwell Automation Publication ICSTT-RM446N-EN-P - April 2018
Chapter 4
AADvance Functional Safety System Implementation
Input Module Safety Accuracy
The input modules determine the channel state and the line fault state by
comparing the input reported values with user programmed threshold values.
When triple analogue input modules are used and active, the system adopts the
median value. When dual modules are used, the lowest reported value is used.
The discrepancy between the redundant channels' measurements are
monitored to determine if they are within the safety accuracy limit.
When the safety accuracy within a channel is detected outside the following
limits then that channel is set to a fail-safe state.
• Digital Input Module = 1 v
• Analogue Input Module = 0.2 mA
When the safety accuracy between channels exceeds the following limits then a
discrepancy alarm is set for the input channel
• Digital Input Module = 2 v
• Analogue Input Module = 0.4 mA
In both situations the following safe values are reported by the variables:
Digital input modules
• Input state FALSE
• Line fault TRUE
• Discrepancy TRUE
• Channel fault TRUE
• and the voltage value is 0mV
Analogue input module
• process value = a calculated value based on a count value of 0 (51 counts
= 0.2mA)
• line fault TRUE
• Discrepancy TRUE
• Channel Fault TRUE
• Count value 0
ATTENTION:
In safety critical applications, the discrepancy alarms shall be
monitored by the application program and be used to provide an alarm to
the plant operations personnel.
