Rockwell Automation 1715-OF8I Redundant I/O System User Manual - Manual (Page 216)

216 Rockwell Automation Publication 1715-UM001C-EN-P - March 2014
Chapter 6 1715 Redundant I/O System in SIL 2 Safety Applications
• Each time a valid packet is received from the controller, the output module
resets the CRTL. If the CRTL ever times out, the output module assumes
the fail-safe state.
• It is recommended that the CRTL remain at the default of four times the
RPI so that one invalid packet does not put the system into the safe state.
For example, if the RPI = 120 ms, then consider 480 ms as the minimum
CRTL. The information in the next section helps determine the maximum
setting for the CRTL.
Determining the Appropriate CRTL Value
Use the following method to confirm whether the default value is acceptable or
you need to change the CRTL value for your application.
The value of CRTL for the I/O connections is governed by this equation:
where CRTLeuc is the process safety time for the equipment under control (euc).
IMPORTANT The default fail-safe state for all 1715 modules is de-energized.
EXAMPLE Consider a system function using one sensor and one actuator
given the following parameters:
• CRTLeuc: 10,000 ms
• Sensor delay: 250 ms
• Time for actuator (an ESD valve) to fully operate: 1750 ms
In this example, the setting of CRTL for the I/O connections is
less than or equal to 3000 ms.