Allen-Bradley 1756-L6 Series, Reference Manual, Page: 43 / 134

Rockwell Automation Publication 1756-RM001I-EN-P - May 2012 43
ControlLogix I/O Modules Chapter 5
Requirements When Using ControlLogix Digital Output Modules
Wiring the two types of digital output modules differs, depending on your
application requirements (these wiring methods are explained in detail in later
sections). However, regardless of the type of ControlLogix output module used,
you must follow these general application requirements when applying these
modules in a SIL 2 application:
• Proof tests - Periodically perform a system validation test. Manually or
automatically test all outputs to make sure that they are operational and
not stuck in the ON or OFF state. Outputs must be cycled from ON to
OFF or OFF to ON. For more information, see
Proof Tests on page 20 .
• Examination of output data echo signal in application logic – The
application logic must examine the Data Echo value associated with each
output point to make sure that the requested ON/OFF command from
the controller was received and acted upon by the module.
In Figure 15 , a timer begins to increment for any miscompare between the
controller’s output and the module’s Data Echo feedback. The discrepancy
timer must be set to accommodate the delay between the controller output
data and the module’s Data Echo response. The time value chosen needs to
consider various system RPIs and network latency. If a miscompare exists
for longer than that time, a fault bit is set.
Figure 15 - Data Echo Discrepancy Timer Logic
The control, diagnostics and alarming functions must be performed in
sequence. For more information on faults, see
Chapter 8 , Faults in the
ControlLogix System .
Timer Done
Data Echo Output Bit
Data Echo Output Bit
Timer
Fault
Application Logic
Actuator
Alarm to Operator
Fault
No Faults
Secondary
Output
Fault
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