Rockwell Automation 1791DS-IBxxxx Guard I/O DeviceNet Safety Modules - Manual (Page 35)

Rockwell Automation Publication 1791DS-UM001J-EN-P - May 2013 35
Understand the Operation of Safety Functions Chapter 2
In this code, the actual muting status bit is used in the first rung, and
the DN bit of the timer is used in the muting status parameter of the
instruction. The actual test output data bit is used in both the first and
last rungs.