Siemens SIMATIC S7 Functional Safety Manual page 202

Glossary
Light test / light time
The test or time in which a "1" signal is deliberately created to confirm that "1" can be
controlled or detected when needed. The signal modules described in this manual do not
deliberately conduct light tests that can affect your program or the output load. The SM 1226
F-DQ 4 x 24 VDC produces ON test pulses up to the Maximum readback time switch on test
on each of the P and M switches individually, but do not intentionally turn on both P and M
switches at once for a test when commanded process value is "0". In the unlikely event of an
undetected fault on the opposite switch, the test pulses can result in energy applied to the
load.
M
Refers to the 24 VDC power circuit 0 VDC reference. In the context of the F-DQ DC module
outputs, M can refer to the switch output connecting the load to M.
Module fault
A fault that affects all channels of a fail-safe signal module (SM). The fail-safe SM attempts
to passivate all channels in the module.
See also Channel fault.
Non-equivalent (input)
A 1oo2 input channel where one signal input interprets high voltage as a process value "1"
and the redundant signal input interprets high voltage as a process value "0". A common
configuration is complementary normally-open and normally-closed switches connected to
the same process event.
Overvoltage category
A definition of transient voltage threat due to lightning strikes and other sources, generally
associated with how closely the circuit is coupled to outdoor electrical wiring. Category III
represents a higher voltage level threat than Category II.
P
Refers to the 24 VDC positive supply. In the context of the F-DQ DC module outputs, P can
refer to the switch output connecting the load to P.
Passivate / passivated / passivation
The fail-safe CPU or fail-safe SM has identified a channel or module as faulty. The fail-safe
CPU or fail-safe SM supplies the fail-safe process value "0" instead of any detection or logic
evaluation that can result in process value "1".
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S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA