Siemens SIMATIC S7 Functional Safety Manual page 200

Glossary
Channel fault
A fault that causes one process value to be passivated, such as a detected wiring fault on
one input. Other channels in the module can continue to support a safety function.
See also Module fault:
● SM 1226 DI 16 x 24 VDC:
– When configured as 1oo1, a detected fault related to one digital input terminal results
– When configured as 1oo2, both associated digital input terminals are represented by a
● SM 1226 DQ 4 x 24 VDC and SM 1226 DQ 2 x Relay: These modules always operate as
1oo2, with two DC switches or two relays required to be ON to supply energy to the load.
A detected fault associated with either switch or either relay results in an attempt to turn
off both switches or relays, and a report to the user program that the channel is
passivated.
CRC signature
The CRC (Cyclic Redundancy Check) signature is a cyclic redundancy checksum that
confirms the integrity of the PROFIsafe message contents and sequence.
Cyclic interrupt / cycle time
The start-to-start user configured time delay between executions of the safety program.
Dark test / dark time
The test or time in which a "0" signal is deliberately created to confirm that "0" can be
controlled or detected when needed. On inputs, the F-DI performs a dark test by briefly
turning off the sensor supply. On DC outputs, the F-DQ DC performs a dark test by briefly
turning off one of the P or M output switches.
Discrepancy time (of inputs)
The configured discrepancy time during which you expect 1oo2 inputs to disagree due to
mechanical and electrical differences in input signals. The F-DI interprets differences
between inputs lasting longer than the discrepancy time as a channel fault.
Equivalent (input)
A 1oo2 input channel where both signal inputs interpret high voltage as a process value "1"
and low voltage as a process value "0".
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in the process value being set to "0". The 1oo1 configuration is exposed to the risk of
a single undetected fault (for example, a sensor stuck ON).
single process value. A detected fault related to either digital input in the 1oo2 pair, or
a failure of the two inputs to compare, results in the process value being set to "0".
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA