Technical specifications
A.1 General technical specifications
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA
139
When you suddenly apply 24 VDC power to the S7-1200 CPU or digital I/O signal modules
(SM), including fail-safe SM, short-term current flows can occur which briefly mimic the effect
of "1" signals at process outputs and inputs. Digital outputs can trigger to ON state for
approximately 50 microseconds at power application. The SMs deliberately test functional
safety P- and M-switch outputs ON at different times for up to the user-configurable
"Maximum readback time switch on test" (500 microseconds to 5 ms) during the power up
sequence and as a cyclic bit-pattern test. This test pulse can energize the load in the
presence of a switch or wiring fault on the opposite side of the load circuit. Such short pulses
are typically not a hazard for electromechanical loads, but you must consider the effect.
High-speed electronic receiver circuits can detect short pulses and improperly interpret the
short pulses as deliberate "1" signals.
Short term current and voltage pulses can occur on 24 VDC I/O circuits near the time that
power is applied.
Such short term pulses can cause unexpected activation or position changes in machinery,
resulting in death or serious injury to personnel and/or property damage.
If your installation includes receivers which can be responsive to short pulses as described
above, you should apply measures such as power sequencing or progressive removal of
safety lockouts to assure that unexpected machine operations do not occur.
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