Theory - Siemens SIMATIC PCS 7 OSx Hardware Manual

Theory

In the normal non-alarm condition, a 24 VAC or 24 VDC supplies power to
Channel #2 common, connector 2L, through the normally-closed contacts of
the manual silence switch and Output Channel #1. When Output Channel
#2 experiences a pulsed alarm condition, the normally-open contact is
momentarily closed, supplying power to the relay. With the relay energized,
both of the relay's normally-open contacts close. These contacts allow the
annunciator both to sound and to latch on the circuit that provides power to
the relay. This occurs even when the contact on Output Channel #2 returns
to the normally-open condition.
To silence the annunciator, either press the manual silence switch or silence
the alarm from the operator keyboard or screen. This opens the
normally-closed contact on Output Channel #1.
NOTE: If you have difficulty with your system, contact the Siemens
Energy & Automation, Inc., Technical Services Group in the U.S.A. at
423–461–2522, or outside the U.S.A. at 49–911–895–7000.
SIMATIC PCS 7 OSx 4.0.0 Hardware (PA RISC) Alarm Applications/Failover Option A-5