Nortel DMS-100 Series Reference Manual page 60

6-2 OAS alarm and Control Circuits
the critical system lamp on the alarm control and display panel (ACD) to
alert operating company personnel.
The alarm system hardware generates the dead system alarm when the alarm
circuits of the OAU and standby MTM or ISM detect a loss of
communication with CC. Multiple faults in the hardware of the message
system can cause this alarm condition. A loss of sanity in the CC software
can also cause this alarm condition. These conditions affect call processing
and result in a loss of communication. The loss of communication has the
following impact:
The alarm system uses two SD points to generate the OAU alarm. One SD
point is in the OAU and the other is in the standby MTM or ISM. The alarm
system uses the same SD points to generate the dead system alarm. Each
SD point controls the OAU alarm relay and a normally-open dead system
(DS) relay.
The critical bell on the audible alarm panel and the critical system lamp on
the ACD are connected to ground through break contacts in the DS relay in
the OAU and standby MTM or ISM. The -48V battery feed connects to the
critical bell from the OAU shelf and the critical system lamp from the
standby MTM or ISM shelf. Both alarm SD points are released when the
OAU and standby MTM or ISM detect a loss of communication with CC.
This SD point release closes break contacts of both dead system relays,
which generates the audible and visual critical system alarm indications.
Enhanced dead system alarm
The enhanced DSA generates the same audible and visual alarms as the
basic DSA. The enhanced DSA also detects the loss or degradation of call
processing.
With the enhanced DSA, the OAU attempts to make a call to CC about
every 5 s. If CC does not answer a call within 20 s, the OAU activates the
required critical alarms. Central control counts the calls the OAU makes. If
CC does not detect any calls in 20 s time period, the alarm system generates
a critical no call processing alarm (NCPALARM) at the External (EXT)
alarm level.
297–1001–122 Standard 06.01 August 1999
The generation of an alarm by the alarm system software in the CC
cannot occur in the normal way.
If the message system has multiple faults, the affected maintenance
subsystem can detect the faults.
The affected maintenance subsystem can alert maintenance personnel
visually at the MAP terminal visual display unit (VDU).
Activation of the corresponding audible and visual alarm hardware
cannot occur.