Discrete Alarms - DPS Telecom NetGuardian 240T Hardware User Manual

6.6

Discrete Alarms

Dry Contact
NetGuardian case
– Batt.
Discrete alarm points can connect as a dry contact or a contact to ground
The NetGuardian 240T features up to 40 discrete alarm inputs (16 with 216T G2 Build Option) — also
called "digital inputs" or "contact closures". Discrete alarms are either active or inactive, so they're
typically used to monitor on/off conditions like power outages, equipment failures, door alarms and so
on.
The NetGuardian's discrete alarm points are single-lead signals referenced to ground. The ground side
of each alarm point is internally wired to ground, so alarm points can connect either as a dry contact or
a contact to ground.
In a dry contact alarm: The alarm lead brings a contact to the ground lead, activating the alarm.
In a contact to ground alarm: A single wire brings a contact to an external ground, activating the alarm.
You can reverse the polarity of each individual discrete alarm point, so that the alarm is activated when
the contact is open. This is done with a software configuration change.
GND
Alarm
Contact to Ground
NetGuardian case
– Batt.
Note: Make sure that grounds have a common reference —
this is usually done by tying grounds together.
GND
Alarm
13