14
Discrete Alarms
5.6
Contact to Ground
– Batt.
Alarm
Dry Contact
– Batt.
GND
Alarm
NetGuardian case
NetGuardian case
Note: Make sure that grounds have a common reference—
this is usually done by tying grounds together.
GND
Discrete alarm points can connect as a dry contact or a contact to ground
The NetGuardian features up to 20 (16 optional) discrete alarm inputs — 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.
5.6.1
Externally Powered Build Option
With externally powered TTL build option, discrete alarms 1-16 are TTL, each supporting a range
of 0-12 VDC. The bias pin can still be used with dry contact closures to provide a 12V wetting
voltage for any of the 16 TTL alarms. However, if the bias pin is not used, then an external 0-12
VDC voltage can be applied to the alarm input pin to trigger alarms.
NOTE:
When bias pin is not in use, the external voltage must be at a voltage potential relative to
the chassis earth ground.
