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SB-293 Satellite Board
General Information on Inputs
A controller input detects a state change generated by a device outside the controller that may
prompt a response from the controller. Input devices that generate a state change may be
normally-closed or normally-open. This section provides a brief description of normally-closed
versus normally-open inputs.
Normally-Closed
A normally-closed input device continually keeps a circuit active or complete. A state change is
generated when the normally-closed input device is forced open, breaking the circuit. In an
access control system, a door switch is a typical example of a normally-closed device. While the
door remains closed, the switch remains closed. When someone opens the door, the door switch
is opened, breaking the circuit and generating a state change. The controller then responds to the
state change and generates an output (such as sounding an alarm if the door is a secure door).
Normally-Open
A normally-open input device continually leaves a circuit open, or incomplete. A state change is
generated when the normally-open input device is forced closed, completing the circuit. In an
access control system, a request-to-exit (RTE) button is a typical example of a normally open
device. In an access control installation, an RTE button is located on the secure side of a door.
While there is no one there pressing the button, the switch remains open. When someone desires
to exit through a secure door, they press the RTE button, closing the circuit and generating a
state change. The controller then responds to this state change and generates an output (such as
unlocking the door to allow egress).
General Information on Safety versus Security with Door Locks
When installing a door lock there are two things to consider: safety versus security, or should the
door be “fail-safe” or “fail-secure.”
Fail-Safe Door Lock
Fail-safe means that if the power should fail at a door (perhaps due to a power outage or
equipment failure), the door will automatically unlock allowing entrance or egress. Power is
required to keep the door locked. A fail-safe door ensures people will be able to enter and exit a
secured area through that door in the case of an emergency. A typical fail-safe application may
use a magnetic lock. In this application, the controller energizes the lock relay, causing the lock
relay to change its state. In its new state the normally-closed circuit is opened breaking the power
to the magnetic lock and allowing the door to be opened.
Fail-Secure Door Lock
Fail-secure means that if the power should fail at a door (perhaps due to a power outage or
equipment failure), the door will automatically lock and not allow entrance, but will continue to
allow egress. Power is required to unlock the door. A fail-secure door ensures a secured area
remains secure regardless of the situation. A typical fail-secure application may use a door strike.
In this application, the controller energizes the lock relay, causing the lock relay to change its
state. In its new state the normally-open circuit is closed activating the release mechanism for the
door strike on the door to be opened.
Quick Start Guide
SB-293
