Keri Systems SB-293 Quick Start - Manual (Page 16)

SB-293 Satellite Board
1530 Old Oakland Road, Suite 100 01837-003 Rev. 2.3
San Jose, CA 95112 USA
(800) 260-5265 (408) 451-2520 FAX (408) 441-0309
Web: http://www.kerisys.com E-mail: [email protected]
Page 16 of 16
Quick Start Guide
SB-293
8.0 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.
8.1 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).
8.2 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).
9.0 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."
9.1 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.
9.2 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.