SB-593 Satellite Board
Unit 17 Park Farm Industrial Estate
01925-001 Rev. 1.2
Buntingford, Herts SG9 9AZ UK
TEL: 0870 444 7234 FAX: 0870 444 7240
Web: http://www.kerisystems.co.uk E-mail: [email protected] Page 16 of 16
Quick S
tart Guide
SB-593
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.
