The System Components

A door access control system has five core components. Understanding each helps you evaluate proposals, diagnose problems, and understand the capabilities and limitations of any system you're considering.

1. The Reader

The reader is the device mounted on the exterior (unsecured) side of the door. It reads the credential — whether that's a card, fob, fingerprint, or mobile device — and sends the credential data to the controller. Readers come in several form factors: proximity only (read-only), keypad (PIN entry), combined card + PIN, and biometric. The reader itself makes no access decision — it just sends the credential to the controller.

2. The Controller

The controller is the brain of the system. It receives the credential from the reader, compares it against its credential database, checks the schedule and door rules (time zones, holidays, access levels), makes the access decision, and either sends an unlock signal to the door hardware or denies access. Controllers are typically installed in a secured equipment room or electrical closet — not accessible to occupants.

Controllers range from single-door controllers (one controller per door) to enterprise controllers managing 8, 16, or 32+ doors from a single unit. The controller also logs every access event: credential presented, access granted or denied, door held open, door forced, system status changes.

3. The Door Hardware

The door hardware is the electromechanical device that physically locks or unlocks the door on the controller's signal. Two main types:

  • Electric strike: Replaces the standard door strike (the plate the latch bolt engages). When the controller grants access, the electric strike releases the latch, allowing the door to swing open. Fail-secure (stays locked on power failure) or fail-safe (unlocks on power failure) versions available. Electric strikes are the most common choice for entry doors.
  • Magnetic lock (mag-lock): An electromagnet mounted to the door frame with a steel armature plate on the door. When energized, the magnetic force (typically 600–1,200 lbs holding force) keeps the door closed. When the controller grants access, power is cut and the door opens. Mag-locks are fail-safe by nature (power loss = door opens), making them appropriate for emergency exit doors where the door must be freely openable during a fire alarm.

4. The Request-to-Exit Device (REX)

People leaving a secured area don't present a credential — they use a REX device. This can be a push-button, a motion sensor aimed at the door from the inside, or a touchbar on the door. The REX signals the controller to release the door hardware for exit without logging a credential event. This is how secured rooms and suites maintain control of who enters while allowing free egress.

Life Safety and Access Control

Access control doors must never prevent egress during an emergency. Fire codes require that all access-controlled doors be capable of free egress — typically via a push-bar, REX, or fire alarm release that unlocks all doors. Your installer must coordinate with your fire alarm system to ensure doors unlock on fire alarm activation.

5. The Management Software

The software (either local server-based or cloud) is where credentials are programmed, access levels are assigned, schedules are set, and event logs are reviewed. Good management software allows: adding or revoking credentials immediately from any computer, setting time-based access (employees can only enter 7am–7pm), grouping users into access levels (all employees get lobby, only management gets server room), generating audit reports, and receiving alerts for forced doors or access denials.

The Access Event Sequence

  1. 1
    Credential Presented

    User presents card, fob, or phone to reader. Reader sends credential data (encrypted) to controller over RS-485 or Ethernet (Wiegand or OSDP protocol).

  2. 2
    Decision Made

    Controller checks credential against database, checks current time against access level schedule, checks door status. Decision takes milliseconds.

  3. 3
    Door Released

    If access granted: controller sends unlock signal to electric strike or mag-lock. Door hardware releases. Strike or lock re-engages after a configured period (typically 5–10 seconds).

  4. 4
    Event Logged

    Every transaction — granted, denied, door forced, door held — is logged with timestamp, credential, door ID, and decision. This log is the access control audit trail.

Alarm System Integration

Access control systems integrate with alarm systems in several important ways: arming/disarming the alarm when the first/last person enters or exits, triggering an alarm when a door is forced or held open, and locking down all doors when the alarm trips. This integration requires that the access control system and the alarm panel be from compatible manufacturers or connected through an integration module. It's an important question to ask before purchasing either system.