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Back to: Advanced Physical Security Integration (APSI)

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Lesson 6.1: Hard-Wired Integration (I/O) & Relay Logic

Module: 6 – Unified Integration Logic

Prerequisites: Lesson 1.3 (Ohm’s Law) & Lesson 5.4 (Circuit Wiring)

Estimated Time: 45–60 Minutes

1. Learning Objectives

By the end of this lesson, you will be able to:

  • Define “I/O Integration” and why it is the “Universal Language” of hardware.
  • Differentiate between Dry Contacts (Switch only) and Wet Contacts (Voltage supplied).
  • Wire a Form C Relay using Common (C), Normally Open (NO), and Normally Closed (NC) terminals.
  • Design a hard-wired handshake between two disparate systems (e.g., linking a Fire Alarm to an Access Control door).

2. The Universal Translator: The Relay

Software integration (API) is great, but it requires drivers, licenses, and network compatibility. Hard-Wired Integration is physics. It always works.

If you want System A (Fire Alarm) to talk to System B (Access Control), you use a Relay.

  • System A sends an electrical pulse to a coil (The Output).
  • The Relay physically clicks a metal switch.
  • System B sees the switch close (The Input) and reacts.

3. Form C Relays: The Standard

Almost every security device (Camera, Controller, Motion Sensor) has a “Relay Output” or “Alarm Output.” These are typically Form C Relays.

They have three terminals:

  1. C (Common): The entrance. This is where you put your wire coming in.
  2. NO (Normally Open): The “Off” state. The circuit is broken. It only connects to Common when the relay is active.
  3. NC (Normally Closed): The “On” state. The circuit is connected. It breaks connection when the relay is active.

The Logic:

  • Triggering a Siren: Use NO.
    • Normal: Circuit open. Silence.
    • Active: Circuit closes. Power flows to siren. Scream.
  • Holding a Maglock: Use NC.
    • Normal: Circuit closed. Power flows to lock. Door locked.
    • Active: Circuit opens. Power cut. Door unlocks.

4. Dry vs. Wet Contacts

This is the #1 way technicians blow up boards.

A. Dry Contact (The Standard)

  • Definition: Just a switch. No electricity comes out of the terminals.
  • Usage: You are just “shorting” two wires together for the other system.
  • Example: Pushing an elevator button. You aren’t powering the elevator; you are just closing a circuit to tell the elevator controller to move.

B. Wet Contact (Powered Output)

  • Definition: The terminal outputs Voltage (e.g., 12VDC) when triggered.
  • Usage: To power a small device directly (like a strobe light or sounder).
  • The Danger: If you connect a Wet Output (12V) from a Camera into the Dry Input of an Access Panel, you will feed 12V directly into a sensitive logic chip and fry the board.
  • Field Test: Always put a multimeter on the output terminals before connecting wires. If you see voltage, it is Wet. If you see 0V (but continuity changes), it is Dry.

5. Practical Application: The “Forced Door” Integration

Scenario: You have an Access Control System (ACS) and a separate Burglar Alarm (IDS). You want the Burglar Alarm to scream if someone forces open the back door.

The Hardware:

  1. ACS Controller: Has an “Aux Relay” mapped to the “Forced Door” event.
  2. IDS Panel: Has a Zone Input available (Zone 5).

The Wiring:

  1. At the IDS (Input): Wire Zone 5 and COM to a cable.
  2. At the ACS (Output): Connect the other end of the cable to C (Common) and NC (Normally Closed) on the Aux Relay.
    • Why NC? Because Alarm Zones expect a closed loop (Lesson 5.1).
  3. The Logic:
    • Normal: The ACS relay is closed. The IDS sees a closed loop. (Zone Secure).
    • Event: Burglar kicks door. ACS detects “Forced Door.” ACS fires relay (Opens it).
    • Result: IDS sees the loop break. IDS triggers the siren.

6. The Fire Alarm Interface (FAI)

This is the most critical I/O integration in the building.

  • Requirement: When the Fire Alarm (FACP) goes off, all Maglocks must cut power instantly.
  • The Hardware: The FACP provides a Dry Contact relay that opens during a fire.
  • The Wiring: You route the Power Wire (Positive leg) of your Maglock power supply through the FACP relay.
    • Power Supply (+) -> FACP (Common).
    • FACP (NC) -> Maglock (+).
  • Operation:
    • Normal: Relay is closed. Power flows through FACP to lock.
    • Fire: FACP relay opens. The power wire is physically cut. The lock drops.