Module 1: Wiring, inputs and outputs
Controllers and interface boards, locks, relays, supervised inputs, Wiegand vs OSDP, voltage drop, power and life safety.
- Identify the parts of a controlled door
- Wire locks and inputs correctly
- Choose between Wiegand and OSDP
- Size cable and power
- Understand how the fire alarm and life safety codes shape every door
This course explains principles. Wiring values, cable distances, and life safety requirements vary by product, firmware, code edition, and the Authority Having Jurisdiction (AHJ). Always confirm against the installation manual and the locally adopted code before you terminate a wire or sign off a door.
1.1 Controller and interface architecture
Most enterprise systems split the job between an intelligent controller (stores the database, makes decisions, talks to the server) and interface modules (land the reader, lock, and sensor wires). They communicate on an RS-485 bus or over IP.
| Device | Role | Notes |
|---|---|---|
| Synergis Cloud Link (SCL) | Genetec's intelligent controller appliance | Connects to Security Center's Access Manager and drives downstream hardware such as Mercury boards and Axis A1601 controllers. |
| Mercury intelligent controller (e.g. LP1502, LP4502, MP series) | Controller with its own database | Used under Synergis Softwire on SCL, and by many other platforms. |
| Mercury interface boards (MR52, MR50, MR16in, MR16out) | Field I/O | MR52: 2 readers, 8 supervised inputs, 6 relays. MR16in and MR16out add inputs or relays. |
| Axis A1601 | IP door controller | 2 reader ports (Wiegand or RS-485/OSDP), Form C relays, supervised door-monitor and REX inputs; can run standalone or be enrolled on an SCL. |
| Axis A1001 (legacy) | Earlier Axis IP door controller | Same concept; A1601 is the current platform. |
| HID VertX / Edge | Legacy IP controllers | Supported by Security Center with a reduced feature set (e.g. no two-person or escort rule). |
1.2 Door hardware: locks, fail-safe and fail-secure
Fail-safe means the lock is unlocked when power is removed. Fail-secure means it stays locked when power is removed. These terms describe the secured side only: a properly installed door always allows free mechanical egress from the inside.
| Lock | Power behaviour | Typical use | Watch out for |
|---|---|---|---|
| Electromagnetic lock (mag lock) | Always fail-safe: holds only while powered | Glass doors, retrofits, high holding force | Strict egress rules (sensor plus manual release, fire alarm release). Draws current 24/7. |
| Electric strike | Fail-safe or fail-secure (often field-selectable) | Most office doors with a mechanical lever on egress side | Must match the lockset latch; check fire rating if on a fire door (fire doors need fail-secure strikes). |
| Electrified lockset / mortise | Fail-safe or fail-secure | Higher security, cleaner look | Needs power transfer hinge or door loop; REX switch often built in. |
| Electrified exit device (panic bar) | Usually fail-secure (electric latch retraction) | Egress doors, assembly occupancies | High inrush current on latch retraction; often needs a dedicated power supply. |
Rule of thumb: stairwell doors and fire-rated doors are usually fail-secure on the secured side so the door stays latched for fire integrity, while re-entry requirements for stairwells may require the lock to release on fire alarm. Your AHJ decides.
1.3 Relays, contacts and flyback diodes
A controller's lock output is normally a Form C relay with three terminals: COM (common), NO (normally open) and NC (normally closed). "Normally" means with the relay de-energized.
| Wiring | When relay is idle | When relay fires (access granted) | Use for |
|---|---|---|---|
| COM + NO | Circuit open, no power to lock | Circuit closed, lock powered | Fail-secure strike (power to unlock) |
| COM + NC | Circuit closed, lock powered | Circuit open, lock unpowered | Mag lock or fail-safe strike (remove power to unlock) |
- Dry contact
- The relay is only a switch. You bring lock power from a separate supply through COM. Most Mercury relays are dry.
- Wet contact
- The controller supplies voltage on the relay. The Axis A1601 can put 12 or 24 V DC on COM via a jumper, within its rated current.
- Relay rating
- Check volts and amps (Axis A1601 relays are rated about 2 A at 30 V DC). Exceeding it welds or burns the contacts.
- Flyback diode
- A lock coil is an inductor. When power is removed, it kicks back a voltage spike that pits relays and resets electronics. Put a diode (e.g. 1N4007) across the lock coil, reverse-biased: band (cathode) to +, anode to −, as close to the lock as possible. Use an MOV for AC locks.
Many locks and power supplies have suppression built in; check before adding one, and never install a diode backwards on a DC lock or it shorts the supply the moment the lock is energized.
1.4 Door inputs: door position switch and REX
| Input | What it tells the controller | Key settings |
|---|---|---|
| Door position switch (DPS / door contact) | Whether the door is physically open or closed | Door forced open (opened without grant or REX), door held open (open longer than the held time), relock on close. |
| Request-to-exit (REX) | Someone is leaving legitimately, so do not raise forced-open | Unlock on REX (on or off), time to ignore REX after a grant, ignore REX while door open. |
| Auxiliary inputs | Tamper, power-supply AC fail and low battery, glass-break, panic buttons | Map to alarms or I/O linking. |
| Auxiliary outputs | Sounders, strobes, door-held buzzers, interlocks, elevator floor relays | Trigger from events, schedules or threat levels. |
REX devices come in two families: passive infrared (PIR) motion sensors above the door, and mechanical switches in a push-bar or lever. On a mag-locked door the REX usually must also cut lock power directly, not just signal the controller (see 1.9). On a strike door with a mechanical lever, set Unlock on REX to off: the person opens the door mechanically and the REX only shunts the forced-open alarm.
1.5 Supervised inputs and end-of-line resistors
An unsupervised input only sees open or closed, so a cut wire or a jumper looks like a normal state. A supervised input adds end-of-line (EOL) resistors at the device so the panel reads a resistance, giving four states:
| State | Resistance (1kΩ/1kΩ example) | Meaning |
|---|---|---|
| Normal (contact closed) | 1 kΩ (parallel resistor shorted) | Door closed |
| Active (contact open) | 2 kΩ (series + parallel) | Door open |
| Short | near 0 Ω | Trouble: someone jumpered the wires |
| Open circuit | infinite | Trouble: cable cut |
Resistor values (1k/1k, 2k/2k, 2.2k/4.7k, etc.) are set per input in the software and must match what is installed. Mercury's newer boards also report ground fault and foreign voltage. A supervision mismatch shows up as a constant trouble state the day the door is commissioned.
1.6 Reader wiring: Wiegand versus OSDP
| Wiegand | OSDP (RS-485) | |
|---|---|---|
| Direction | One-way, reader to panel | Two-way, panel polls reader |
| Wires | D0, D1, LED, buzzer, power, ground (6+ conductors) | A, B (twisted pair), power, ground (4 conductors) |
| Distance | About 500 ft (150 m) typical | Up to 4,000 ft (1,200 m) on RS-485 |
| Supervision | None: a dead reader looks like an idle one | Constant polling; offline and tamper reported |
| Encryption | None: data can be sniffed and replayed | OSDP Secure Channel (AES-128) |
| Multi-drop | One reader per port | Multiple addressed readers on a bus (check panel limits) |
| Feedback | LED and beeper lines only | Text, LED colours, firmware updates, status |
For OSDP, use shielded twisted-pair cable rated for RS-485 (around 120 Ω characteristic impedance), terminate the bus ends where the manufacturer requires it, ground the shield at one end only, and give every reader a unique address and matching baud rate. OSDP is an SIA standard (also IEC 60839-11-5).
1.7 Cable, distance and voltage drop
| Circuit | Typical cable | Typical limit |
|---|---|---|
| Wiegand reader | Shielded 6–8 conductor, 22 or 18 AWG | About 500 ft |
| OSDP reader / RS-485 panel bus | Shielded twisted pair, 24 AWG, plus power pair | Up to 4,000 ft data; power limited by voltage drop |
| Lock power | 18 AWG or heavier, 2 conductor | Set by voltage drop, not a fixed number |
| DPS, REX, aux inputs | 22 AWG, 2 or 4 conductor | Long, low-current |
| Controller network | Cat 6 | 328 ft (100 m) per Ethernet segment |
Lock circuits fail from voltage drop more than anything else. The round-trip formula:
Worked example
A 12 V DC mag lock drawing 0.5 A on 18 AWG (about 6.4 Ω per 1,000 ft) at 200 ft drops 2 × 200 × 0.0064 × 0.5 = 1.28 V, leaving about 10.7 V at the lock, an 11% drop. Most locks want within 10%, so go to 16 AWG, use 24 V, or move the power supply closer.
Try it: lock circuit voltage drop
Resistance values are approximate for solid copper at room temperature; use the cable manufacturer's datasheet for real designs.
1.8 Power supplies
- Use a listed access control power supply with battery backup, sized for the sum of all locks, readers and boards plus about 20% headroom.
- Keep lock power separate from controller and reader power where possible, so lock inrush and spikes do not reset the electronics.
- Use a fused or PTC-protected distribution board so one shorted lock does not drop every door.
- Supplies with a fire alarm interface input can drop selected fail-safe outputs on alarm.
- Wire the supply's AC-fail and low-battery outputs to controller inputs so you hear about power problems before the battery dies.
- Battery standby time is a design decision: calculate standby current times hours required, and remember mag locks draw continuously.
1.9 Fire alarm tie-in and life safety (high level)
Life safety always overrides security. NFPA 101 (Life Safety Code), the IBC and the ADA Standards drive most door requirements in the US; the edition adopted by your jurisdiction and the AHJ's interpretation are what count.
- Free egress: a person inside must be able to leave without a key, card, special knowledge or effort.
- Sensor-release electrical locks (typical mag lock door): a sensor on the egress side releases the lock, plus a manual release device marked PUSH TO EXIT, roughly 40–48 in. above the floor and within 5 ft of the door, that cuts lock power directly for at least 30 s, independent of the access control electronics.
- Electrically locked egress doors must release on fire alarm activation and on loss of power. Do this in hardware: fire alarm relay to the lock power supply, not only through software.
- Delayed egress hardware (typically 15 s, up to 30 s if the AHJ approves) needs signage and an irreversible release cycle, and only where the occupancy allows it.
- Fire-rated doors must stay positively latched: use fail-secure, fire-rated strikes or locksets, never a mag lock as the only latching.
- ADA: operable parts such as readers, push buttons and hardware within the accessible reach range (commonly 15–48 in., with door hardware 34–48 in.), operable without tight grasping or twisting.
Common mistakes
- Wiring a mag lock on COM/NO, so it only locks when access is granted.
- No flyback diode on a DC lock, leading to random controller resets and burnt relays.
- EOL resistors installed at the panel instead of at the device, which defeats supervision.
- Unlock on REX enabled on a strike door with a mechanical lever, unlocking the door every time someone walks past the PIR.
- Running OSDP on untwisted alarm cable, or forgetting termination, causing intermittent reader offline events.
- Fire alarm release done only in software; if the server or controller is down, the doors stay locked.
- Lock power drawn from the controller's reader or aux output, exceeding its rated current.
Check your understanding
1.A mag lock must unlock when the relay fires. Which relay terminals do you use?
2.Is a mag lock fail-safe or fail-secure, and why can't it be the other?
3.A supervised door contact reads near 0 Ω. What does the panel report?
4.Name two security advantages of OSDP over Wiegand.
5.A 24 V strike draws 0.25 A at 300 ft on 18 AWG (6.4 Ω/1,000 ft). What is the voltage drop?
6.Why should the fire alarm release be hardwired to the lock power supply?
7.Where does the flyback diode go and which way does it face?