> For the complete documentation index, see [llms.txt](https://help.security.gallagher.com/knowledge-hub/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.security.gallagher.com/knowledge-hub/products/cc-hardware/input-and-output-io-devices.md).

# Input and output (I/O) devices

Compare Gallagher I/O device families, functions, and upgrade compatibility.

Gallagher controllers connect to monitoring and control equipment through I/O devices. This includes current HBUS and OSDP hardware, plus legacy GBUS and Local BUS hardware.

### The unassigned-function principle

Inputs and output relays in Gallagher Command Centre **do not have pre-assigned functions**. Any input can be used for door monitoring, detector monitoring, plant monitoring, and so on. The actual function is assigned during Command Centre configuration.

This is the foundation of Gallagher's flexible I/O model. The same hardware serves access control, perimeter intrusion alarms, plant interlocks, and life-safety supervision interchangeably.

### I/O protocol families

<table><thead><tr><th width="89">Family</th><th width="219.9998779296875">Status</th><th>Application</th></tr></thead><tbody><tr><td><strong>HBUS</strong></td><td>Current</td><td>Default for new installs. See <a href="/knowledge-hub/products/cc-hardware/input-and-output-io-devices/io-devices-hbus.md">IO Devices - HBUS</a> for the full module catalogue and topology rules.</td></tr><tr><td><strong>OSDP</strong></td><td>Current, hub bridge</td><td>When third-party OSDP devices (card readers, keypads, biometric scanners) need to integrate with Gallagher controllers. See <a href="/knowledge-hub/products/cc-hardware/input-and-output-io-devices/io-devices-osdp.md">IO Devices - OSDP</a>.</td></tr><tr><td><strong>GBUS</strong></td><td><p>Legacy</p><p>Support stopped at v9.00</p></td><td>Field-devices on older sites. See <a href="/knowledge-hub/products/cc-hardware/input-and-output-io-devices/io-devices-legacy.md">IO Devices - Legacy</a>.</td></tr><tr><td><strong>Local BUS</strong></td><td><p>Legacy</p><p>Support stopped at v8.00</p></td><td>Older field-devices. See <a href="/knowledge-hub/products/cc-hardware/input-and-output-io-devices/io-devices-legacy.md">IO Devices - Legacy</a>.</td></tr></tbody></table>

> ⚠️ **Command Centre upgrade traps:** A site running GBUS expansion hardware cannot upgrade its Command Centre server past **vEL9.00** without first replacing the I/O hardware. A site running Local BUS hardware is capped at **vEL8.0**. Always confirm the I/O protocol family before quoting a CC upgrade for an established site.

### HBUS at a glance

HBUS is Gallagher's proprietary RS-485-based protocol. Key characteristics:

* **Uniquely serialized devices.** Devices identify and configure themselves when connected to a controller.
* **Automatic firmware management.** The Controller pushes the matching firmware version to the device, with no user interaction.
* **Flexible function assignment.** See the unassigned-function principle above.
* **Daisy-chain topology.** See [IO Devices - HBUS](/knowledge-hub/products/cc-hardware/input-and-output-io-devices/io-devices-hbus.md) for cable, distance, and termination rules.

### OSDP

**OSDP (Open Supervised Device Protocol)** is an open, RS-485-based communication protocol used in physical access control systems to connect access control panels with peripheral devices like card readers, keypads, and biometric scanners.

* The Security Industry Association (SIA) developed the protocol.
* It is increasingly replacing legacy Wiegand.
* It adds security, supervision, and bidirectional communication.

Gallagher's OSDP integration is delivered via the **OSDP 8 Port Hub (C300678, install note 3E5775)**, an HBUS-side hub that bridges OSDP-attached peripherals to Gallagher Controllers.


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