OSDP Device Communication

Introduction

It’s hard to believe that the physical industry continued deploying readers with antiquated insecure Wiegand protocol. The rest of the world has incorporated seamless secure communications on just about every piece of hardware sold. Unlike the Wiegand protocol, the Open Supervised Device Protocol (OSDP) allows for two-way communication. OSDP is huge step forward in allowing readers and other security devices to become intelligent and have the ability to fully communicate with the rest of the world.

Each OSDP bus requires one Access Control Unit (ACU) and can support up to 128 Peripheral Devices (PD). Each PD is identified by an address between 0-127. All communications are initiated by the ACU and the PDs respond when their address is included in the packet. To facilitate reporting events that occur at the PD, such as a card read, the ACU must poll each PD regularly. 

Topology

OSDP typically communicates with readers on a RS-485 2-wire bus. There is no difference with the installation of RS-485 needed for OSDP. It extends the same wiring to downstream boards for readers. Most integrators will be familiar with the requirements for the installation of 2-wire RS-485. There are three common topologies that can be used to connect OSDP devices.

  1. The point-to-point connection type used with Wiegand readers is supported.
  2. Having RS-485 allows for many more options for wiring. A typical RS-485 bus can have multiple device connected on short stubs.
  3. A RS-485 hub can be added for a star topology similar to ethernet networks.

While it is common for OSDP to communicate on RS-485, the standard is not dependent on a specific physical layer. It is possible to have devices connected using standard TCP/IP over ethernet or wireless. The controller in the diagrams shown above can be replaced with an ethernet->RS-485 bridge and it then connects to the bridge. Additionally, there is no limit to the number of OSDP busses that can be configured on a controller. If the controller has enough processing power, more busses and PDs can be added.

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