Introduction

OSDP describes the communication protocol for interfacing one or more Peripheral Devices (PD) to a Control Panel (CP). The OSDP specification describes the protocol over a two-wire RS-485 multi-drop serial communication channel. Nevertheless, this protocol can be used to transfer secure data over any physical channel.

LibOSDP complies with v2.2 of the OSDP specification. This page pulls excerpts from the specification that are crucial to understanding the protocol.

Physical Interface ​

Half-duplex RS-485 — one twisted pair, shield/signal ground.

Signaling ​

Half-duplex asynchronous serial: 8 data bits, 1 stop bit, no parity bits, with any one of the following baud rates: 9600, 19200, 38400, 57600, 115200, or 230400.

Character Encoding ​

The complete 8-bit character is used. All possible bit patterns may appear within a message.

Channel Access ​

The communication channel is used in "interrogation/reply" mode. Only the CP may spontaneously send a message. Each message sent by the CP is addressed to one and only one PD.

Timing ​

The transmitting device must guarantee a gap of at least two character times before accessing the communication channel. This idle-line delay is required to allow signal converters and/or multiplexers to sense that the line has become idle.

The PD must send a single reply to each addressed message within 200 ms. If the PD is temporarily unable to process a command, it must send osdp_BUSY. When the CP receives osdp_BUSY, it may re-send the same command as if it had timed out.

The typical REPLY_DELAY should be less than 3 milliseconds. If a device is overwhelmed it can send a BUSY message.

Message Synchronization ​

A PD obtains message synchronization by waiting for an inter-character timeout and then looking for a Start-Of-Message (SOM) code. The device receives and stores at least the header fields while computing the checksum/CRC. If the checksum is valid, only the PD matching the address field processes the message; all other PDs monitor the packet by counting the remaining bytes to anticipate the start of the next packet.

An inter-character timeout while receiving aborts the receive sequence, after which the PD re-syncs using the method above. The nominal inter-character timeout is 20 milliseconds.

Packet Structure ​

See the packet structure documentation.

Peripheral Device Capabilities ​

OSDP PDs must advertise a list of predefined capabilities to the CP in response to the osdp_PDCAP command. See the comprehensive list of PD capabilities.

Commands ​

See the list of commands supported by LibOSDP.

Secure Channel ​

OSDP can run an encrypted, authenticated session over the bus. See Secure Channel for the handshake, SCBK handling, and install mode.

Frequently Asked Questions ​

Common protocol questions are answered in the FAQ.