OSDP describes the communication protocol for interfacing one or more Peripheral Devices (PD) to a Control Panel (CP). The OSDP specification describes the protocol implementation 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 v2.1.7 of the OSDP specification. This page pulls excepts from the specification that are crucial points when trying to understand the protocol.

Physical Interface

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


Half duplex asynchronous serial 8 data bits, 1 stop bit, no parity bits, with either one of 9600, 19200, 38400, 115200 or 320400 baud rates.

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 the “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.


The transmitting device shall guarantee a gap of a minimum of two character times before it may access the communication channel. This idle line delay is required to allow for signal converters and/or multiplexers to sense that the line has become idle.

The PD shall send a single reply message to each message addressed to it within 200 ms. If the PD is unable to accept the command for processing due to temporary unavailability of a resource required to process the command, then the PD shall send the osdp_BUSY reply. When the CP receives the osdp_BUSY reply, it may, at its discretion, choose to re-send the same command as it would if the command delivery 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

The general procedure for a peripheral device (PD) to obtain message synchronization is to wait for an inter-character timeout then look for a Start-Of-Message (SOM) code. The device should then receive and store at least the header fields while computing the checksum/CRC on the rest of the message. If the checksum is good, only the PD that matches the address field processes the message. All other PDs, however, should monitor the packet by counting the remaining portion of packet to be able to anticipate the start of the next packet.

If there is an inter-character timeout while receiving the message the PD shall abort the receive sequence. Once aborted, the PD should re-sync using the method described above.

The nominal value of the inter-character timeout shall be 20 milliseconds. This parameter may need to be adjusted for special channel timing considerations.

Packet Structure

See packet structure documentation.

Peripheral Device Capabilities

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

for more details.


See list of commands supported by LibOSDP.