Pelco-D/P are two, relatively similar communication protocols for motorized CCTV cameras.
The devices often feature switches for selecting the protocol within the same hardware and can be implemented in the control keypads, recorders or software, e.g. on a PC. The protocols control camera pan, focus, zoom, etc.
However, they are used less and less. With the high-resolution analogue systems (HD-CVI, AHD, HD-TVI), the developers have introduced new proprietary motorized camera control solutions. Pelco commands are sent with the video signal and require an extra pair of cables.
The signal is transmitted one-way over RS-485 at a distance of up to 1 km and the controlled device does not respond to the commands. Both systems use a 1-byte address field and allow control of up to 256 devices connected to the bus bar. The data rates are not predefined and can be selected from: 1200, 2400, 4800, 9600 bps. Wireless versions are also available.
The protocols support an extended command set, i.e. pre-sets - a set of commands defined by the user for automatic camera control. A motion path and speed, camera return point, fixed camera position, system reset, path function on/off can be used in automatic mode.
Both protocols are relatively simple. The protocol uses a 7-byte or 8-byte frame.
Pelco-D protocol format:
Sync byte – fixed to 0xFF. Address – logical address of the camera being controlled; can control up to 256 devices. Checksum – eight bit modulo 256 checksum calculated from bytes 2 to 6. Command 1, Command 2 – command for the device; see table below for details. Data 1,Data 2 – speed of camera pan and tilt; values from 0 (stop) to 0x3F (high speed); 0xFF – turbo speed.
Fixed to 0
Sense – meaning of bit 3 and 4 in command 1.
Pelco-P protocol format:
End of text
Sync byte – fixed to 0xA0. Address – logical address of the camera being controlled; control up to 256 devices. End of text – fixed to 0xAF. Checksum – xOR sum calculated from bytes 1 to 7. Data 1, Data 2 – command for the device; see table below for details. Data 3, Data 4 – speed of camera pan and tilt; from 0 (stop) to 0x3F (high speed); 0xFF – turbo speed.