ginngi engineering

A controller for controlling a PTZ camera using the Pelco Coaxitron protocol was implemented, which consisted of a custom FPGA, a Linux based device driver and protocol stack. The Coaxitron protocol controls a PTZ camera via the existing video coax cable rather than a dedicated RS485 cable, thus reducing installation costs. Commands are sent to the camera from the controller on specific lines of the video frame. To meet the tight timing requirements the design required a custom FPGA interfacing to software elements running under Linux.

The design consisted of a physical and protocol layer. The protocol module implemented a Pelco-D command suite and was written in C++ under Linux in a Davinci DM6446 processor, which communicates to the physical layer via a custom device driver. The physical layer consists of a custom FPGA, which accepts commands from the protocol layer and translates them to a physical signal. According to the Coaxitron standard video is not present on the coax cable on specific lines of the video frame; instead, the camera listens during that time for commands from the controller, which are used to control the PTZ functions. The FPGA determines when the specific video line time-slot is present and then transmits the respective command to the camera using a modified asynchronous bit stream. The FPGA was implemented in VHDL and simulated under ModelSim. On the board level a circuit was designed to drive the coax cable during the PTZ transmission window, controlled by the FPGA.