Can we help? - Mind your protocol!

Q: Over the past couple of months I have been looking into installing a dome camera telemetry system. My research established that there are two main types of systems available. One works on coaxial cable, the others use data cable. Firstly, could you give me an overview of these two types of system and secondly, could you confirm whether they are compatible with one another?

A: I'll start by giving you a brief description of the main formats.

Coaxial
Coaxial telemetry is transmitted along the same cable as the video signal. The multiplexer or telemetry controller sends data to the receiver in the form of pulses. These FSK pulses (frequency shift key) are situated on the back porch of the composite 1v-p-p signal.

The total distance of the transmitted signal is dependent on the grade of cable used, so careful consideration should be given to this in the planning stage.

RS485
RS485 has become the industry's standard workhorse for multi- point data transmission. It allows multiple pieces of equipment to communicate bi-directional over a single twisted pair cable.

It uses the voltage difference between the two terminals to signal binary code. Being a balanced transmission system, termination resistors are used in the end of line device.

With data transmission distances of more than 1200 metres, it makes it an ideal communication platform for telemetry systems large or small.

The above are the two main ways of transmitting the telemetry signal, that is the signal from the base station (transmitter) to the camera (receiver).

However, to answer your 'Mix and Match' question, even within these two methods of transmitting the telemetry signal, there are many variants. Separate manufacturers will use different protocols (meaning the language they use to communicate with each other).

For example, a RS485 transmitter manufactured by one company does not always work with a RS485 dome manufactured by another company.

It is possible to acquire a protocol converter for certain manufacturers' equipment. These devices connect inline and will convert a coax signal into a RS485 signal, and so on, allowing you to match one manufacturer's transmitter to another manufacturer's receiver. Please check with your supplier for the range available.

However, many multiplexer units have leading manufacturers' dome protocols built in. These are operated using a keyboard with an integrated joystick, and conveniently eliminate the need for any additional conversion equipment.