CCTV - PTZ wiring

[Sally2000]

Two of my cameras have PTZ control with the usual RS485 signalling. I wired them with twisted pairs, and they work properly using 9.6kbps signalling.

I am going to upgrade two other cameras, with PTZ control. They are about 30m and 40m from the DVR, and to install new twisted pairs to them would mean a lot of work. I do however have unused pairs on existing 8-core alarm cables going to each of these locations, so it would be very convenient to use them. These are not twisted pairs as specified for RS485.

Does anyone have experience of running RS485 over conventional alarm wiring, or have any comments?

 

[sparkymarka]

Worth a go ! What have you got to lose ?

 

[Sally2000]

"What have you got to lose?"

I can always try it when the cameras arrive, but in the meantime it would be good to know if anyone else has done it successfully or otherwise.

 

[bernardgreen]

One circuit RS485 at 19.2 Kb ( IIRC ) about 100 metres using non twisted pair did work for a data link but extra error detection was incorporated. Other circuits in the same cable were DC control circuits ( driving relays ) and when these switched state they did affect the data due to back EMFs from the relay coils.

 

[Sally2000]

Thank you Bernard, that's interesting.

The RS485 protocol was designed to operate at much higher speeds, and twisted pair would be essential then. But of course camera control needs only low speeds. I will certainly try the new cameras with the existing spare pairs.

I assume you had diodes across those relay coils?

 

[bernardgreen]

I assume you had diodes across those relay coils?

Not initially but they were added along with series resistors at the switch end. The resistor slows the discharge of the capacity of the cable and thus reduces the EMF pulse.

On very long cables with a significant capacitance to discharge that series resistor between switch and cable also reduces wear on the switch contacts.