A common phenomenon occurs when debugging monitoring projects: the image is normal, but the dome camera cannot rotate. The problem mostly lies in the reverse data link. This article explains the role and wiring method of the RS485 reverse channel in the video optical transceiver.

Why does video return need a data channel?

The control instructions of the dome camera are sent from the back-end keyboard or video recorder, and the direction is opposite to the video. A reverse channel is needed to bring the instructions back to the front-end along with the optical fiber. like8-channel video + 1-channel reverse 485 data video optical transceiverThis type of model multiplexes one channel of reverse data and eight channels of video on the same fiber. There is no need to lay another fiber for the control line. The link and management are simple, and one less device space is occupied on the computer room side.

485 is connected to the decoder like this

• Connect hand in hand along the line, A to A, B to B, no reverse connection within the same section;
• The short cable from the decoder to the optical transceiver should be as short as possible and run separately from the power cable;
• Add a matching resistor of about 120 ohms to the end of the bus. It does not need to be installed when the line is short;
• Only one dome machine or one bus is connected to the 485 channels along the way. When the number of dome machines is large, it should be expanded by the distributor first.

When wiring, record the number of each channel and the address of the dome machine in the table and take photos for files. Later maintenance does not have to rely on guesswork, and the table can be restored when changing machines.

Troubleshooting sequence for reverse channel failure

First switch to a known normal channel and try once to determine whether it is a circuit or equipment problem; then check the polarity and terminal resistance; if there is still no connection, useThree-in-one data optical transceiverThis kind of transparent data equipment has a separate link, and by comparing the command sending and receiving results, the fault scope can be quickly locked. In practice, reverse link abnormalities are mostly caused by reversed polarity and lack of terminal resistance on long lines. These two items should be checked first. EB-LINK technical support can assist in interpreting the on-site situation during working days. The phone number is 0755-83179002.

The reverse channel is a channel with low cost and great experience. It takes ten minutes more to record during the wiring stage and saves one hour during the debugging stage.