The lighting industry often uses a data protocol called DMX512 which can control up to 512 channels of either lighting or effects over a single twisted pair cable operating with the RS485 electrical standard.
When using long runs of cable, lots of connected devices, branching out in different directions or crossing between zones that may have a ground differential due to different power sources, the use of an isolator/buffer is recommended to ensure integrity of the data.
The isolator/buffer works by using two separate sections of circuitry with independent power supplies. The receiving circuitry has a standard RS485 receiver chip which converts the incoming data to a logic signal that is then used to drive an opto-isolator. The receiving side of the opto-isolator then drives one or more RS485 transmitter chips to output a freshly amplified and electrically isolated data signal.
Note that this simple arrangement is only suitable for communication in a single direction. Bidirectional communication requires much more sophisticated circuitry that can interpret information on the network and change data direction as needed. (As used in the newer RDM lighting standard that is backwards compatible with DMX.)
For reference, the RS485 standard is designed to transmit data for long distances at modestly high speeds with good rejection of external interference. It's widely used in industrial equipment as an alternative to canbus. The main features are the use of a differential pair of wires with a screen that is usually grounded at one end of the network. Data on the twisted pair is transmitted as binary data with one polarity representing a "1" and the other polarity representing a "0". The switching of polarity ensures a very decisive state change and the way the receivers determine the data from the polarity with a wide tolerance to the ground reference means that common mode interference (electrical noise induced onto both wires) is less of an issue.
Maintaining a strong signal level is important as the DMX512 protocol has no error detection. If the signal is degraded or electrical reflections occur on long cable runs then it can result in spurious data, which is very visual when it is being used to control dimmers, lights and effects. It manifests itself as flickering or random movement and colour from intelligent lighting fixtures.
Despite being aimed at the entertainment industry, these units could also be used with other low speed (less than 1Mbit) unidirectional RS485 networks.
19 июн 2015
