Modern lighting equipment includes dimmers, flashing lights, moving lights, colored lights, and gobos (GOes Before Optics). These lighting systems are often controlled over long distances—up to 4000 feet—using the DMX512 communications protocol.
What is DMX512?
The DMX512 standard, which specifies an 8-bit asynchronous serial protocol and 250-kbps data rate, is designed to carry repetitive control data from a single controller to one or multiple receivers. The control data on the primary link consists of up to 513 slots that are sent in packets over a balanced transmission line.
Information, in an 8-bit format, is sent sequentially to the various nodes. Values range from 0 to 255, where 0 indicates the off condition and 255 indicates the on condition. A break condition lasting two frames indicates the start of a sequence of 512 values. A high level for at least 8 µs indicates the start of the first byte.
The DMX512 standard specifies a system using ground-referenced transmitting devices and isolated receiving devices. The receiving devices are isolated to protect expensive lighting equipment from harmful current surges.
Figure 1 shows a discrete implementation of an isolated DMX512 receiver. An isolated power supply is generated by a transformer driver driving the primary side of a transformer. The output of the transformer is rectified and regulated to create an isolated 5-V supply. The data and control signals for the RS-485 transceiver are isolated using optical isolation.
The ADM485 RS-485 transceiver converts a control signal received on the A and B pins into a serial output on the RXD pin. This signal is optically isolated and connected to the UART input of the ADuC7020 precision analog microcontroller. The ADuC7020 software decodes the message and outputs logic-level signals to the digital-to-analog converter (DAC). The ADTL084 JFET-input op amps buffer the DAC outputs to provide 0 V to 10 V signals.
The ADuC7020 software sends a response to indicate that the message was received correctly. This signal is optically isolated from the ADM485, which outputs a signal on the A and B pins.
Figure 1. Discrete DMX512 receiver block diagram.
Configuration using ADM2487E
Figure 2. ADM2487E DMX512 receiver block diagram.
The ADM2487E transceiver, suitable for half-duplex or full-duplex communication on multipoint transmission lines, operates with data rates up to 500 kbps. It provides 2.5-kVrms isolation—certified to Underwriters Laboratory (UL) and VDE standards—and ±15-kV ESD protection.
For half-duplex operation, the transmitter outputs and receiver inputs share the same transmission line by externally linking transmitter output pin Y to receiver input Pin A, and transmitter output pin Z to receiver input pin B. Designed for balanced transmission lines, the ADM2487E complies with TIA/EIA‑485‑A-98 and ISO 8482:1993 standards.
Current-limiting and thermal shutdown features protect against output short circuits and situations where bus contention might cause excessive power dissipation. Fully specified over the –40°C to +85°C industrial temperature range, the ADM2487E is available in a 16-lead, wide-body SOIC package.
The ADM2487E features an open- and short-circuit fail-safe receiver input design, eliminating the need for external biasing resistors. The low-input-current receiver design (125 µA) enables up to 256 nodes to be connected on the same bus.
Replacing the discrete DMX512 receiver implementation with the ADM2487E implementation provides a space-saving, more robust, more reliable system.
Why is Isolation
Figure 3. Galvanic isolation allows information flow, prevents ground-current flow.
The ADM2487E integrates galvanic isolation with the transceiver to create robust protection against harmful current surges. An ideal solution to implement an isolated DMX512 receiver, it will reduce the overall form factor, improve reliability, and increase robustness.
Hein Marais [email@example.com] graduated from the University of Stellenbosch, South Africa, in 2001 with a Bachelors degree in electronics. He started his career in the South African Air Force working on electronic warfare systems. In 2003, he joined Grintek Communication Systems, where he was involved in the design of software and hardware for military radios. In 2007, Hein joined ADI, where he is currently a product applications engineer for the High Speed Interconnect group, working on RS-485 and LVDS products. (return to text)
Download this article as a PDF: Using Isolated RS-485 in DMX512 Lighting Applications
Copyright 1995- Analog Devices, Inc. All rights reserved.