Hardware Example

The following chapter represents an application design example for explanation proposals and is not part of the product standard. The customer must design his own solution, choose its most appropriate components and validate the final product according to the legislation and the DMX and RDM specifications.

DMX+RDM Responder Hardware Example

This example shows the design of a DMX+RDM Receiver using a non-isolated receiver.

The standard requires the manufacturers to label this type of receiver as NON-ISO.

Block A: Connectors

Connectors

The official DMX connector is the XLR-5. Exceptions include RJ45, miniature connectors, and screw terminal connectors. However, despite its popularity and widespread use, XLR-3 is not part of the DMX standard and should not be used.

XLR-5 connectors are typically used in professional equipment, while XLR-3 connectors are more common in cost-sensitive devices. XLR-3 is generally cheaper than XLR-5. Using an XLR-3 connector has the drawback of making your product compatible with standard microphone cables, which are specifically designed for low-frequency analog audio—not digital signals. As a result, microphone cables are not suitable for DMX.

DMX receivers (with or without RDM) use two XLR‑5 connectors in daisy-chain configuration with the following configuration:

• DMX Out: Female connector
• DMX In: Male connector

And the following pinout:

• Pin 1: Singal-Common
• Pin 2: Pair A, Data −
• Pin 3: Pair A, Data +
• Pin 4: Pair B, not used
• Pin 5: Pair B, not used

Cable

The DMX cable screen must be connected to the pin 1 of the XLR-3 or XLR-5 connector and not to its shell. Do not connect the cable screen to the connector shell.

Use only twister pair cable to carry the DMX signal.

Do not use microphone cable, as it has been designed to carry low-frequency signals, and it will degrade the DMX signal, increasing the chances of spurious flickers on the LED fixtures.

Terminator

If your device is the last one on the DMX bus, a line terminator should be placed at its DMX Out connector. This reduces signal reflections and prevents DMX signal degradation, which can cause flickering on long cable runs among other problems.

The line terminator is a 120 Ω, 1 W resistor connected across the balanced signal lines (Data + and Data −). You can also find DMX connectors (with no cable) that have this resistor integrated internally.

Place only line terminators at the beginning and at the end of the DMX bus. No more than two terminators should be used on the same bus. Usually, DMX controllers include an internal terminator, so adding one at the beginning of the bus is often unnecessary.

Block B: Protection

The protection stage is influenced by several factors, including the intrinsic robustness and protection features of the RS485 transceiver chip, the product's budget, and its required reliability, among other considerations. Refer to your transceiver's documentation to determine the appropriate protection requirements.

In the schematic, a bidirectional 400-W transient suppressor diodes (CDSOT23-SM712) are used to protect against surge transients.

Block C: Transceiver

DMX operates over the RS485 electrical standard. Therefore, an RS485 transceiver is required to convert the differential RS485 signals to TTL-compatible voltage levels before entering the IS3720.

Either 3.3 V or 5 V transceivers can be used, as the IS3720 RX/TX and DIR pins are 5 V tolerant. However, 5 V devices are preferred because they offer better noise immunity on the DMX bus and allow longer cable runs.

Grounding-Earthing

Do not connect the pin 1 (Singal-Common) of the XLR-5 connector directly to the earth (the mains earth), as this can create dangerous current loops.

Signal-Common should be connected to earth through a 100 Ω (2 W) resistor to limit potential current flow through the DMX cable. This is especially important in large installations.

Block D: IS3720

The IS3720 is very simple to integrate into your design.

A 100 nF and 2.2 μF decoupling capacitors should be placed as close as possible to the supply pins (VDD and VSS).

The RX/TX is the UART data pin for DMX+RDM communication. This pin requires an external pull‑up resistor. Since DMX+RDM communication is half‑duplex, this pin connects to both the Driver Input and Receiver Output of the transceiver.

The DIR pin connects to both the Driver Enable and Receiver Enable pins. The DIR pin also requires a pull‑up resistors. This pin goes high when the IS3720 is transmitting data and remains low at all other times.

Use the same pull‑up voltage as the transceiver supply. If you use a 5 V transceiver, connect the pull-up resistors to 5 V. If you use a 3.3 V transceiver, connect the pull‑up resistors to 3.3 V.

The I2CSPD pin defines the I2C speed. Connect this pin to VSS for a speed of 100 kHz. For 400 kHz, it should be pulled to 1.65 V, which is half of 3.3 V. This can be achieved with a simple resistor voltage divider using 3.3 V and VSS. For 1 MHz, the pin must be connected to 3.3 V. This pin is not 5 V tolerant.

Block E: I2C-Serial Interface

For proper operation of the I2C-serial interface, pull‑up resistors to 3.3 V or 5 V are necessary. Typical resistor values are 4.7 kΩ for Standard Mode (100 kHz) and 2 kΩ for both Fast Mode (400 kHz) and Fast Mode Plus (1 MHz).

Block F: Application

Here is the rest of your product design. Typically, a microcontroller interfaces with the IS3720, but a microprocessor or a single-board computer, such as a Raspberry Pi, can also be used as long as they are equipped with an I2C-serial interface.

Your task here is to read the register DMX_START_ADDRESS, then read the corresponding DMX_CHx channels data registers and map their values to your LEDs.