Isolated RS-485 Transceivers

A standard RS-485 transceiver sends and receives digital signals between digital equipment. They use positive and negative signals limited to 5 VDC levels. One can connect them as simple point-to-point configuration or as multi-point connections with two or more devices communicating. RS-485 transceivers allow high-speed communication in electrically noisy environments, as is usual within industrial plants.

Each of the two output lines on an RS-485 transceiver uses square waves to send serial data to another distant transceiver. A capacitive line offers high impedance to high-speed transmission, distorting the rise and fall of the signals. A capacitive line is one where the line carrying the signals is close to the signal ground.

Rather than use capacitive lines, RS-485 transceivers use a balanced line where the two output lines carry voltages of opposite polarity all the time. In balanced lines, the signal rise and fall times are much better, resulting in transmitting high-speed signals over longer distances.

Using +5 VDC and -5 VDC for each of the two output lines alternately an RS-485 transceiver can offer either non-inverting or inverting signals on its output lines. When the output is non-inverting, its polarity is the same as that at the input of the transceiver. For the inverting pin, the polarity is always opposite to that at the input.

In an industrial application, using isolated RS-485 transceivers is the normal practice, as the interconnecting cable often must pass through an environment with high voltages present. The isolation prevents any high-voltage spike inadvertently appearing on the interconnecting cable and passing on to the circuit driving the transceivers.

Analog Devices offers two types of isolate RS-485 transceivers. The ADM2867E is signal and power isolated up to 5.7 kV rms, while the ADM2561E has isolation levels up to 3 kV rms.

Both transceivers pass radiated emission testing, conforming to the requirements of EN55032 Class B standard. The tests use a double-layer PCB with two small 0402 size external ferrite beads on isolated ground and power pins.

Both devices feature integrated but isolated DC-DC converters generating low EMI. The isolation barrier offers immunity to system-level EMC standards. On the A, B, Y, and Z pins of the RS-485, a family of isolator devices offer ±15 kV air and ±12 kV contact ESD protection complying with the IEC6100-4-2 standard. Cable invert pins on the device allow users to reverse cable connections to quickly correct the connection while maintaining fail-safe performance on the receivers.

A double-layered PCB reduces the design time and material costs while providing Class B radiated Emissions. The cable invert feature reduces debug time during system install by allowing users to easily correct installation errors. With a greater than 8 mm creepage and clearance, and the IEC 6100-4-2 ESD and 5.7 kV digital isolation, the RS-485 transceivers from Analog Devices can maintain signal integrity even when the signals are passing through the harshest of environments.

Isolated RS-485 transceivers are useful for industrial automation, communication, building and infrastructure, and in aerospace and defense, mainly because of their cable invert feature, high isolation, low EMI/EMC capabilities, good surge protection, and improved ESD safeguards.