CN0031: Half-Duplex, Isolated RS-485 Interface

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Circuit Note PDF, 09/2010 (pdf, 88 kB)
Benefits & Features
  • Fully isolated signal and power traces
  • RS-485 half duplex tranciever circuit
  • Low power with reduced board space
Products Used
  • Programmable Logic Controllers/ Distributed Control Systems
  • Field Instruments/ Smart Transmitters
  • Communications


This circuit provides a half-duplex, isolated RS-485 interface using the ADM2485 high speed, isolated RS-485 transceiver and the ADP3330 high accuracy linear regulator. This circuit achieves signal and power isolation and at the same time decreases board space and power consumption.

Figure 1: Half-Duplex Isolated RS-485 Interface Using ADM2485 and ADP3330 (Simplified Schematic)


The ADM2485 is a half-duplex RS-485 transceiver that can easily achieve a fully isolated RS-485 PROFIBUS compliant node. The ADM2485 integrates a transformer driver that, when used with an external transformer and LDO, generates an isolated 5 V power supply to be supplied between VDD2 and GND2.

The oscillator outputs D1 and D2 of the ADM2485 drive the center tapped transformer T1. A pair of Schottky diodes and a smoothing capacitor are used to create a rectified signal from the secondary winding. The ADP3330 linear voltage regulator provides a regulated 5 V power supply to the ADM2485 bus-side circuitry (VDD2) as shown in Figure 1.


When the ADM2485 is powered by 3.3 V on the logic side, a 1CT:2.2CT transformer T1 is required to step up the 3.3 V to 6 V, thereby ensuring enough headroom for the ADP3330 LDO to output a regulated 5 V output.

If the ADM2485 is powered by 5 V on the logic side, a 1CT:1.5CT transformer T1 is required to ensure enough headroom for the ADP3330 LDO to output a regulated 5 V output.

The external transformer used with the ADM2485 must have a center-tapped winding. The turns ratio of the transformer must be set to provide the minimum required output voltage at the maximum anticipated load current with the minimum input voltage. Table 1 shows suitable transformers for the ADM2485 circuit of Figure 1.

Manufacturer Primary Voltage 3.3V Primary Voltage 5V
Coilcraft DA2304-AL DA2303-AL
C&D Technologies 782482/35C 782485/55C

Table 1: Transformer Suppliers


Product Description Available Product Models to Sample
ADM2485 2.5 kV Signal Isolated, High Speed (16 Mbps), Half Duplex RS-485 Transceiver w/ Transformer Driver ADM2485BRWZ
ADP3330 High Accuracy Ultralow I(Q), 200 mA, SOT-23, anyCAP® Low Dropout Regulator ADP3330ARTZ-2.5-R7 ADP3330ARTZ-2.85R7 ADP3330ARTZ-2.75R7 ADP3330ARTZ3.3-RL7 ADP3330ARTZ-3-RL7 ADP3330ARTZ-5-RL7 ADP3330ARTZ-2.75RL ADP3330ARTZ-3.6-R7
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