Designing a High Anti-Interference Isolated RS485 Interface for Allwinner T153

1、Overview

In industrial control, smart building, power monitoring and other scenarios, the RS485 bus is widely used for its advantages of simple wiring, long transmission distance, and multi-node support. However, the electromagnetic environment on industrial sites is complex, with numerous interference sources such as motor start-stop, relay operation, and lightning surges. Ordinary non-isolated RS485 interfaces are highly prone to communication abnormalities or even chip damage caused by excessive common-mode voltage and ground potential differences. Based on the Allwinner T153 SoC, this article details how to design a highly anti-interference isolated RS485 interface across three dimensions: power isolation, signal isolation, and interface protection.

2、Power Isolation Design

2.1 Isolated Power Supply Selection

Power isolation is the energy foundation of the entire isolation system. This design uses the B0505S-1WR3 fixed-voltage input isolated power module, with 5V input, 5V output, and 1W power rating, belonging to the fixed-voltage input DC-DC isolation module category. The module features high integration and achieves 1500Vdc electrical isolation without peripheral components, providing an independent VDD_485 power supply for the RS485 side and fundamentally cutting off the DC path between the two grounds.

The selection of the 1W power level is well-considered: the typical operating current of the SIT3088EESA transceiver is approximately 300μA, and combined with the power consumption of the digital isolator and peripheral circuits, the total power consumption is far below 1W, leaving sufficient margin. Meanwhile, a 270Ω dummy load resistor R320 is connected in parallel at the module output to ensure stable operation under light-load conditions and prevent output voltage drift.

Figure 1: Power and Signal Isolation Schematic

2.2 Input and Output Filtering Design

Both the input and output terminals of the isolated power module require a complete filtering network. On the input side, VCC_5V0 first passes through an LC filter circuit composed of a 6.8μH inductor L320 and a 4.7μF capacitor C320 to suppress high-frequency noise on the power line. A 4.7μF/16V ceramic capacitor C321 is then connected in parallel near the module input pins to provide a local decoupling path.

On the output side, two 10μF capacitors C322 and C323 are connected in parallel to VDD_485, with the combination of large and small capacitance values covering ripple suppression across different frequency bands. A well-designed filter not only ensures power quality but also inhibits coupling of the module's own switching noise into the system side through the power line.

2.3 Ground-to-Ground Safety Capacitor Design

A 1nF/2KV safety capacitor C327 is connected across GND and 485_GND. The function of this capacitor is to provide a low-impedance discharge path for high-frequency common-mode interference while maintaining isolation characteristics at DC and power frequency. The 2KV voltage rating ensures the capacitor will not break down under surge impact, while the 1nF capacitance value discharges high-frequency noise without affecting isolation withstand voltage due to excessive leakage current. This capacitor is a critical component for EMC capability and must not be omitted.

3、Signal Isolation Design

3.1 Digital Isolator Selection

Signal isolation uses the BL7121AH dual-channel digital isolator to achieve electrical isolation of UART signals. The BL7121AH is a high-capability digital isolation chip supporting data rates up to 150Mbps, fully meeting the requirements of commonly used RS485 baud rates such as 115200bps and even higher.

The design also reserves multiple compatible alternative models, including CA-IS3722HS, ADUM121N1BRZ, NSi8121N1, and CBMuD1201HASS, covering different speed grades and supply chain options. Among them, ADUM121N1BRZ supports a wide voltage range of 1.8V~5.5V, NSi8121N1 supports 2.5V~5.5V, and CBMuD1201HASS is an economical choice with a 50Mbps speed grade.

3.2 Channel Configuration

The UART8 interface of the Allwinner T153 provides three signals: TX, RX, and RTS, therefore two dual-channel isolators are needed: U322 handles the RTS direction control signal, and U323 handles the TX and RX data signals. The RTS signal is used to control the transmit/receive direction of the RS485 transceiver and is key to half-duplex communication.

The VDD1 side (system side) of U322 is connected to VCC_3V3, and the VDD2 side (bus side) is connected to VDD_485, with completely independent power supplies on both sides. OUTA corresponds to the RTS output channel, bridging the system side to the bus side and controlling the enable of the RS485 transceiver. U323 is similarly configured with VDD1 connected to 3V3 and VDD2 connected to VDD_485, with OUTA for the TX direction (system to bus) and INB for the RX direction (bus to system), achieving isolated transmission of full-duplex signals.

3.3 Peripheral Circuit Design

Each isolator is equipped with 0.1μF decoupling capacitors (C331, C330, C333, C332) on both side power pins, which must be placed as close to the chip power pins as possible to provide the shortest high-frequency current path.

100Ω resistors (R329, R330, R331) are connected in series at the signal input terminals, serving current limiting and impedance matching functions while suppressing high-frequency ringing on signal lines. 

An additional 4.7K pull-up resistor R345 is added on the RTS signal side to ensure RTS remains high during power-up or reset,