I. General Principles

(I) Purpose

To standardize the protection design of RS485 communication interfaces and power systems, improve reliability in complex environments, and prevent equipment damage and communication failures caused by factors such as electrostatic discharge and surges. Applicable to the design, selection, commissioning, and acceptance of related systems.

(II) Core Principles

Follow IEC 61000, TIA/EIA-485-A, and other standards.

Protection is graded according to "front-end energy dissipation, mid-end clamping, and back-end current limiting."

Protection does not affect communication and power performance.

Use mature industrial-grade components.

RS485 and power protection are designed in a coordinated manner, with unified grounding.

II. Core Protection Design

(I) RS485 Dedicated Protection

💡 Supplementary Notes: TVS diodes and ESD protection are mandatory; C35 and C34, and gas discharge tubes (GSM090D) can be reserved if space permits; on PCBs with limited space, smaller components can be used, but performance must meet requirements.

1. ESD Protection

Interface front-end TVS diode (6.5~10V bidirectional)
Transceiver A/B pins supplemented with secondary protection, parasitic capacitance ≤ 40pF
A/B and ground, and TVS diodes in parallel between A/B to form differential mode + common mode protection

2. Surge Protection

Three-level architecture:
Front-end uses GDT/MOV for energy dissipation
Middle-end TVS diode clamping (power ≥ 1.5W)
Rear-end uses 10~22Ω current-limiting resistor/PPTC for current limiting

(II) Power Supply Protection

Supplementary Notes: Varistors can be omitted if size is limited; component selection needs to be determined based on the actual circuit voltage!

Power supply protection process:

First, it passes through a PTC fuse

Then through a varistor and TVS diode

Then through a diode, and finally into the internal power supply

1. Surge protection

Three-level architecture:
Front-end GDT/MOV (current ≥ 20kA) discharges energy
Middle-end TVS diode (power ≥ 5W) clamps
Rear-end series current-limiting resistor + common-mode inductor for filtering and current limiting

2. Overvoltage/Overcurrent/Reverse connection

Varistor, Schottky diode
Select a wide-voltage input power module
Output terminal series matching PPTC/fuse
Preferably select a power module with protection functions

(III) General protection

1. EMI protection

RS485: Use shielded twisted pair cable (coverage ≥ 90%)
Power line: Use shielded cable, both ends grounded (grounding resistance ≤ 4Ω)
Equipment uses metal casing, shielded interface
RS485 side parallel high-frequency capacitor + common-mode inductor
Power supply side EMI filter

2. Grounding Design

Signal ground, power ground, shield ground, and enclosure ground are laid out separately. They ultimately converge to a main grounding terminal connected to the earth. Grounding paths are short and thick. Multiple devices connected to the same grounding network are used.

3. Wiring Layout

RS485 uses a daisy-chain topology with 120Ω terminating resistors at both ends. Protective devices are placed close to the interface. The circuit board is divided into signal and power areas. The grounding copper foil is wide.

III. Component Selection

All components are industrial-grade (-40℃~85℃) or wide-temperature range (-40℃~125℃). Parameters are matched to the system, with priority given to models integrating protection functions. RS485 and power protection device levels are matched.

Summary: The protection design of RS485 communication interfaces and power systems must adhere to the core principles of graded protection, collaborative design, and unified grounding. The appropriate protection level must be selected based on the application environment. From component selection to wiring layout, each step must be carefully controlled to ensure long-term stable operation of the equipment in complex electromagnetic environments.

Key Points Recap:

📊 Three-Level Protection: Front-end energy dissipation → Mid-end clamping → Back-end current limiting

📊 Protection Class Matching: RS485 and power supply protection levels must be consistent.

📊 Industrial Grade Devices: -40℃ minimum, wider temperature range preferred.

📊 Unified Grounding: Signal ground, power ground, and shield ground should be separate and then combined.

Following this specification can effectively improve the system's anti-interference capability and reliability, and reduce the field failure rate.