Terminal Resistor is a resistor used in electrical bus systems (such as CAN bus, RS-485, Profibus, etc.). Its main purpose is:
Function:
1. Prevent signal reflection:
In a bus system with high-speed or long-distance transmission, if the signal is "broken" at the end or not properly terminated, signal reflection will occur, resulting in communication errors. The terminal resistor absorbs signal energy and prevents reflection.
2. Impedance Matching:
The bus has a certain characteristic impedance (for example, the CAN bus is about 120Ω), and the terminal resistor is usually set to the same value to ensure that the impedance of the entire line is consistent and improve communication stability.
Common application scenarios:
| Communication protocol | Common terminal resistance values | Application scenarios |
| CAN bus | 120Ω | Automobile, industrial automation |
| RS-485 | 120Ω | Industrial control, access control system |
| Profibus | 220Ω + 390Ω (complex structure) | Industrial fieldbus |
| Ethernet (early) | 50Ω | Coaxial cable (rarely seen) |
Physical form:
- Resistor itself (soldered on PCB)
- Pluggable terminal resistor (convenient switch, such as a dip switch on the terminal block)
- Network terminator: There will be an independent small module on the RS-485 interface
Common terminal resistor connector types and applications
| Interface/bus type | Termination resistance value | Connector type | Termination resistance form | Application scenario |
| CAN bus | 120Ω | DB9 / M12 / Molex / Phoenix terminal block | Integrated in the connector or external module | Automotive electronics, industrial equipment, robots |
| RS-485 | 120Ω | RJ45 / DB9 / terminal block / Phoenix | Pluggable or jumper-set terminal module | Industrial controller, access control, building system |
| Profibus-DP | 220Ω + 390Ω*2 | DB9 (with terminal resistance switch) | Integrated in DB9 plug, or external switch control | PLC, industrial communication |
| Ethernet coaxial (early) | 50Ω | BNC connector | BNC Terminator (connector with resistor) | Legacy networks, broadcasting systems |
| Modbus RTU | 120Ω | RS-485 connector (terminal block / DB9 / RJ45) | External terminal resistor module or onboard jumper | Electricity meter, water meter, industrial field control |
| LVDS/high-speed digital communication | 100Ω or 120Ω | High-speed connectors such as board-to-board / FPC / SMA | Onboard or cable end package | Industrial cameras, FPGA boards, LCD interfaces |
