Calculadora de Terminación RS-485
Calcula las resistencias de terminación, polarización y longitud máxima de bus RS-485/RS-422.
Fórmula
R_term = 120 Ω; R_bias = (Vcc − 0.2) / 0.054
Cómo Funciona
RS-485 is a robust serial communication standard designed for reliable data transmission over long distances in industrial and networking environments. The standard requires proper termination to minimize signal reflections and maintain signal integrity. Termination involves placing resistors at the end of a transmission line that match the characteristic impedance of the cable, typically 120 ohms for twisted pair cables. Without proper termination, electrical signals can bounce back and forth along the cable, causing signal distortions, timing errors, and potential data corruption. This phenomenon, known as signal reflection, occurs when an electrical signal encounters an impedance mismatch at the end of a transmission line. Termination resistors absorb these reflections, ensuring clean and accurate signal transmission across the network.
Ejemplo Resuelto
Consider a 500-meter RS-485 network using CAT5 cable with a characteristic impedance of 120 ohms. To calculate termination: 1. Measure cable length: 500 meters 2. Determine cable impedance: 120 ohms 3. Place 120-ohm resistors at both network ends 4. Use ¼ or ½ watt resistors to handle network power 5. Connect terminators between DATA+ and DATA- lines Result: Properly terminated network with minimal signal reflection and reliable communication.
Consejos Prácticos
- ✓Always use matched termination resistors at network endpoints
- ✓Choose resistors with 1% tolerance for best performance
- ✓Consider power rating when selecting termination resistors
- ✓Use parallel termination for optimal signal integrity
Errores Comunes
- ✗Omitting termination resistors completely
- ✗Using mismatched resistor values
- ✗Placing termination resistors in the middle of the network
- ✗Ignoring cable length and impedance characteristics
Preguntas Frecuentes
What is the standard termination resistance for RS-485?
120 ohms is the standard termination resistance for most RS-485 networks using twisted pair cables.
How many termination points are needed in an RS-485 network?
Typically, only two termination points are required - at the physical ends of the network.
Can I use different resistance values for termination?
While 120 ohms is standard, other values can be used if they match the specific cable's characteristic impedance, but consistency is crucial.
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