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How is the value of a termination resistor determined?
The basic rule when choosing a termination resistor is that the value of the resistor must equal the characteristic impedance of the twisted pair cable. Termination resistor prevents signal rebound, like waves hitting a wall.
What happens when you add a 120 ohm resistor?
By adding a 120-ohm resistor at the end of an RS485 transmission line, the signal will be dampened by the resistor instead of reflected into the bus. If the value of the resistor does not match the characteristic impedance of the cable, reflection may occur.
Do you need a 120 ohm resistor for a short cable?
For a very short cable, a single 120 ohm resistor would normally work. But may not be needed depending on propagation delay compared to your transfer rate. For short cables, you can often get by without any termination unless the baudrate is very high.
Where are the termination resistors on a RS485 cable?
In most RS485 configurations, two termination resistors are used. One of each is placed at the extreme end of the twisted pair cable. However, if the system is configured in a way where only one device is transmitting and the transmitter is located at one end of the cable, only a single termination resistor is needed on the far end of the cable.
Why do you need a termination resistor on a PCB?
When you send a pulse along a twisted cable that is left unterminated, it will bounce back and distort the subsequent pulse. This is signal reflection can be prevented by adding a termination resistor at the end of the line.
Where are termination resistors located on a twisted pair cable?
One of each is placed at the extreme end of the twisted pair cable. However, if the system is configured in a way where only one device is transmitting and the transmitter is located at one end of the cable, only a single termination resistor is needed on the far end of the cable.