Termination Resistors: Their Function and Necessity on PCBs

A termination resistor across a white background

 

I once dabbled with creating apps for iOS devices. It was not a wise move for someone who is deeply rooted in electronics. Needless to say, I gave up barely after 6 months, returning to what I did best. It was months later that I got a rude shock when I was credited with an annual fee as I forgot to terminate the iOS developer membership. 

In electronics, you don’t get charged for not terminating anything, except when you’re missing out termination resistors in a differential pair transmission line. The consequence is distortion in the transmission line as the signal bounces back as it reaches the end of the line. 

What Is A Termination Resistor 

By definition, a termination resistor is a single resistor placed at the end of an electrical transmission line. In electronics, you’ll encounter termination resistors when you’re working with differential pair signals, such as the RS 485. It is a simple component that ensures signal integrity on the bus, especially when high-speed transmission is involved. Furthermore, termination resistors are used to avoid signal reflections. 

How Termination Resistor Works 

In all simplicity, a termination resistor placed at the end of the transmission line prevents the signal from bouncing back. If you need a clearer picture of this phenomenon, you need to understand the character of a twisted pair transmission model.

Any twisted pair cables are defined by its inductance and capacitance along the line. You can visualize it as multiple capacitors and inductors connected in parallel and series respectively. 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. 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.

 

 A wave in the sunset, reflecting light like signal reflection

Termination resistor prevents signal rebound, like waves hitting a wall.

 

The characteristic impedance of twisted pair cables is not a magic number. Instead, it is derived from physical characteristics of the cable itself. In RS485, the twisted pair cable used is defined to have a characteristic impedance of 120 Ohm. 

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.

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. 

Do You Need Termination Resistors On The PCB 

At this point, you may wonder if there are instances where the transmission line would work fine without any terminating resistors. Theoretically, a differential pair signal will be distorted, if the reflection signal loses significant amplitude before the next bit is sampled by the receiver.

In other words, you’ll probably be fine when transmitting at a very low speed. But that defeats the purpose of differential pair cable, which is supposed to provide reliable communications over long distance without sacrificing speed. 

The bottom line is, make it a practice to install termination resistors on differential pair transmission lines. The cost of the resistors and installation is insignificant compared to the potential problems caused by signal rebound.

 

Mechanical engineer working or technician on wiring

It’s best to leave termination resistor installation to technicians.

 

As for whether you need to place the termination on the PCB, I would say that you don’t. You will not need a termination resistor if the particular hardware is not the first or the last on the transmission line. 

Besides, system installers are usually trained to install termination resistors, and you wouldn’t want two similar resistors in parallel at one end of the line. It halves the value and creates a potential signal rebound.

With Cadence's toolset for PCB layout, analysis, and production you’ll be more than equipped to handle any board need, especially placing termination resistors. OrCAD PCB Designer has the capacity to accomplish any design task you need with both speed and intelligence based on smart DRC mechanics and routing tools. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

About the Author

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