resistorsseries
Why is a series resistor used in high speed signal data lines? Is there any reason to include a resistor or simply protection from the receiver end?
For example, I'd like to understand the reason behind the low value series resistor placed in processor-to-SDRAM data and address lines.
While the answers by @Passerby and @MichaelKaras pretty much cover it, there's one more thing to add:
Humans perceive light intensity non-linearly: At very low intensities, we are very sensitive to a even slight variation in brightness. On the other hand, at higher intensity, the unassisted human eye is pretty much incapable of discerning differences in intensity.
This really interesting graph demonstrates this excellently:
(source)
Essentially the ability to perceive change in intensity is very high when most of the vision is attributable to the rods of the eye (scotopic vision), and drops very low when the cones are doing the sensing (photopic vision), i.e. at slightly higher luminance.
Less critical but good to know: LEDs illuminate somewhat non-linearly versus current, with the graph dropping off the linear as current increases. This is most noticeable with red.
(source)
So, long story short:
The human eye cannot notice even large intensity changes at the higher levels of light that an LED generates at higher currents. Using half or even less than half of nominal rated current (20 mA typical for indicator LEDs, 50mA or more in high power LEDs) will thus work perfectly fine for most indication purposes.
In my designs, 5 mA is my preferred current for all indicator LEDs: Try it, it works great!
You may want to read about the impedance and capacity of a wire/transmission line. I will try my best to translate to English as I learned most of this in German ;)
Every wire not only has a resistance, but also and impedance and capacity. These add up to the [electrical impedance] (https://en.wikipedia.org/wiki/Electrical_impedance). If you look at the effects of impedance and capacity, you will notice that these scale with frequency.
You can always use a serial matching end, it just depends on the configuration. If you have a wire with a impedance of 50 Ohm (typical HF wire) or 100 Ohm (like CAT5 Network cable), you need a matching end. This end is a 'network', wich will also get the frequency, so a normal resistor 50Ohm will work at 1MHz, but will have a mismatch (and a reflection!) at 1Ghz (which is why extra expensive frequency tolerant resistors exist). To counter this, you can measure the values of your resistor (C and L) and attach additional resistors/inductors/capacitys to counter the effect.
So your network on the end of the cable has to match your cables impendance at the given frequency. How you archive this matching is your choice. A series resistor is the common choice for 'household' electronics in the sub GHz area, above there are special solutions.
Best Answer
A series resistor between a high-speed driver and a trace is to match total impedance of driving source (impedance of driver + series resistor) with characteristic impedance of transmission line (PCB trace). If the source impedance is not matched to transmission line, multiple reflections would occur, such that the reflected waves would go back and forth, and disrupt signal levels at receiver end, vastly reducing signal quality of the channel and killing signal margins.
In modern (good) silicon designs, however, there is no need in impedance-matching resistors, because normal high-speed interfaces nowadays have controlled impedance, and board designers learn to design traces with perfect match. The resistors are used only in poorly-designed silicon chips where the driver impedance control and properly controlled termination is not implemented.