Can someone give me a fairly simple description of the reason behind signal bounce as it relates to a bus network? I know what it is, and how it affects the network, but I don’t have an understanding of the physics behind why a signal will travel unchecked across the network without a terminator at each end.
Thanks.
As a wave propagates down a transmission line, the voltage and current of the wave are related by a characteristic of the cable - the voltage divided by the current will be a constant, and this value is called the characteristic impedance of the cable. So this wave of voltage and current is heading down the cable. The wave doesn’t yet know what impedance is on the other end, because it hasn’t reached that point yet. Then it gets to the end, and it’s left open without a terminator. The current doesn’t have anywhere to go when it gets there, and the buildup of charge at the end sends a wave travelling in the opposite direction down the cable, the same size in voltage and current as the one going the other direction, but now this reflected wave is added to the voltage/current that was already on the line. Then when that wave gets to the other end, it will reflect again if a terminator is not on that end.
If a terminator had been there, the termination would have absorbed the current with no reflection. I hope this helps, it’s kind of hard to explain electromagnetism and transmission lines, assuming very little electrical kowledge.
That definitely does help. I do have some electrical knowledge, so at least so far, what you said makes sense. The only thing I didn’t get was why the built-up charge is added to anything. Isn’t the charge just the original signal?
Same thing happens with RF, except that we refer to it as a Standing Wave Ratio (SWR), and work hard to keep the SWR below 1:2.
Vlad/Igor
KF4VCC
As the wave travels down the line from sending to receiving end it is slightly reduced in strength along the line. When it gets to an open-circuit end it is reflected. The open circuit constitutes an exceedingly high impedance and little current can flow. The result is that there is a relected current wave with a polarity opposite the incident wave. The reflected current subtracts from the incident current leaving almost zero current to go into the open end. That minute, residual current results in a little radiation from the end but most of the current is reflected back toward the sending end with the result that Curt C outlined.