Regarding HCSL I wonder why there is source termination at all? If in case of source termination, at receiver side there is no termination or a high impedance input, then the input reflects nearly all the power/voltage. That can hardly be the goal!? I would assume, that end termination would be the better chioce. What am I missing here?


Source termination makes sense to me if sender and receiver are so close to each other, that the line is not yet a transmission line. Then one would like to terminate at the source in order to keep the RC timeconstant low (R: sender impedance, C: line + receiver impedance). That is only a guess. I hope there is anyone out here to clarify.

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1 Answer 1


You are correct that, in the case of source termination, the power is reflected back from the receiver to the source. This is the goal, assuming the source is property terminated at 50 ohms.

Here is some more detail: (1) End Terminated Case: A 800mV rising edge is sent down the transmission line. Because the receiver is terminated with 50 ohms, the receiver sees a 800mV rising edge. No energy is reflected back to the source, at least ideally. Other signaling formats which are end terminated include LVDS.

(2) Source Terminated Case: A 400mV rising edge is sent down the transmission line. Because the receiver is unterminated (open), all the power is reflected back towards the source, and the receiver sees a 400mV + 400mV = 800mV rising edge. The power reflected back towards the source is terminated by the 50 ohm source resistance. If source were not properly terminated, then you'd get additional reflections between source and load, which is never desirable. Other signaling formats which are source terminated include LVCMOS.

Both termination styles are valid and (ideally) result in the same signal seen at the receiver. Both termination styles work with long transmission lines.


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