Suppose we have a parallel termination like this on a high-speed trace:
If the line impedance \$ Z_o \$ and \$R \$ are matched, then there are no reflections from load to cause any ringing at the driver output or at the receiver input.
But if the source impedance \$R_s\$ of the driver doesn't match with the line impedance \$ Z_o \$, shouldn't there be reflections still at the driver output from the line? So isn't that impedance matching necessary as well?
I am asking this because, in serial termination scheme, we match the source impedance to the line impedance. But in parallel termination scheme, I just can't see this concept. So, does that mean we have to mix both the schemes to ensure signal integrity on a high-speed trace?