Signal Integrity and series termination

A couple of things...

1) Don't use 20 ohm resistor packs. Why? Because this is usually, less a data integrety issue, than it is an EMC issue. You're going to be in the EMC lab, trying to get this thing to pass compliance, and you'll want the flexibility to tweak each resistor's value. You might be able to put a 65 ohm in there. Not "theoretically correct", but if you're scope tells you it's not rounding off your signal too much to cause signal integrity issues, but is rounding enough so you pass compliance, than that's what you'll end up with. You won't know though, till build it and look at it on the scope.

2) Think about the use case. What's going to happen? Is it a data logger, that writes to RAM constantly, but rarely reads from it? Or is it program memory, where you're reading from RAM constantly, but rarely writing to it. Position your resistors based on who drives the bus most often. If it really is 50/50, then just pick one. It doesn't really matter that much.

This is because series termination is not used for bi-directional termination. You can, of course, terminate both ends but this is not optimal. It is best if using passive termination to terminate both ends using ac parallel termination or at least dc parallel termination.