Some times I need to confirm a pattern is present on the output of the 328/1284 or shift register.
Using a circuit similar to this, you can enter a pattern on the DIP switch and monitor A=B output with your scope trigger i/p.
With a logic analyzer, it frees up 7 inputs for other things.
Gee I haven't seen a '688 in use for about 20 (make that 30) years. 
Good idea though if the inputs needed for triggering do not need to be traced as well.
Rob
20 (make that 30) years
I guess I am dating myself.
Me to