A discussion on another forum created an interesting point. The question was regarding the proper signal to use when a player "apparently" scores a TD, but there is a foul on the play. Is the TD signal appropriate? Or, just signal T/O? I believe most conventional thinking was/is to signal T/O, report the foul, and proceed with completion of the penalty. But there are/were some folks that would say to signal the TD, then report the foul and complete the penalty.
For that forum, and for NCAA prior to the play clock rule change for 2018, that was pretty much just an academic issue - it has/had no impact on the game. But, in NCAA for 2018 (and beyond!), it seems to me that it will be helpful to the PCO to definitely signal T/O, then proceed with the foul administration. Again, I am assuming the 40-second play clock for the Try is to start immediately upon the TD signal, although I am not at all certain of that. (None of my 'sources' still in FBS football seem to know that answer, either, yet, but I'll keep pestering them until they divulge the answer). On that assumption, though, when the PCO sees a TD signal, he will start the play clock, and it will run until he sees somebody signal T/O. But, somebody will need to signal T/O. In the absence of a T/O signal, the play clock should run. I would hope someone on the field would notice the play clock running and signal T/O. But, it is possible that might take some moderate amount of time, which could be confusing to the teams and the spectators.
So, the best solution is definitely to signal T/O in lieu of TD. Of course, if you don't have, and don't see, the flag, and you are covering the BC as he crosses the goal/catches the pass in the end zone, then you may not know about the foul. But, all the more reason to use good "timing" and check around before signaling the score. If you see somebody signaling T/O, then just hold up on the TD signal, and report to the R to make sure he knows there was an apparent score on the play. The official with the foul call needs to make a "big" T/O signal and whistle, to be sure everybody knows about the foul. Meanwhile the PCO has seen the T/O signal immediately upon the conclusion of the play, and is just holding, waiting for the results of the foul administration. Nice and smooth.
Robert