8" disk drive project

I want to use a QUME 842 or SA800 in an altair disk subsystem. Both seem to have the same interface. The FD400-5x0-5x1 manual seems to show the drives being quite similar. The first thing I see is that there are extra signals. "Trim Erase", "Write Enable", and "Write Data" where the Shugart interface only has "Write Data" and "Write Gate". I assume that gate and enable are the same thing. The FD Pertec drives also have "IN", "OUT", and "STEP" instead of "STEP" and "DIRECTION", but that is not a big deal. That can be generated in software. What I don't see is any input to perform the function of head current. How does the shugart interface handle this? Can I ignore the trim erase feature? How do I generate a write busy signal? I'm willing to make some circuits to fake these signals if required. Thanks, Grant