@KoFi, post #1
IBM then jumpered all drives to use /DS2 on pin 12. And being standard Shugart drives (or equivalent) they expected their motor enable on pin 16 just as Shugart intended. So you have drives that, regardless of their actual role, expect one signal on pin 12 and one on pin 16, and don't pay any attention to what is going on at pins 10 or 14.
@snifferman, post #2
@KoFi, post #1