Acorn floppy interface question

Hi all, This is going to seem like a weird question...... Does anyone know why on almost all of the Acorn machines with disk interfaces (including at least one non-Acord designed one), they gate the drive select signals with the motor on signal. e.g. drive select is inactive unless the moror is also on ? This seems to be a peculiarity of Acorn as none of the other machines I am familliar with do this (Dragon, CoCo etc), even when they use the WDxa7xx type controlers. Though I guess cirtainly with the Atom / BBC A/B it may be a requirement of the 8271, it seems odd that it was carried over to the machines that only ever had 177x controlers. Cheers. Phill. -- Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric ! "You can twist perceptions, but reality won't budge" -- Rush.