Using a Qume 242 Floppy Drive with a ZFDC Board

["G. Beat" <gregor...@gmail.com>]

Excerpt from related article.
http://nemesis.lonestar.org/computers/tandy/hardware/model16_6000/floppyfix.html

For both the 5.25" and the 3.5" drives, you must be able to disable a behavior that is the default for IBM-compatible computers but is alien to these older non-IBM-compatible computers. In particular, IBM-compatible drives use the READY line from the floppy drive to not only signal whether the drive has media and is spinning at the correct speed, but the drive also uses the READY signal to indicate if the diskette in the drive has been changed since the last drive operation was performed.

The 8" floppy drive 50-pin cable actually has a separate signal to report disk changes (called DISK CHANGE), but the mini-diskette cabling system that dates back to the mid-1970s did not include this signal. The drives used in the Tandy Model I, III, 4/4D/4P and color computer systems were all unable to report that diskettes had been changed, so the operating systems had to rely on fairly primitive and unreliable methods to detect when a diskette had been changed, and mainly had to rely on the human to not change diskettes unless the operating system or the application said it was time to do so.

When designing the IBM Personal Computer, IBM apparently wanted to utilize the mini-diskette drives already made by several vendors (including Tandon, eventual maker of the drives used in the original IBM PC and already a supplier and licensor of 5.25" drive designs to Tandy) and the established mini-disk cabling system, but IBM also wanted the operating system to be able to find out about media changes.

To accomplish this goal, drives made for the IBM PC (and subsequent models "compatible" with the IBM PC) will falsely return a NOT READY signal even if media is in the drive, in the event that the diskette has been removed and replaced recently. The operating system clears this false "NOT READY" indication by forcing the drive to reposition the heads by at least one track while ignoring the NOT READY status. If the drive was simply trying to report a media change, moving the heads causes the drive to now appear to be READY. If the drive really isn't ready, the NOT READY indication will persist even after the heads are moved.

This is why empty floppy drives in IBM PC (and compatible) computers make so much noise when there is no diskette in the drive if you tell the system to examine that drive. This noise is caused by the BIOS or DOS-compatible operating system disk driver ordering the heads to be moved back and forth several times when a NOT READY error is detected, hoping to clear any disk-change condition that may exist before finally giving-up and returning an error to the software that called the diskette driver.