Floppy drive mod for ensuring media change
detection on Atari ST
Only small fraction of floppy drives
works well with Atari ST machines media (disk) change detection system.
Reason is that Atari used non-standard solution, to spare some lines. Here I will describe concept, and
will give concrete solution for floppy drive Chinon FG-357. What we need for media change
detection? - pin WP (#28) on floppy connector must be active (low) when
no floppy in drive. This is all, but most of drives works not so, and
this line is active only when write-protected floppy is in drive. With simple logic, it can be
achieved on all floppy drives. Unfortunatelly, no general 'pin'
because user must find one specific point on drive - where floppy
presence is detected. Combining it with WP line will give desired
result. For logic we will use Quad
open-collector NAND circuit 74LS03.
On picture above is floppy drive Chinon FG-357, modded for Atari
There is line and logic for HD/DD switching too (long black wire out). For media change detection we
use 2 NAND gates: Black wire connected to one pin near to right bottom
on Low when floppy is in, and on High when no floppy in drive. It goes
to pin 9 of IC, and there is one resistor of 4K7 to GND. On other input
of gate (pin 10) goes inverted signal from drive A select line (pin 10
of floppy connector) . There is of course pull-up resistor ( 4K7 ) for
open-collector. Output of gate (pin 10) goes simple to WP line ( pin 28
of floppy connector ). Because it is low active, it will go to low
always when drive is selected and no floppy in it.
Only lower 2 gates on schematic are required for media change detection
circuit. Third gate is for HD/DD switch control.
On photo, white line connected to JP4 is for HD/DD switching - it goes
on pin 4
of IC (there is 10K to GND too), and AND-ed with inverted drive A
signal goes to Atari (black wire out)- there is no line for it in
standard Atari floppy cable, so you need to lead it somehow to Atari
and drive 8/16 MHz clock switch with it.
By different drive you need to find point on circuit where floppy
presence is detected. If it works inverted than above (so Low when disk
is out), then you need to invert it first - there is free gate for it