Since questions about replacing, upgrading floppy drives or FDC on Atari machines are among most frequent in this forum, I think that we need some general guide about.
I will start with simplest and probably most 'popular' task: replacing drive with some other (usually called PC floppy drive) .
Generally, Atari ST and PC floppy drives are almost same, compatible. Main difference is that those for PC are set as logical unit 1 (B), and for ST as logical unit 0 (A). So, you need to set PC drive as 0 when attaching to ST. Here comes first trouble: many new drive has no jumper for that (no jumpers at all). Then we have couple ways:
1: swap lines 10 and 12 on floppy cable
2: Shortcut pins 10 and 12 on floppy drive connector solder side. Good only when has 1 drive total.
3: Shortcut on small board on external ST floppy case - or make swap there
With this simple mod, in 99% cases drive will work with ST.
But we may expect another trouble, especially with newer drives:
ST will not detect floppy change, what may cause data corruption on floppies. This is serious problem, but solution is not so simple. Atari used some non-standard way by disk change detection, to spare some lines - to override problem with newer drives we must ensure that 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. So, some additional logic is reuired. There is no general solution, because many depends from used drive. Here is a solution for drive Chinon FG-357 :http://ppera.07x.net/atari/flomodam.html
There are some lists about ST compatible floppy drives (with correct disk change detection), and maybe someone can add it here. Epson SMD 380 and 340 are compatiible, and you may find them in many later Atari machine. 340 is HD, while 380 is only DD capable.
Later I will describe how to solve work with HD (high density, 1440KB) floppies in any ST.
Here are some pin layouts, taken from Atari ST Profibuch:
You do not have the required permissions to view the files attached to this post.