Atari ST(E) and floppy drives

[ppera]

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:

[ppera]

Among ST(E) machines only Mega STE has built in circuit for high density (1.44MB) floppies, in further text - HD floppies.
However, many Mega STE machines require additional changes: for instance mine had DD floppy drive built in. So, I needed to replace it with some HD capable. After it changing pos. of DIP Switch #7, and HD floppies were usable with. By some other versions different changes may be required.

Modding ST and Mega ST machines: for work with HD floppies FDC chip (1772) must be clocked with 16MHz instead 8 . It can be achieved in many ways. On attached pic. is one of the simplest and best. It is design of Djordje Vukovic. Has good side that FDC is clocked with 16MHz only when HD floppy is accessed, so less overheat will happen. There stays that it is for 1 drive. Right. But if we connect to point marked by MO not motor on, but drive sel A signal, it will be good with 2 floppies too. To point HD we need to connect HD sense signal from floppy drive. It is often on floppy connector pin 2, but not always, so some search on drive may be required. In worst case, some manual switch will do job.

Experiences say that about 50% FDC chips in diverse ST machines is capable for stable work on 16MHz. So, if you are unlucky will need to replace it. Best solution would be Ajax chip, but price is high. Some series and brands are known as better - for instance marked with 02-02. Or VLSI brand.

Modding STE: principle is same, but we can't take 16MHz from shifter. Usual solutions are usage of 16MHz oscillator on adapter. Cheaper way is to use frequency doubler: it can be made with 74HCT86 (XOR gates) + 1 gate - we need 4 gates in row for correct delay by HCT chips. But it may vary by brand.

Steprate problems: clocking FDC with 16MHz results in halved steprate times. So, regular 3 mS will became 1.5 mS, what drives can not follow. Solution is to set steprate to 6 mS - there is many simple proggy for that. TOS 2.06 sets all steprates to 6 - it is primitive solution, what works, but we have awful noise by using DD floppies.

[techie_alison]

Excellent!!

[jd]

yeh, nice one

[VincentGR]

ALPS floppy works great.