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Amiga 500 Internal Floppy Drive in Atari ST?

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raoulduke

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I have a modified floppy drive in my ST, because the original drive was dead. After months of storage the modded drive (inexplicably) doesn't work. I resoldered the mods to no effect. I have a dead Amiga 500 with a drive going to no use at the moment. Will that work without modification in the ST? I think it's probably just a DS0 drive with a Shugart connector; since I'm not even sure plugging in unmodded internal drives poses risk of harm to either drive or machine. But I've found no documentation either way.
 
An Amiga drive will probably not work (or even fit) but you can use a standard PC 3.5 inch floppy drive on the ST. There are some signal differences as the ST uses the write protect notch to detect disc changes.
On the web is more info to be found: http://www.atari-forum.com/viewtopic.php?t=14049
 or http://www.atari-wiki.com/index.php/Replace_internal_720k_DD_Floppy_Drive_with_1.44MB_HD_Floppy_Drive
The original ST drive might be repairable though, often there are just some small (SMD) electrolytics bad on the motordrive board (underside) which can be replaced and the drive will work again.
 
Thing is the mod for PC drives to Amiga (I said in my first post that the dead drive is a "modded drive" - I forgot to say modded PC drive) is basically the same (http://eab.abime.net/showthread.php?t=30944), no? Unless people think there's real risk of harm to the ST (and to a lesser extent drive) I'm inclined to try it.

*It's probably (still not in front of me) an Epson SMD-400. So it may be a jumpers issue - http://atariage.com/forums/topic/229673-falcon-floppy-replacement/ - or it may not work. But I don't think there's a risk either way. https://groups.google.com/d/msg/comp.sys.atari.st/kc5lmYrHRiw/TfWduwsv_gEJ

**My drive doesn't power on. (The non-working modded drive did power on.) So is it that the power cables are reversed? I think what these show http://4.bp.blogspot.com/-RXp3p7X22...mwAzG9SY/s1600/Amiga+RPI+Drive+Connectors.JPG is that the red cable is 12v and that it is supposed to be on the left (http://picclick.co.uk/Amiga-500-plus-Panasonic-Internal-Floppy-Disk-Drive-with-122227502627.html). Confusingly, in the ST blue is 12v but it's also on the left side as is standard with virtually all floppy drives. So either the drive itself doesn't work or there's a jumper issue.
 
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Most 3.5 inch floppy drives need a single supply (5 Volts!). If you connected it by accident to 12 Volts it has probably fried the drive electronics and there is a serious chance it damaged the floppy controller in the computer also.
Color coding in non-standard computers can be confusing. In a PC red means +5 Volts, yellow is +12 Volts, orange is 3.3 Volts etc. With older systems you never know and only by measuring you can be sure..
 
But in this case we can be sure. In n the ST, blue is 12 and red is 5

*And yet ironically that's totally reversed. I started to disassemble my external Amiga drive to see if that one would work and noticed that the standard configuration of the power cable is reversed - so 12v is supposed to go on the right side of of Amiga drives - at least those that came standard in the A500 and the external drive. It works fine now thankfully - and surprisingly.
 
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An Amiga drive might need some modification. I believe the Atari ST uses 'normal' 720k PC drives (except most PCs don't use the drive select feature, but use a twisted cable, in which both drives are configured as 'B:'/drive 1, and the cable makes one of them 'A:'/drive 0). The Amiga has some 'special needs', and as such it uses a 'subset' of PC drives. The Amiga also has some small modifications done to the interface, see here: http://eab.abime.net/showthread.php?t=30944
(Of course this shows how to modify a generic PC drive to use it in an Amiga. The drives used in the Amiga came ready-made from the factory, so they won't have patch cables like this running over the PCB).
You might need to 'undo' those to make them work in an Atari ST I suppose (pins 2 and 34 have special use on Amiga). Or perhaps the Atari just ignores those pins, and it works as-is. One way to find out :)
Amiga's internal drive is also configured as 'drive 0', so you wouldn't need to change that for the Atari ST.
 
Scali said:
The Amiga has some 'special needs', and as such it uses a 'subset' of PC drives.
Click to expand...
Actually, it's vice versa. The Amiga strictly adopts the Shugart interface, whereas the PC dropped parts of it, which were later partly re-added, but not how they were supposed to be. IBM ****ed it up, actually.
 
Timo W. said:
Actually, it's vice versa. The Amiga strictly adopts the Shugart interface, whereas the PC dropped parts of it, which were later partly re-added, but not how they were supposed to be. IBM ****ed it up, actually.
Click to expand...

I mean that only a 'subset' of the drives that can be used in PCs can also be used in Amigas (as you say, the PC doesn't implement the full Shugart interface, and as a result, many drives on the market don't either, because they are only made for PC. You need one of the drives that supports the extra features, and depending on how it is wired, it may need to be modded).
I didn't mean that the Amiga implements a subset of the floppy interface that the PC uses (which should be obvious from the modding page I linked. The Amiga actually uses features like detecting a disk. Anyone who ever used an Amiga would be familiar with that: to boot an Amiga, you just insert a disk, and the Amiga auto-detects that you've inserted the disk, and will start booting. PCs can't do that, and you need to put the disk in the drive before you boot up the PC, else it just tries to read anyway, and gives a disk error, at which time you need to manually retry. Sad, really).
 
Timo W. said:
Actually, it's vice versa. The Amiga strictly adopts the Shugart interface, whereas the PC dropped parts of it, which were later partly re-added, but not how they were supposed to be. IBM ****ed it up, actually.
Click to expand...

Wrong. Amiga drives handle drive select signal different than PC and ST. On Amiga DS signal goes low for a short moment, the drive stays selected until SD signal goes low again. On PC/ST it stays selected while DS is low.
 
gertk said:
Most 3.5 inch floppy drives need a single supply (5 Volts!). If you connected it by accident to 12 Volts it has probably fried the drive electronics and there is a serious chance it damaged the floppy controller in the computer also..
Click to expand...

Also wrong. Floppy drives have 4 pin connectors for power 5v, gnd, gnd, 12V. If a drive doesn't need 12V the 12V pin is just not connected on the drives electronics board. So supplying 12V to a 5V only drive does not grill the drive.

It's also not the case that most of drives only require 5V. It depends how old the drive is. Only very new drives only needs 5V.
 
Then maybe this one doesn't need 5V and I was just putting the 12V into nothing. I was virtually certain that Amiga drives and the ST board don't have this type of fault protection, which strongly implies one or the other or both should be dead.

1ST1 are you saying that certain features on the ST shouldn't work properly with the SMD-400 from the Amiga in it?
 
Scali said:
I mean that only a 'subset' of the drives that can be used in PCs can also be used in Amigas (as you say, the PC doesn't implement the full Shugart interface, and as a result, many drives on the market don't either, because they are only made for PC. You need one of the drives that supports the extra features, and depending on how it is wired, it may need to be modded).
I didn't mean that the Amiga implements a subset of the floppy interface that the PC uses (which should be obvious from the modding page I linked. The Amiga actually uses features like detecting a disk. Anyone who ever used an Amiga would be familiar with that: to boot an Amiga, you just insert a disk, and the Amiga auto-detects that you've inserted the disk, and will start booting. PCs can't do that, and you need to put the disk in the drive before you boot up the PC, else it just tries to read anyway, and gives a disk error, at which time you need to manually retry. Sad, really).
Click to expand...

Also PC and ST can autodetect if a disk has been inserted. But they do it a bit different than Amiga. When you let an amiga stay without anything doing, you can hear that it regulary selects the drive and makes a read test. When no floppy inserted it fails, that means there is no read signal, and amiga knows that there is no disk. This even works with unformatted disk, as also unformatted/foreign-formatted disk returns invalid bit stream instead of nothing. Atari ST also regulary selects the drive for disk change detection. It's not making a read test, but it observes the write protect switch in the drive. Every time you put a disk in or take it out, that switch is pushed by the disk, no difference if the disk is write protected or not. Also PC could use both methods to detect disk change, but it was not implemented in BIOS and DOS, but there is software which is using these tricks by their own.
 
gertk said:
but you can use a standard PC 3.5 inch floppy drive on the ST.
Click to expand...

Yes, any PC drive will work. Even HD or ED drives would work as long as you don't use HD/ED disks. That's because the HD/ED disk with it's additional hole will switch the drive in HD/ED mode. That means under others higher current to the write-head on write operations and higher sensibility to the read-head on reading (write/read heads are integrated in one thing). If you format HD disks in DD capacacities in such configurations they get unreadable for real DD drives. What you can do is sending the signal from the HD/ED hole sensor to an addtionals TTL switch on the ST mainboard and let it switch the WD1772 between 8 and 16 Mhz operation. Then ST can read/write HD disks. But you will require TOS 2.06 for clean operation, and not all 1772 support 16 Mhz, better only use 1772-02-02 or ATARI AJAX.

The only thing in general where you have to care about is the drive select signal. On ST they need to be set to DS0 while on PC they are DS1 for the first drive (based on that the PC uses twisted cable). 2nd floppy drive for ST must be set to DS1 like on PC, if you connect it to the ST internal floppy cable. On external connector it depends on your cable. If you put PC drive into ATARI SF314/SF354 it must set to SD0 as the adapter board in these drives do the twist.

You can also attach 5,25 drives to ST. Some drives need little tool in auto folder to slow down steprate as the drive is too slow to move the head. Or if it is 40 tracks drive only, you need other tool in auto folder to tell this to TOS. There are some rare 640/720 kb 5,25 drives which run without trouble, or you also can use 1.2 MB HD drives, they also can format disks in 720kB (9sectors, 80 tracks, double sided) or with a HD kit like described above even 1.2/1.44 MB (difference between 1,2 and 1,44 MB is 16 or 18 sectors). TOS is able to handle all these formats.

ATARI AJAX controller even can support ED drives if you clock it with 32 MHz, but no experience if TOS can handle 2.88 MB disk format. Never seen this.
 
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1ST1 said:
Also PC and ST can autodetect if a disk has been inserted. But they do it a bit different than Amiga. When you let an amiga stay without anything doing, you can hear that it regulary selects the drive and makes a read test. When no floppy inserted it fails, that means there is no read signal, and amiga knows that there is no disk. This even works with unformatted disk, as also unformatted/foreign-formatted disk returns invalid bit stream instead of nothing. Atari ST also regulary selects the drive for disk change detection. It's not making a read test, but it observes the write protect switch in the drive. Every time you put a disk in or take it out, that switch is pushed by the disk, no difference if the disk is write protected or not. Also PC could use both methods to detect disk change, but it was not implemented in BIOS and DOS, but there is software which is using these tricks by their own.
Click to expand...

If you open the lid on a floppy drive, you generally see two simple sensors at the front, one on the left (or two in the case of a HD drive), and one on the right. The left one detects if a disk is inserted, and the right one detects if the disk is write-protected. The Amiga can use the left sensor, a PC cannot (afaik the line is simply not connected on the floppy controller).
There's a 'disk inserted' latch. The 'click' is because the Amiga periodically performs a read to reset the latch (for compatibility I suppose).
See here: http://www.amigahistory.plus.com/noclick.html
 
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Scali said:
If you open the lid on a floppy drive, you generally see two simple sensors at the front, one on the left (or two in the case of a HD drive), and one on the right. The left one detects if a disk is inserted, and the right one detects if the disk is write-protected. The Amiga can use the left sensor, a PC cannot (afaik the line is simply not connected on the floppy controller). This means the Amiga can even detect a disk without the 'click' timer that is enabled by default (that may have been done for compatibility). There was 'no-click' software to enable that. Some games/demos also use this to detect a disk change.
Click to expand...

Wrong. DD drives have only one switch on the one side. HD/ED drives have two switches. One is for write protection, the other is for DD / HD-ED detection.
 
1ST1 said:
Wrong. DD drives have only one switch on the one side. HD/ED drives have two switches. One is for write protection, the other is for DD / HD-ED detection.
Click to expand...

I'm talking about the drives in my Amigas.
Anyway, here's the pinout for PCs and Shugart, also with some remarks on the Amiga: http://pinouts.ru/HD/InternalDisk_pinout.shtml
The 'DCD' line (Disk Change Detect) on pin 2 of the Shugart interface simply doesn't exist on the PC interface.
 
On PC and ST Pin 2 is used for DD/HD+ED detection. 3.5 inch drives send the signal to the disc controller, 5,25 drives get the signal from disc controller (as they don't have a HD switch).

I think Pin 2 for disc change was special to Amiga, not shugart standard. In those diagrams with shugart bus pinout I only can see variations where it was used for HD select/detection or unused. I have an old Toshiba 5,25 floppy 640/720 kb drive from Olivetti M24, where Pin2 can be jumpered to drive the "head load", that means the read/write head lowers to the disc only when this signal is active. It also can be set to lower the head by default, or with drive select or with motor on.
 
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1ST1 said:
I think Pin 2 for disc change was special to Amiga, not shugart standard.
Click to expand...

Could be. At any rate, that's a difference from regular PC/Atari ST drives.
At any rate... PC also does weird stuff with the drive-select.
And another thing I noticed... The way floppies are 'driven' is not always the same either. That is, I had problems with the drives in my Commodore PC20-III, since it apparently doesn't use a standard PC controller, but something more like an Amiga one.
The difference is in how the motor enable and drive select lines are driven. As you can see in the Shugart pinout, there's only one motor enable. So the controller will drive a generic motor enable line, and the combination with the drive select line will determine which drive's motor is enabled.
A PC has a specific motor enable line for both drives. So what happened when I connected two 'normal' PC drives, is that one drive's motor would never spin up.
I found that the stock drive in the PC20-III had a special jumper to select a 'motor on drive select' function or something like that. So it would just always turn on the motor on drive select.
 
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