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How many floppy drives can a SWIM chip control? (just curious)

olePigeon

Well-known member
Obviously at least three, in the case of the SE and LC.

Would it be possible to interface with the SWIM chip via NuBus? Could you have a NuBus card that has 4 more connectors (2x external, 2x internal) plus whatever controller you'd need to interface with the SWIM, and then have 6 drives total on, say, a Macintosh II?

Or would it need its own SWIM chip?

Sorry. Just curious.
 

Spidey01

Well-known member
In the TechNote for my Duo, I see there's a chip select in the pin out but don't see any real detail about the SWIM II itself.


Going to hazard a wild guess of 2 depending on implementation details. Not that I'd expect to ever see an Apple II that crammed full of nothing but floppy controllers.
 

olePigeon

Well-known member
But the SE and LC can have three floppy drives. Well ... actually, I've never tried all three ports at the same time. I just assumed.

Apple II could daisychain, though. You could have 6 floppy drives on a single controller. It also technically had an IWM and not a SWIM, but I don't know what the difference is.
 

jeremywork

Well-known member
My recollection is rusty, but I had thought the Macintosh implementation of IWM/SWIM only allows for one drive per port, which is why the daisy chain port on external drives is often plugged with a plastic cover. Apple IIs IWM could use the daisy chain feature for up to two 5.25 and two 3.5" drives on one port, but not Macs. The SuperDrive card for Apple IIs included a SWIM which should address two 1.4M drives on a daisy chain.

The LC has no external port, so you can have two internal drives, but none external.

The SE/SE-FDHD and the Portable both have two internal drive ports in addition to their external port, and will allow three drives to function simultaneously if all populated.

I remember a mod to enable two internal drives on an SE/30 but this method utilized the enable pin for the external port, effectively just moving the external drive to the second internal bay.

One of my eBay finds from years ago included a very nifty DB19 port installed in a Nubus blank, with a cable running to an internal floppy connector. It's come in quite handy to be able to plug a known working external drive into a II series case or a later Quadra/Powermac when I haven't examined the internal drive yet, but this contraption still doesn't add *more* drives.
 
But the SE and LC can have three floppy drives. Well ... actually, I've never tried all three ports at the same time. I just assumed.

Apple II could daisychain, though. You could have 6 floppy drives on a single controller. It also technically had an IWM and not a SWIM, but I don't know what the difference is.
SE can control 3 (2 internal and 1 external) without an issue.
 

olePigeon

Well-known member
One of my eBay finds from years ago included a very nifty DB19 port installed in a Nubus blank, with a cable running to an internal floppy connector.
Applied Engineering made one of those so you could run one of the internals on the Macintosh II to the rear. They advertise it in their booklet.

I've been on the lookout for one of those for my NASA IIfx. The steel chassis covers up the second floppy slot, and I didn't want it to go to waste. The cable should be easy enough to make, but I've run into a problem with the bracket. However, we just got a giant CO2 laser cutter at work for the STEM lab. So I'm hoping it'll be strong enough to cut a NuBus bracket.

Although, I'm also wondering if I could make a connector mid-way on the cable so I could snake it out the unused lock slot.
 
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olePigeon

Well-known member
I wonder what would be involved in making a NuBus card that added 1x external and 2x internal ports. I love me some floppy drives.
 

MrFahrenheit

Well-known member
I seem to recall in school, they had a Mac Classic or SE setup, and they had the internal floppy drive and 2 external floppy drives. I could be wrong, but I seem to remember it that way. They even had the stick-on label of (1) and (2) on the two external drives.
 

olePigeon

Well-known member
There's no daisy chaining on the Macs. So if they had one, perhaps they rerouted the internal one like mentioned above. Or perhaps it was a Color Classic with a //e card.
 

Crutch

Well-known member
The LC has no external port, so you can have two internal drives, but none external.

The SE/SE-FDHD and the Portable both have two internal drive ports in addition to their external port, and will allow three drives to function simultaneously if all populated.

Indeed, Wikipedia (at a somewhat unusual level of detail) explicitly states that the SE and the Portable are the only Macs that can natively support three floppy drives, and I believe this is one instance where Wikipedia is correct!

It is indeed physically not possible to connect three floppy drives to an LC, because it lacks an external floppy port.
 

jeremywork

Well-known member
Applied Engineering made one of those so you could run one of the internals on the Macintosh II to the rear. They advertise it in their booklet.

I've been on the lookout for one of those for my NASA IIfx. The steel chassis covers up the second floppy slot, and I didn't want it to go to waste. The cable should be easy enough to make, but I've run into a problem with the bracket. However, we just got a giant CO2 laser cutter at work for the STEM lab. So I'm hoping it'll be strong enough to cut a NuBus bracket.

Although, I'm also wondering if I could make a connector mid-way on the cable so I could snake it out the unused lock slot.
Thanks for this tidbit, I had assumed it was a high quality homebrew job but it makes more sense to be one of the AE units. There's no identifying information on it though, so I may never have found out.

It seems they cut into a II/IIx/IIfx blank to fit the connector...
A mod to make it fit through the lock slot would be cool :)

IMG_9406.jpg
IMG_9408.jpg
IMG_9407.jpg
IMG_9409.jpg
 

sfiera

Well-known member
According to my understanding, the IWM has two enable pins (/ENBL1 and /ENBL2), which allow it to natively control two different floppies, so the simple answer is two. But beyond that, it’s possible for software and hardware on either side of it to cooperate in a way that enables additional drives. I’m not sure if the total count of supported drives is well-defined.

The IIgs supports six drives, but only two of each, and only in specific orderings. The newer, smarter drives (like 3.5” floppy drives) must be earlier in the chain than older drives (like 5.25” floppy drives). I think that the way this works, 3.5” drives understand and ignore 5.25” commands, and pass them downstream, or consume 3.5” commands and suppress them downstream. That way, a 3.5” and a 5.25” drive can share the /ENBL1 signal and both be “floppy 1”.

Apple II daisy-chaining involves putting both /ENBL1 and /ENBL2 on the DB-19 connector; drives connect the upstream /ENBL2 pin to the downstream /ENBL1 pin (and maybe the reverse?). Macs assign /ENBL1 to the internal connector and /ENBL2 to the external connector (Mac Plus schematic). However, there’s an entirely different way that Macs can daisy chain HD20 drives using the Phase3 pin.

I’m not sure how exactly the SE/LC could support three floppies. It’s possible they have an IWM variant with three enable pins. It’s also possible the enable pins come from somewhere else—I don’t know if floppies actually need their /ENBL signal to come from the IWM, or if it would be possible to enable them some other way.
 

jeremywork

Well-known member
According to my understanding, the IWM has two enable pins (/ENBL1 and /ENBL2), which allow it to natively control two different floppies, so the simple answer is two. But beyond that, it’s possible for software and hardware on either side of it to cooperate in a way that enables additional drives. I’m not sure if the total count of supported drives is well-defined.

The IIgs supports six drives, but only two of each, and only in specific orderings. The newer, smarter drives (like 3.5” floppy drives) must be earlier in the chain than older drives (like 5.25” floppy drives). I think that the way this works, 3.5” drives understand and ignore 5.25” commands, and pass them downstream, or consume 3.5” commands and suppress them downstream. That way, a 3.5” and a 5.25” drive can share the /ENBL1 signal and both be “floppy 1”.

Apple II daisy-chaining involves putting both /ENBL1 and /ENBL2 on the DB-19 connector; drives connect the upstream /ENBL2 pin to the downstream /ENBL1 pin (and maybe the reverse?). Macs assign /ENBL1 to the internal connector and /ENBL2 to the external connector (Mac Plus schematic). However, there’s an entirely different way that Macs can daisy chain HD20 drives using the Phase3 pin.

I’m not sure how exactly the SE/LC could support three floppies. It’s possible they have an IWM variant with three enable pins. It’s also possible the enable pins come from somewhere else—I don’t know if floppies actually need their /ENBL signal to come from the IWM, or if it would be possible to enable them some other way.
Looks like the SE uses an additional GLU chip to control both internal drives on /ENBL1 (assume the portable does the same,) still only two /ENBL lines on the IWM itself.

⌘⇧1 - Eject Drive 1
⌘⇧2 - Eject Drive 2
⌘⇧0 - Eject Drive 3
~need to test this but if memory serves...
Screen Shot 2021-10-03 at 10.52.44 PM.png
 

olePigeon

Well-known member
@jeremywork So that GLU just goes between the ENBL1 , /HI, and the two drives (Lo and Hi.) Seems like if you had another chip, you could do the same for the external port, no? Enable 2 drives external? Looks simple enough.

If that's the case, I wanna find me a SE donor board and give it a whirl.
 
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sfiera

Well-known member
That should be all the hardware that's needed, but you're almost certainly going to need a ROM that knows how to handle that extra enable bit too.
 

trag

Well-known member
It looks like there's a bit on the VIA that needs to toggle as needed. I'm not sure if anything but the 3-floppy machines drive that VIA bit.

It's an interesting question.
 

olePigeon

Well-known member
Would be neat to figure it out. Could potentially enable additional floppy drives, especially external drives for computers that don't have a floppy port on the rear.
 

NJRoadfan

Well-known member
The IIgs gets its extra 3.5" drive enable line from a pin run off the VGC. The daisy chain board in the 3.5" drives handles forwarding the enable signals to the 5.25" drives so that's why 3.5" drives have to be first in the chain.

The SWIM based Apple II 3.5" Controller supports two Superdrives plus Smartport devices like the Unidisk 3.5" drive.
 
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