• Hello, Guest! Welcome back, and be sure to check out this post for more info about the recent service interruption and migration.

IIsi with external 400k floppy possible?

sunder

Well-known member
Is it possible to connect an external 400k floppy drive intended for use with an original Mac 128 to a IIsi?

http://docs.info.apple.com/article.html?artnum=5911

Says it wants at least an 800k floppy drive, but is this due to OS limitations wanting HFS, or is there some reason it wouldn't work?

All I need it to do is use it make disk images/copies with Disk Copy 4.2 or copyIImac, not to actually format, or access a file system.

I guess what I'm really asking is, is there any danger to the IIsi in hooking up a 400k external drive?

 

Anonymous Freak

Well-known member
A 1.4 MB drive works just fine with 400 k floppies in 6.0.8. You just need to hold down the Option key while selecting 'Erase Disk' from the Special menu, and it formats as MFS instead of HFS, as Aoresteen's link says.

And using Disk Copy, even System 7.5 can read and write 400k disk images. I'm pretty sure Disk Copy can read/write 400k disks/images even Mac OS 8.1, even though the Finder can't mount the disks!

 

sunder

Well-known member
Oh, I know.

That would work in most cases (using the Superdrive to read 400k floppies) we tried that, but about half the disks wouldn't read, but with an 800k drive it did read some cleanly. Unfortunately the guy who was using his Mac II with an 800k drive had a hard drive crash, so he sent the disks to me for recovery.

So there are physical differences between 400k/800k drives and 1.44M drives that may make a big difference here.

For one thing, if the original drive used to write these was even slightly misaligned, the smaller r/w head in the 1.44M drive wouldn't see the whole band, but while the 400k head would see a bit more of it.

Also, the 400k drive uses a variable speed motor - so some tracks are read at different speeds than others in order to store more data. The 1.44M drives don't vary the motor speed, but rather vary the shift register clock rate that's used to read from the drive head.

I've actually had great luck moving data between a 1.44.M drive to a 400k drive. i.e. from a 7100 or a TAM, or a IIsi to a Lisa and back without any problems, but this is for disks that are 20 years old and have been stored in a non-controller environment (i.e. garage)

So that's why I'm trying to connect a 400k external drive to a IIsi, but wanted to check if I'm risking the IIsi's SWIM chip or other electronics. :) I don't want to damage my poor IIsi in the process.

 

sunder

Well-known member
So, I didn't need to do this at all, the disks read just fine in my TAM's superdrive.

I may revisit this idea later just to see if it would work, but for now I'm happy I got at the data.

 

Scott Baret

Well-known member
I thought I read somewhere that a 400K drive won't work on anything with a SuperDrive...but alas I have no 400K external to test with.

 

Anonymous Freak

Well-known member
I know that back in the day when 5.25" floppies moved from 360 KB to 1.2 MB, there were lots of problems with low-density disks in high-density drives, but I have never had such issues with 3.5" disks. (During the transition from 360 KB to 1.2 MB, it was fairly common to see computers with one of each type of drive, for better compatibility. My old IBM AT is that way.)

 

Aoresteen

Well-known member
I know that back in the day when 5.25" floppies moved from 360 KB to 1.2 MB, there were lots of problems with low-density disks in high-density drives, but I have never had such issues with 3.5" disks. (During the transition from 360 KB to 1.2 MB, it was fairly common to see computers with one of each type of drive, for better compatibility. My old IBM AT is that way.)
In the early days (1984/85/86) there was a huge difference between Single Sided and Dual Sided single density 3.5" disks. I had a lot of 400K disks that would not format at 800 k when I got my 800k drive. Then the 2 MB 3.5" disks DSDD (1.4MB formatted) were introduced and you had problems again with 800 & 400k drives using them. By 1988 things settled down and you could use 1.4MB MS-DOS disks and reformat them to 800 k ok. I must have recycled a 100 of those free AOL disks that eveyone was getting. I'd get a bunch of them and have a formatting session and I used a hair dryer to peel the labels off. [8D]

In 1984 I went into a computer shop and paid $5 a SINGLE blank TDK 3.5 inch disk. 8-o They were $25 for a box of five. You could also buy 3 inch disks but that's another story!

 

tomlee59

Well-known member
I discovered the hard way why many 400K disks didn't work well as 800K disks: The 400K drives can destroy one of the surfaces. The business side has a head; the other side rests on a post. That post can be abrasive enough to kill that surface. Maybe a clean, new 400K drive wouldn't cause this problem, but how many of us have such a thing? :)

Years ago, I took a few fresh 400K disks out of a box and had no problem formatting them as 800K. However, if I first formatted their brethren in a 400K drive, the yield as 800K disks plummeted. I could actually see score marks where bits had been ground away.

 

Anonymous Freak

Well-known member
Well, there are actual, measurable, physical differences between the three types:

Early 400K disks were truly one-sided. There was no magnetic coating on the other side! (Unlike single-sided 5.25" disks, which often had the coating on both sides, just in case someone tried to use it 'upside down', which is impossible with 3.5" disks.)

"Good" single-sided drives should never destroy a double-sided disk, because they just have a felt-covered pressure pad opposite the read/write head. Unless the pressure pad breaks, a double-sided disk should work just fine (when formatted as single-sided,) in a single-sided drive, and back again.

The move to high density, however, presented problems. High density disks use a much weaker magnetic field than low density disks. If you use a "High Density" disk that has never been formatted in an 800K (properly called "Double Density", but more often called "Low Density",) drive, it will likely format just fine. The stronger magnetic field of the 800K drive may not 'hold' for as long as the weaker magnetic field that the HD disk was designed for, but it should work for short-term uses.

The problem arises that once you have done that, an HD drive doesn't always have enough 'oomph' to negate the DD format completely. This means that re-formatting an HD disk to HD can fail, and essentially ruin the disk for any future use.

Likewise, an 800K disk is meant for the stronger magnetic field, and often had 'coarser' particles that may not even be tightly-packed enough to be formatted as HD, even if the disk had never had a DD format done to it.

In short: 400K disks should always assumed to be purely one-sided, and never attempted to be formatted as anything else. 800K disks can be reformatted as 400K and should have no problems whatsoever (unless you put it into a 400K drive that is physically broken so that it scratches the 'back' side.) 800K disks that have never been formatted (especially later-manufactured ones,) CAN be formatted as HD, and then re-formatted 800K again, but formatted 800K disks should never be re-formatted as HD. HD disks can be formatted to 800K, but it can be unstable, and once formatted as 800K, will be VERY unstable upon re-formatting to HD. But, HD 3.5" drives should be able to read and write (and even format,) 800K and 400K disks with no problems whatsoever, because HD drives can do both strengths of magnetic field, when they know which strength to use.

The problem with 5.25" disks and drives is that most HD drives didn't have the ability to operate at the stronger field strength of lower-density disks. This meant that a 360K disk that was formatted on a 1.2MB drive may work just fine in 1.2MB drives, but not in native 360K drives. Reading was usually not a problem, only writing.

 

tomlee59

Well-known member
I've never seen a 400K disk with no coating on one side. Very interesting!

Yes, the felt pads are supposed to be soft enough not to abrade away the media, but apparently they aren't always in such good shape.

 

Scott Baret

Well-known member
Sort of off topic, but the 5.25" disks made me remember this since I have an IBM PS/1 with both 3.5" and 5.25" high density drives...

If you use DOS, your disks will always be formatted to the highest capacity your drive is capable of (unless you use a really old version of DOS that doesn't recognize larger capacities). In other words, if you type format b: to format your 360K 5.25" disk, it will format it as a 1.2MB.

Yet another problem Macs of that vintage don't have...

 

Anonymous Freak

Well-known member
There are command-line options for format that tell it to format the disk at the correct capacity.

format a: /f:

where is the commonly referred to size. Valid options are 160 or 180 for single-sided double-density 5.25" drives, 320 or 360 for double-sided double-density 5.25" drives, 720 for double-density 3.5" drives, 1.2 for high density 5.25" drives, 1.44 for high density 3.5" drives, and 2.88 for extra-high density 3.5" drives. (3.5" 'mode 3' drives can use 1.2 as well.)

 

Also, using /8 is a shortcut telling the system to format a disk to 360 K in a 1.2 MB drive. (i.e. "format a: /8")

 

Aoresteen

Well-known member
Sort of off topic, but the 5.25" disks made me remember this since I have an IBM PS/1 with both 3.5" and 5.25" high density drives...
If you use DOS, your disks will always be formatted to the highest capacity your drive is capable of (unless you use a really old version of DOS that doesn't recognize larger capacities). In other words, if you type format b: to format your 360K 5.25" disk, it will format it as a 1.2MB.

Yet another problem Macs of that vintage don't have...
Actually this is controlled by the BIOS where the type and capacity of the drive is stored. DOS simply gets the data from the BIOS (on POST it is stored in a Low Global). You can set a 3.5 1.4mb drive up as a 720K drive in the BIOS and DOS will treat it as such.

 

Scott Baret

Well-known member
I think I've seen this option in my PS/1's BIOS, called "configuration utility" on that particular system. I'm dusting it off again this weekend and will try that.

 

Mac128

Well-known member
The reason you can't use an original 400K disk with a SWIM is that the 400K drive depends on the system clock signal (PWM) to adjust speed on pin 20. Since the 800K disk nor the Superdrive depend on this signal, adjusting their own speed internally, the SWIM uses a completely different signal on pin 20 which has nothing to do with disk speed (in particular it is +5V). Therefore, the 400K drive simply will not work on a SWIM Mac and could possibly be damaged.

 
Top