View Full Version : Model 25 and 1.44MB Drives

March 6th, 2017, 10:16 AM
I've been told countless times that the Model 25 8086 system can not work with 1.44MB drives. Well it's time to put this myth to bed: https://www.youtube.com/watch?v=H4y0TPFV_qI

I will add this, the "big button" 1.44MB drives do in fact fail to read 1.44MB disks on the 25, but the "small button" drives like the one I installed here, work just perfectly. Format, read, write, and use. I've even installed games from their original 1.44MB disks so it does read 1.44MB disks formatted in other machines too.

EDIT: Just to add, I did this test to 12 model 25 8086 machines, some color, some black and white, some newer, some older.. It worked just fine in literally every case.
EDIT2: Adding video URL showing that the 286 Model 30 contains identical disk controllers and gate arrays as an XT model 25, solidifying the fact that the XT class PS/2's can in fact use high density just fine: https://www.youtube.com/watch?v=r56Aoa17560

As to why big buttoned drives don't work, I honestly don't know, unless my drive is simply broken. (Updated: Verified the big buttoned drives do NOT in 1.44MB mode on the XT-25's--So it's definitely the small buttoned pin-header type floppies that do).

March 6th, 2017, 11:50 AM
I've got 2.88MB drives installed in my beast at home -- they function at 1.44MB capacity, but otherwise she doesn't really complain.

The 8530-286, on the other hand, they complain about the 2.88 drives...

March 6th, 2017, 11:56 AM
I have never even thought of trying a 2.88MB drive, but yea that definitely wouldn't work.. It's a curious case about 1.44MB support though.

The tech ref manual makes no mention of 1.44MB support, even in theory, the controller speed says it shouldn't, but it does. And I went out of my way to find other model 25's to borrow to test, and yea, it works on every single one.. And on the "big button" 1.44MB drives, in all case it did not work. Only the small button drives work.

It's not supported, but it does work. Like all documentation, it's very possible there was a last minute change that allowed them to do it and the manuals simply were never updated. My model 25, my father bought in 1987, so I know it's a very early machine.

March 6th, 2017, 12:24 PM
I've got some of these old stock, i've been moving them on ebay.

If the original drive is dead, i've been subbing the 2.88 drive -- they've kept better over time than the 720 or 1.44 drives.

I must have 20+ out there with the larger capacity drive. I haven't had any feedback where they have ceased to function or anyone that's had problems with them.

I did notice some peculiar behavior after adding an XT-IDE card into mine, but I can't confirm it's actually an incompatibility with the drive itself or if it's how the XT-IDE card is interfacing with the system. I haven't troubleshooted it yet -- waiting for the right rainy day..

March 6th, 2017, 12:27 PM
Good to know, thanks. If I ever need to replace the drive, I'll do that. I do need to take my 1.44MB drive apart, and clean it, lube the rails, etc. It's due for a good cleaning..

What's the issue with the XT-IDE you're having? I'm using a rev 2 prototype, and have had "some" issues with it, but it was a bad IC. Replacing it resolved it. I also had to clean the ISA slot and add a little deoxit because it was intermittently having system lockups. Since the deoxit, no more freezes.

March 6th, 2017, 12:30 PM
From what i've initially observed, the B drive light activates when there isn't any actual disk activity. At times it's bright, but it's typically dim. After some time, they start to act up, not reliably reading the data on the floppy disk. General failure, i think it said. I can't recall. But it was booting fine to the industrial disk module that I'm presently using as the HDD.

There are some photos up on my profile of the unit. I have to post the new photos with the XT-IDE card.

March 6th, 2017, 12:34 PM
Hmm now that is pretty unique, I've never seen that, but I don't have a second floppy in mine. I wish I did and had the faceplate for one.. The only other drive I have is a big buttoned 720KB drive, but it uses a different connector than the drive and ribbon I currently have installed.

That particular issue, you might want to check to make sure the ribbon isn't pinched somewhere, I can't see how the XT-IDE could be causing that, but I could be wrong there.

March 6th, 2017, 12:44 PM
Any parts you need, i've got them, drop me a message.

I was actually watching your video on the VGA mod rework with the pass-through in the DB15. That was smart with the pass-through.

I noticed that PE3 you were using for ethernet, and it gave me an idea. I was attempting to extend the isa bus for a third card nestled inside, but I got sidetracked in the move. I had one of these in stock, and now I no longer have an excuse.

March 6th, 2017, 12:50 PM
Thanks I may consider it, to add a second B: drive (I have no money or income at the moment and will have to put it on hold for now).

The passthrough design I did, allowed me to also hide a KVM switch in the back, and use the model 25's monitor as a second monitor to my desktop which runs Linux. I just setup 640x480 60Hz non-interlaced, and it shows a beautiful picture, even when watching movies (though phosphor persistence is certainly interesting in fast movies). I have a trick for 1280x960 60Hz interlaced (as it's electrically the same as 640x480 60Hz non interlaced), but I plan to do a write up on that, and share the timings used. But I have to admit, it's hard to impossible to read anything in that mode.

The PE3 works very well, and in fact I use it right at boot time so it gets the clock synced with a time server. Who needs a real time clock chip if you have the internet? lol

March 17th, 2017, 09:13 PM
I took a look in the PS/2 Model 25 Technical Reference hosted on Bitsavers. This part was very interesting:

Page 5-59 ... Bits 7,6 Data Rate Selected .. 500Kbps and 250Kbps are valid options.

The 500K bps data rate is used by high density drives. The technical reference says that the machine uses a "gate array" to implement the diskette controller. Gate array is IBM's fancy term for "custom logic". They probably used the same chipset for all models of the Model 25, hence your discovery that it supports high density diskettes.