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IBM PC/XT and 3,5" floppy support?

romanon

romanon

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Hi, I saw some pictures, where XT has 3,5" floppy drive. Is it possible with original XT type floppy disk controller? Even type 1 of IBM AT 5170 has no full BIOS support for 720k 3,5" floppy drive.
So i tried it on my 5150 with common XT floppy controller.

fotka.jpg

Results:

Booting from 3,5" HD drive with 720k floppy - YES
Writing to 3,5" HD drive with 720k floppy - YES
Reading from 3,5" HD drive with 720k floppy - YES
Formating 3,5" HD drive with 720k floppy to 720l - NO
Formating 3,5" HD drive with 720k floppy to 360k - YES
 
You can connect a 1.44 Mb 3.5" floppy drive to the original XT 5160 floppy controller and use it in 720K mode, To format the 720k floppy you will need to add a dos driver, See this page on Modem7's website. If you want to use HD 1.44 floppy's you will need to use an HD floppy controller and Bios.
 
Malc said:
You can connect a 1.44 Mb 3.5" floppy drive to the original XT 5160 floppy controller and use it in 720K mode, To format the 720k floppy you will need to add a dos driver, See this page on Modem7's website. If you want to use HD 1.44 floppy's you will need to use an HD floppy controller and Bios.
Click to expand...

Assuming one could find such a drive (and enough disks), could one also connect a 720K 3.5" FDD to an original PC/XT FDC?

Has anyone tried this?

(I'm just thinking that using the right tracks and media might be less iffy – or at least more authentic.)
 
ropersonline said:
Assuming one could find such a drive (and enough disks), could one also connect a 720K 3.5" FDD to an original PC/XT FDC?

Has anyone tried this?
Click to expand...

Yes, I've tried that on my 5160, and it worked.
720k 3.5" drives work pretty much the same as the stock 360k 5.25" drive. The only difference is that they have 80 tracks instead of 40. If you configure that with the driver, it should work fine (reading always works fine, it's writing/formatting that's the tricky part).
 
Last edited:
Scali said:
Yes, I've tried that on my 5160, and it worked.
720k 3.5" drives work pretty much the same as the stock 360k 5.25" drive. The only difference is that they have 80 tracks instead of 40. If you configure that with the driver, it should work fine (reading always works fine, it's writing/formatting that's the tricky part).
Click to expand...

The driver's only needed if you have the first XT BIOS. The later BIOS includes code in POST to detect 80-track drives and configure the equipment word appropriately.
 
ropersonline said:
Assuming one could find such a drive (and enough disks), could one also connect a 720K 3.5" FDD to an original PC/XT FDC?

Has anyone tried this?

(I'm just thinking that using the right tracks and media might be less iffy – or at least more authentic.)
Click to expand...

Yes, In the IBM PC 5150 you'd need to use the Dos driver "driver.sys" to format the 720K floppy, Same goes for the IBM XT 5160 with the first bios, But the later XT 5160 bios had support for 720K floppy, Back in the day 720K 3.5" floppy drives were an optional add on, you'll be lucky If you can find a good working one at a reasonable price, Some people think they are made of gold and charge accordingly, I'd stick with a 1.44 3.5" drive, shed loads of those still around.
 
You can also use a 1.44MB 3.5" FDD in an XT if you get either a multi-fuction adapter (even one for an AT) and can get 1.44 support with a TSR, but can't boot from it. You can get a bootable 1.44 FDD into an XT using an 8-bit FDD adapter with its own BIOS chip, but those adapters are expensive and a little harder to come by. For example I recently saw a mini/micro XT adapter on eBay that can support upto 4 FDD drives in mixed sizes. I use one in my XT to support both 1.44 bootable FDD and a 1.2MB 5.25 HH drives and it works, but also ran into a conflict because I also had an XT-IDE adapter (which I used to add a pair of CF drives) whose address collided with the FDD adapter and had to do some trickery to the XT-IDE to fix that collision.

Regards,
Mike
 
Last edited:
Malc said:
To format the 720k floppy you will need to add a dos driver, See this page on Modem7's website.
Click to expand...

There is an easier way. Just use SETBPB35 -- unlike DRIVER.SYS, it doesn't take up any RAM:

ftp://ftp.oldskool.org/pub/tvdog/tandy1000/misc/setbpb.zip

This program will set the device characteristics for the A or the B diskette drive to indicate that the drive is a 720K drive instead of a 360K drive. It requires DOS 3.2 or later.

The only command line parameter is the drive letter (A or B).

For example, to indicate that your B drive is a 720K 3.5" diskette drive:

SETBPB35 B

Run it from the DOS command prompt or add it to your AUTOEXEC.BAT file.
 
Chuck(G) said:
If you have a version of MSOOS that supports it, you can use "DRIVPARM" in your CONFIG.SYS and not use up memory with DRIVER.SYS. It also doesn't change the drive letter.
Click to expand...

I'd forgotten all about DRIVEPARM, It always annoyed me with the drive letter change using DRIVER.SYS, I need to re-config my XT so i'll use that, Thanks.

vwestlife said:
There is an easier way. Just use SETBPB35 -- unlike DRIVER.SYS, it doesn't take up any RAM:
Click to expand...

Never seen that one before, That will come in handy too, Thanks
 
Really stupid question:

SETBPB and DRIVPARM can't be used to announce an A: drive as 720K and boot from it, right?

Or is there some way booting still works right up to the point where DRIVPARM/SETBPB are encountered in CONFIG.SYS/AUTOEXEC.BAT, at which point the PC groks, oh, this is 720K?
 
If you're booting off a 720KB disk then it doesn't matter. The OS will boot off the first 40 tracks anyway, and once it starts it'll realize the media in the drive is formatted 80 track. SETBPB and DRIVPARM load after the OS has already started booting to let it know the drive is 80 track - so when you type "FORMAT B:" it doesn't think it's a 40 track drive (360KB) etc
 
Booting from a 720K floppy will work fine, The driver is required to format a 720K floppy as a 720K floppy in a 5150 or a 5160 with the first bios, Without the driver the 720K floppy will be formatted as a 360K.. If your 5160 has the latest bios it has support for 720K floppy's so the driver is not required.
 
Malc said:
If your 5160 has the latest bios it has support for 720K floppy's so the driver is not required.
Click to expand...

I never understood the 720K support, because there is no way to tell an XT it has a 720KB drive - no switch, no CMOS.
Does anyone know what the deal with that is? Does it try and seek to track 50 or similar on start?
 
It's actually pretty easy to tell the difference--and several BIOSes have used it. The key is the track 0 sensor. If you step out 50 cylinders from home, the sensor will activate in less than 50 steps toward 0, as the 360K head carriage will hit a stop. I never liked that, but hey it works.

Electrically, there's no practical difference.
 
Chuck(G) said:
It's actually pretty easy to tell the difference--and several BIOSes have used it. The key is the track 0 sensor. If you step out 50 cylinders from home, the sensor will activate in less than 50 steps toward 0, as the 360K head carriage will hit a stop. I never liked that, but hey it works.
Click to expand...

Award's 8088/8086 BIOS does that -- if you have a 40-track drive installed, during POST you'll hear the loud grunting sound of it hitting its end stop as the BIOS tries to make it seek more than 40 tracks.
 
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