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720kb floppy drive suddenly turned into a 360kb drive !?

Overmann

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Apr 21, 2017
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Hi all,
I wrote this in another thread called "Adding a 5.25" to Commodore PC20-III" but thought it was interesting and annoying enough to warrent a thread of it's own (as it was a bit of a digression in the other thread).

The problem is that my 3.5" floppy drive no longer wants to format 720kb disks. It happily reads them but when i type Format A: it starts formatting 360kb, and if I specify format a: /F:720 or format a: /T:80 /N:9 it reports back that the parameters are incompatible.

The computer is the Commodore PC20-III from the thread above. I have been messing around trying to use a ISA controller card to get second floppy drive going in the machine, but had no luck. I eventually gave up, but then I found my 720kb drive had stopped working properly as well. I imagine something in the BIOS has been turned around and it now understands my drive as a 260kb drive, but I have no idea how to change it back.

Does anyone have any ideas?
 

cwathen

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I'm not familiar with the machine but as it's an XT-class computer I'd be very surprised if it has any form of CMOS setup. All configuration should just be jumpers and switches - are you sure you put everything back the way it was before after you tried adding a card? And are you sure a card was required for a second drive? Again, I'm not familiar with the machine but most PC compatibles with standard Shugart style controllers should support two floppy drives. If the cable doesn't have two connectors it should just be a matter of changing it for one that does.

If all else fails you could try using DRIVPARM to force the drive to be treated as 720K. Add to config.sys:

Code:
drivparm=/d:1 /c /h:2 /s:9 /t:80

Have you done any work with CONFIG.SYS and/or are you still using the same DOS that you were before? I've come across a couple of XT clones with 3.5" 720K drives as standard but which required DRIVPARM or DRIVER.SYS to recognise them as they used a controller expecting 5.25" 360K drives. If you didn't know that you'd run into problems using a generic DOS disk rather than the original supplied disk with the necessary configuration set.
 
Last edited:

Overmann

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Thank you for the reply! I did not make any alterations to the jumpersettings (which I probably should have as it was probably set to only look for one drive. That might be why I didn't get it to work) :p
The floppy controller on the motherboard can indeed handle two drives but it's very particular about which drives and they need to have an additional PCB on the connector to tell the controller which drive it is etc. I didn't have such a drive but I had an ISA controller that could address the Floppy controller as secondary (or somesuch).

I didn't make any alterations to my system software either. I'm still running the same installation as I was and the config.sys is the same. I installed DOS 6.22 from floppies that did not come with the machine and it worked fine, so it should not require drivparm, but I will absolutely try it. You never know!
 

SomeGuy

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XT class computers usually have no built-in BIOS support for 720k drives. They always assume any drive attached is a 360k drive.

To format a 720k disk using the DOS formatter on an XT class computer (that does not have any any special high density FDC card) one must specify the drivparm command or load driver.sys.

However, DOS will usually still READ 720k disks just fine without those.

Another option is just to use a better floppy disk formatter program. The one provided with DOS is very brain-dead.
 

Overmann

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That is very interesting. So perhaps I never actually formatted a floppy on this machine. I was using my 486 to transfer games so it is entirely possible. How do I go about loading driver.sys? Is there a particular driver.sys I need? I've not heard of this before? Will loading running drivparm in config.sys do the same thing?
 

SomeGuy

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Driver.sys is included with DOS 3.x. If you have DOS 3.x that is what you would need to use to add 720k support. Although 3.30 has a sort of hidden drivparm command built in.

Later versions of DOS use the built in drivparm command instead of driver.sys.
 

Overmann

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That is strange. I still get the same results. I've written drivparm out exactly as above in config.sys but it still won't format 720kb.
 

SomeGuy

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You said your drive was drive A:

The drive parameter should be "/d:0". (Because programmers don't realize that negative one people start counting from zero.)

Also you might need to specify the "/f:2: parameter (format-type = 3.5" 720k).

So try -
drivparm=/d:0 /c /f:2 /h:2 /s:9 /t:80
 

Overmann

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That did it @SomeGuy! Thank you a WHOLE bunch!:D
Could I use this to specify the second drive if I ever god it up and running, or would DOS/"BIOS" correctly assume that it's a 360kb 5.25" drive?
 

vwestlife

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The easiest way to enable the full capacity of 720 KB 3.5" drives in old PCs and XTs is to use SETBPB35:

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. 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
 

DDS

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When one says "DOS version x does y" one may or may not be correct for all variations if version x. When Microsoft and IBM started playing tug of war with the future of their shared operating system, things got a little bit strange. Some features included in MSDOS version x did not appear in IBM's PCDOS until a later numbered version if at all.

And then there's FreeDos.
 

vwestlife

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IBM's PC DOS introduced DRIVPARM in version 3.2, but MS-DOS didn't have it until 3.30A (OEM release).

But many OEMs had their own special code to support 3.5" floppy drives even in versions older than when plain vanilla MS-DOS began supporting it. For example, the version of MS-DOS 2.11 that comes in ROM in the Tandy 1000HX supports its 3.5" floppy drives even though 3.5" drive support didn't officially exist in DOS until 3.x. And Zenith's MS-DOS 3.3 Plus has a special configuration utility to allow it to recognize the full capacity of 3.5" 720K drives in an XT-class system, with no DRIVPARM or DRIVER.SYS needed.
 
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