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View Full Version : Use a High Density FDD in an older PC



Erik
November 18th, 2010, 07:41 AM
The original IBM PCs and XTs were made before High Density FDD's caught on, and as a result of this, no intentional support were added in neither hardware or software. In order to get HD drives to work on a PC or XT, we have to work around therse limitations.
Hardware
The original IBM Floppy disk controller was based around a simple microcontroller for controll and a bunch of custom logic for interfacing. This card was intended to be used with Double-Density drives, and it's unable to interface with HD drives due to timing issues (unless the HD drive emulates a DD drive when DD media is present). In order for the computer to properly interface with HD drives, the controller must be replaced with a more recent one with support for HD drives.
Controllers as found on AT-style multifunction cards works well, as they mostly only use the 8-bit partion of the card's 16-bit bus. With a HD-compatible controller, the computer will at least be able to interface with HD drives.
Software
The next limitation is the BIOS. By default, the BIOS can't really make too much sense of more recent formats, even though it may be able to read and write correctly to DD disks of any size. HD disks may still be unreadable, and will in that case just generate errors when access is attempted. In order to solve this problem, a more recent replacement of the BIOS diskette routine package (Int 13h) must be installed.
Some HD controllers come with a replacement in a physical ROM chip on the card itself. This replacement is automatically loaded on boot, and all you have to make sure is that the switches on the card are set correctly.
If this is not the case, a replacement will have to be installed by a software driver after the machine boots, usually as a part of the OS initialization. An example of such a driver is '2M-XBIOS.COM' from the 2M FD-tool packagehttp://en.wikipedia.org/wiki/2M_(DOS). If the machine has an HDD, this will be no problem since the driver is not needed for the HDD to boot properly, but if the machine only has floppy drives, a startup disk of Double-Density has to be used in order to install the driver. This disk must of course be formated as a startup disk, and it must also contain the driver. You should make sure to read how to set up the driver before installing it (this information usually provided with the driver).
Category:How_To

Chuck(G)
August 18th, 2011, 11:27 AM
I don't understand what "HD FDD" means. HD = "hard disk"; FDD = "floppy disk drive" or maybe "fixed disk drive"--the use of the two acronyms together is confusing. Why not just spell it out? "Hard Disk".

"Fixed disk drive" once had a very specific meaning--a drive whose media could not be removed. The IBM 2314, for example, was not a fixed disk drive--the 2316 packs could be removed.

Mike Chambers
August 18th, 2011, 12:40 PM
high-density floppy disk drive.

Chuck(G)
August 18th, 2011, 01:32 PM
Okay, an Emily Latella moment. :blush:

Nevertheless, "high denisty floppy drive" would clear up a bunch of headscratching.

Mike Chambers
August 19th, 2011, 02:19 PM
lol. agreed, yeah that would help.

circuit
August 18th, 2016, 03:17 AM
I would love a 3.5" drive in my 5160, I didn't want HD so much for the capacity, more because a) They are plentiful and cheap, b) they come in black(!) but I've never found a combination of drive / controller that actually works for me. Has anyone got one running in a 5160 and if so which card / drive?

Stone
August 18th, 2016, 05:58 AM
IIRC, a 1.44 floppy will function in a 5160 as a 720K floppy.

mmruzek
August 18th, 2016, 06:11 AM
I would love a 3.5" drive in my 5160, I didn't want HD so much for the capacity, more because a) They are plentiful and cheap, b) they come in black(!) but I've never found a combination of drive / controller that actually works for me. Has anyone got one running in a 5160 and if so which card / drive?

An 8 bit ISA controller card that I have found works in a 5150 PC to install a 1.44 MB floppy disk drive is from JCC. Here is a listing to one on EBAY. (Keywords JCC 2DR14A 810730). I'm guessing you can find one for much less if you watch the listings and search on the keywords. FWITW: I have tried several floppy disk controllers in a 5150 that 'should' work, but don't!

http://www.ebay.com/itm/JCC-2DR14A-810730-Circuit-Board-T41310-/281997406680?hash=item41a85af1d8:g:d40AAOSwLa9UVKT O

Malc
August 18th, 2016, 06:44 AM
Ouch!, Way too expensive and missing the HD floppy bios Rom.

Malc
August 18th, 2016, 06:51 AM
As 'Stone' said, A 1.44M Floppy drive will function in an XT 5160 as a 720K connected to the original controller, See This Page (http://www.minuszerodegrees.net/5160/floppy_diskette/5160_floppy_drive_support.htm) on Modem7's website.


I would love a 3.5" drive in my 5160, I didn't want HD so much for the capacity, more because a) They are plentiful and cheap, b) they come in black(!) but I've never found a combination of drive / controller that actually works for me. Has anyone got one running in a 5160 and if so which card / drive?

Al Hartman
August 18th, 2016, 09:11 AM
I built one of these. It works great in my 5150, and can even read and write single density TRS-80 disks!

http://www.malinov.com/Home/sergeys-projects/isa-fdc-and-uart