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Replacing 5.25 drives with 3.5 drives on a Compaq Portable I?

bobmcjr

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Feb 8, 2015
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I have recently acquired an original Compaq Portable. It came with the manuals, the DOS boot disk, and 20 blank disks. It turns on, and the screen and keyboard appear to work fine, however it seems that the DOS disk has decayed beyond use. I would like to replace one or both 5.25 drives with 3.5 drives, but I am not sure which type of drive would be compatible, how to install them, or if HD 3.5 floppies are supported, even formatted as 360k.
Ideally I would do something similar to this: http://www.vintage-computer.com/vcforum/showthread.php?42154-Compaq-Portable-I-upgrade
Alternatively, I have tried to find a 5.25 drive for a reasonable price I could stick in a Windows 95 PC and rewrite the boot disk image on that, but found nothing.
Can anyone point me in the right direction? Or is there some other mod I could do to get DOS working?
 

tkc8800

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Jul 1, 2014
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I have the same machine and when I was tinkering with mine, I connected a 3.5" drive to it with no problems. I used a Teac 1.4mb drive and it worked fine. Any 1.4mb floppy drive should work. I think the maximum disk format that will work is 720k so best to use double density disks formatted to 720k. I have disk images of the original Compaq DOS if you need them. If the 5.25" drives in your machine still work, you should be able to connect at 3.5" drive with the correct DOS and then create 5.25" boot disks using the 5.25" drive in the machine.

Removing the drives from the machine is a bit fiddly but not too hard. You need to remove the top and bottom plastic case covers and then remove the top of the inner cage. There are four screws that also need to be removed, these hold the drives in. Then remove the expansion cards. That will give you enough room to pull the dives out from the inside of the machine. I recall when working on mine that I left the 5.25" drives in and just connected the 3.5" drive directly to the controller using a longer floppy cable that I had. This allowed me to copy stuff from the 3.5" drive to the 5.25" drive in the machine.

There's some pictures I took of mine in this thread that shows you the top open and the back of the drives:

http://www.vintage-computer.com/vcforum/showthread.php?44112-Compaq-Portable-restoration
 
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bobmcjr

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Feb 8, 2015
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Thanks for the info. As it turns out, I still have a single DD 720k 3.5 floppy lying around (and a ton of HDs). If the new 5.25s can still hold data, I'll try to make a few boot disks. The compaq DOS image I acquired boots in qemu, so hopefully it work on the Compaq itself. I'll post back with my results once I get a drive.
 

trr94001

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Jan 9, 2014
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Connecticut
I had a second hand Compaq Plus (a Portable 1 with a 10mb hard drive) my freshman year of college. I replaced the 5.25" drive with a double density 3.5" and installed Dos 5. Aside from needing to build a custom cover to fill the gap left by the the half height adapter I had the 3.5" floppy in it all worked without a hitch. The only odd thing was that i couldn't format 720k disks, though I could read and write just fine. At the time I assumed it was a limitation of the controller.
 

PhilA

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Feb 19, 2009
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Location
Cut Off, Louisiana, USA
The only issue I had with changing mine was that the original drives are a slightly different size, and if you want to do as I did mine and put two half height drives in one of the full size slots, a little bit of ingenuity with the big rubber shock absorber pads is required to get things to fit right, as the holes don't quite line up right.

Aside from that, I just stuck a plain old Sony 3.5" drive in place with a cable stolen from another computer and the power adapter came with the drive holder. It works just fine, reads and formats 720kb as mentioned, but MS-DOS does need instruction in the MODE command to make it realize the BIOS is wrong in reporting 320kb drives.


--Phil
 

bobmcjr

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Feb 8, 2015
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I finally opened mine up. It would appear I need a cable like this:
fddcable02.jpg
to hook up a 3.5" drive and one of the 5.25" drives simultaneously, correct? @tkc8800 my existing floppies appear very different to the ones pictured in your post.
And here are photos of the inside of the Compaq and the cards: http://imgur.com/XTrZwbI,wSjk53P,REtW5qQ,iTUDwwj,ysPhubA
The VDU board appears to have an RCA style coax port, and I couldn't find any information on the "MULTI FUNCTION" card.
 

sergey

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Silicon Forest, Oregon, USA
The question "does my PC/XT clone supports 3.5" drives" was discussed multiple times. And the answer is YES, but only in 720K mode (unless using high-density controller + BIOS extension or a driver). It is a PC/XT floppy disk controller limitation. You can use 1.44MB drives, and you might be able to cover density detect holes in 1.44MB diskettes and format them as 720KB... although some reported unreliable operation in this case.

The only information BIOS of IBM PC, XT, and their clones, and anything before IBM AT, including Compaq Portable reports to DOS is the number of floppy drives as configured by DIP switches.
There is no difference between 320K or 360K drives. Also the only reason your DOS will not format 720K disks is that it is too old... The support for 720K drives was officially added in MS-DOS 3.11.

It is possible to mix 5.25" and 3.5" drives on one cable. Be careful with termination resistors. Older 5.25" / 360K drives have removable/disable-able 150 ohm termination resistors installed on the farther from the controller drive (disabled/not installed on the middle drive), while 3.5" drives normally will have permanently installed 1 kohm resistors. I guess there is two ways to go about connecting drives:
1. Replace middle 5.25" drive with 3.5" drive (terminators on both drives, but 1 kohm will not really affect anything)
2. Replace the farther 5.25" drive with 3.5" drive (1 kohm terminator on the 3.5" drive, no terminator on 5.25" drive)
It is not a good idea to have termination resistors only on the middle drive... (it might work unreliably or will not work at all)
 

modem7

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Melbourne, Australia
It is possible to mix 5.25" and 3.5" drives on one cable. Be careful with termination resistors. Older 5.25" / 360K drives have removable/disable-able 150 ohm termination resistors installed on the farther from the controller drive (disabled/not installed on the middle drive), while 3.5" drives normally will have permanently installed 1 kohm resistors. I guess there is two ways to go about connecting drives:
1. Replace middle 5.25" drive with 3.5" drive (terminators on both drives, but 1 kohm will not really affect anything)
2. Replace the farther 5.25" drive with 3.5" drive (1 kohm terminator on the 3.5" drive, no terminator on 5.25" drive)
It is not a good idea to have termination resistors only on the middle drive... (it might work unreliably or will not work at all)
From page 527 of the fourth edition of the book, Upgrading and Repairing PCs:

"Some confusion exists regarding the situation in which you have both 5 1/4-inch and 3 1/2-inch drives installed on a single cable. In this case, the 5 1/4-inch drive must have its terminating resistor configured as appropriate, which is to say either leave it installed or remove it depending on whether the drive is at the end of the cable. Nothing with regards to termination is done with the 3 1/2-inch drive because the terminating resistors are non-configurable. Although it sounds like mixing these might cause a problem with the termination, actually it works out quite well as the math shows ..."
 

bobmcjr

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Feb 8, 2015
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So I got a 3.5" drive and hooked it up. When I turn the compaq on, the light goes on on the floppy and it sounds like it's going to start, but then it just turns off, and the compaq hangs forever. I'm thinking that my disk isn't properly formatted. When I open it up with a hex editor, the header says MS-DOS5.00 and FAT12.

I have disk images of the original Compaq DOS if you need them. If the 5.25" drives in your machine still work, you should be able to connect at 3.5" drive with the correct DOS and then create 5.25" boot disks using the 5.25" drive in the machine.
Are your disk images in a format that can be written without booting into some DOS variant in a VM? The one's I've come across are in ImageDisk format, and not surprisingly, DosBox can't handle them. I intend to use a 720k 3.5" disk.
 

modem7

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So I got a 3.5" drive and hooked it up. When I turn the compaq on, the light goes on on the floppy and it sounds like it's going to start, but then it just turns off, and the compaq hangs forever. I'm thinking that my disk isn't properly formatted. When I open it up with a hex editor, the header says MS-DOS5.00 and FAT12.

Are your disk images in a format that can be written without booting into some DOS variant in a VM? The one's I've come across are in ImageDisk format, and not surprisingly, DosBox can't handle them. I intend to use a 720k 3.5" disk.
Until you get a known good Compaq DOS boot disk, a WinImage-created image of a 720K sized IBM DOS 3.3 boot diskette is at http://www.minuszerodegrees.net/misc/5170_gsetup_720k.imz
 

tkc8800

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So I got a 3.5" drive and hooked it up. When I turn the compaq on, the light goes on on the floppy and it sounds like it's going to start, but then it just turns off, and the compaq hangs forever. I'm thinking that my disk isn't properly formatted. When I open it up with a hex editor, the header says MS-DOS5.00 and FAT12.


Are your disk images in a format that can be written without booting into some DOS variant in a VM? The one's I've come across are in ImageDisk format, and not surprisingly, DosBox can't handle them. I intend to use a 720k 3.5" disk.

The images I have can be used in WinImage. I used a Windows XP machine with WinImage v9 to create my disks. I've uploaded the images to the page below. Compaq DOS 2.12 is a single 360k disk image. Compaq DOS 3.31 is made up of five, 720k disk images. Version 2.12 is the more era appropriate dos for the first Compaq portable 1. 2.12 should work fine on a 3.5" disk if you want to use that version.

http://tkc8800.com/page/CompaqPortable
 
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bobmcjr

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Feb 8, 2015
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Thanks for those. WinImage flat out refused to write the 2.12 360k image (I couldn't figure out how to modify it so it would think it was 720k). Instead I made a hybrid 3.31-2.12 disk with the basic boot stuff from 3.31 and the utilities from 2.12.
After I got it booting, the enter key worked twice, then failed, so I am now in the process of keyboard repair. So far so good. Also, I learned that both drives must be connected for it to do anything.
 

tkc8800

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I'll experiment with it myself and see if I can create a 720k image of the 2.12 version. I have a Windows XP machine with both 3.5" and 5.25" drives installed and WinImage. I recall when I attached a 3.5" drive to my Compaq portable I used a later version of DOS.
 

bobmcjr

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Feb 8, 2015
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I got the keyboard repaired to a usable condition, and everything is working pretty nicely. It would appear that my original DOS disk is still good, and that it's the original Drive A that went bad, as I was able to diskcopy it and boot from my 3.5" drive.
I'd imagine DOS 2.12 uses a bit less ram than disk 2 of 3.31? I'm limited to the stock ram as I can't find the bad ram chip on my Twinhead CT-6050 nor find anything about it on the internet to adjust the dip switches to correct error 4040 201.
But yeah, I'm pretty happy with it so far.
 

bobmcjr

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Feb 8, 2015
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Well I got my portable closed back up with only 10 'extra' screws.
I made a wooden mount for the 3.5" drive in slot 1 and it's held in there pretty solidly.
And as it turns out, this has the exact correct hardware needed to run the 8088mph demo released last month, so that's pretty amazing.
Shame that my twinhead card is broken. I suppose I could get a ramless sixpakplus and transfer all the ram into it since it appears to be a socket/other hardware issue and not a chip problem.
 

PhilipA

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Aug 4, 2013
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Larose, LA, USA
I took a look at the 8088mph demo on mine- the composite output is possibly designed for a lower impedance connection than my TV because the output unloaded is well outside NTSC specs, at nearly 2V p-p. Running it through a carefully adjusted trimpot got full color working on the screen, just one or two of the colors aren't quite "right", like the oranges, but all in all it's pretty dern close!

--Phil
 
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