View Full Version : Krazy XT Motherboard Hijinks (Power LED & Reset Switch)

September 25th, 2006, 06:33 PM
I just did something rather cool to my half 5160 (Clone with real XT/portable/industrial motherboard)........I added the following feathres to the Motherboard...

1.) Reset Switch Capability

By Bridging Pins 20 and 21 of the 8088 CPU, you can hard reset the system, what this does is throws the computer into reset state. Very useful if you are like me and running lots of old DOS games that don't have an exit command.

2.) Power LED

I read up that the power LED pulls the power off a +5 Volt line on a 450 Ohm Pullup Resistor. What I did was put a wire to the ground pad in front of the system memory, and a second wire to a pair of empty holes in front of the memory management chips on the far bottom right corner of the motherboard, and picked the left hole.

Fig 1 - Picture Of The Board


The red and yellow wires protruding from pins 20 and 21 of the 8088 CPU are for the reset switch.

The black wire connects to the large ground tab (with a hole in it for a standoff/screw) in front of the memory sockets, and another red wire goes to one of the two holes for some sort of connector on the lower leftmost corner of the board in the picture.

And of course, the End Result (can't really show Reset without an animated gif). Of course, all these mods are fully reversable. It was a real piece of work getting the reset lines onto the 8088 CPU though.....


Links I Used

in unrelated notes, I also found out Terry was right about there being a custom BIOS, turns out it only accepts 1.44MB drives, which actually rocks since I just had another 5.25" drive kick the bucket today, had to put a new one I luckily found at the thrift in my 286. Good thing I backed up all my floppies to hard disk. So basically, my XT runs dual 1.44M drives now.

Terry Yager
September 25th, 2006, 07:21 PM
I'm glad it's working out for ya. Course, if I'd known it was 'special' I prolly would've kept it. Ya want the rest of the case to go with it, for authenticity? (I'll even throw in the four memory boards that were clogging it up, which is why I swapped it out for a 640K mobo).


September 26th, 2006, 06:06 AM
A reset switch is very handy on these old machines.

As for the BIOS, you could always use my BIOS dumping program and post a copy for the rest of us to inspect:


September 26th, 2006, 08:08 PM
Here the BIOS Dump, plus the program said it found an "extension ROM" at C000 apparently.

September 27th, 2006, 06:03 AM
The BIOS extension at C000:0000 is for your VGA card. A 1990 Oak Technology.

The BIOS date on your 5160 is 11/08/82, which is exactly the same BIOS date as my XT. And the BIOS dump is exactly the same as mine. (The dump is 64K. The real size is actually 40K, so you have to strip the first 24K of data.)

Why do you think this supports 1.44MB drives?

September 27th, 2006, 07:14 AM
Well, there ARE 2 1.44MB drives in it........


September 27th, 2006, 07:58 AM
There might be 2 1.44MB drives, but if the motherboard doesn't know what to do with them what is the point?

When somebody says 1.44MB support, I expect that somewhere the machine knows it has 1.44MB drives and will use them. I can put a 1.44MB drive on an machine, and if the BIOS of the machine doesn't know what a 1.44MB drive is it will use it as an old 5.25 360KB drive.

It's not a custom BIOS ... it matches mine perfectly. So I'm trying to figure out if I'm missing something, or if MadMike has missed something.

September 27th, 2006, 08:09 AM
My XT (01/86 BIOS) can support 720kB floppy drives (although 1.44MB ones will work, but only at 720kB).

September 27th, 2006, 08:17 AM
My XT (01/86 BIOS) can support 720kB floppy drives (although 1.44MB ones will work, but only at 720kB).

Which is what I've pointed out already. The 1.44MB drive can work with older, slower data rate controllers. Any machine can do that.

The question is, is this a custom BIOS and does it really support 1.44MB drives?

September 27th, 2006, 09:21 AM
How about a floppy disk controller that supports high-density?

September 27th, 2006, 09:31 AM
Sure that's fine. But if the BIOS of the machine doesn't know how to talk to the controller, it's just wasted hardware too. The new controller would have to provide a BIOS extension to add support for the higher density floppy drives, or you would have to use a software device driver to do it.

I didn't see a BIOS extension, so maybe there is a high density floppy controller and software. But I don't think it's a custom BIOS ...

We'll find out eventually ...

September 27th, 2006, 05:19 PM
Here's the weird thing....

The floppy drives are both 1.44M formatted models, both can load/format 1.44M floppy diskettes, I have not tried 720K yet...

The strange part isthe drives are on a 16 bit ISA IDE/Floppy controller, no BIOS on it whatsoever either. I've tried 360K and 1.2M drives, neither of those work with it apparently, unless my 360K is bad, which I doubt, since it works in all my other machines. That's half the reason this amuses me so much, I have read that XT's did not ever support 1.44M drives, much less even work with them at full capacity unless one has a special card with a BIOS/BIOS extension on it that supported 1.44M drives.

Here's the odd part though, I notice it only likes to run DR-DOS. It hangs on MS-DOS 3.31-6.22, it also hangs with PC-DOS, but with DR-DOS, it loads up perfectly fine. Either way, it's an interesting Hardware situation to say the least. I had a clone with an original IBM PC BIOS awhile back, put in a 16 bit controller and a 1.44M drive, and the 1.44M would only work as 720K, go figure....

EDIT: Just doublechecked, it does not format 1.44M drives, but it does read em' go figure.

September 27th, 2006, 05:28 PM
If it is a late XT clone with a custom BIOS, what are the odds the manufacturer considered 5.25" drives obsolete and only included support for 3.5" drives into the BIOS?

Aha: mbbrutman already told that the dumped ROM image was identical to his, so ... can you boot something like memtest from a HD floppy to verify whether it is the OS that adds functionality or something else?

September 27th, 2006, 05:29 PM
Quite the mixed bag ..

So you have a 16 bit IDE/Floppy controller running on an XT motherboard. That explains the hardware high density support. But the BIOS of the machine is original and there are no BIOS extensions other than the VGA card that I can see.

It's possible that DR-DOS is providing software support in the absence of BIOS support. I'm not sure if that's a feature of DR-DOS or not.

DOS is supposed to try to make a BIOS call to figure out what kind of drives are on the machine. Old machines like PCs and XTs fail on that specific call because they didn't implement it. Newer machines return what was read from the BIOS. If the call fails then DOS assumes 360K drives, unless it's an OEM DOS that says otherwise.

One possible explanation is that the diskette format type is encoded in the boot record of the floppy. If DOS boots from the diskette or reads the diskette, it can see what format was laid on the diskette and ignore the failed BIOS call.