XT-IDE on PCjr

Hardware questions and modifications

XT-IDE on PCjr

Postby Brutman » Sun Feb 13, 2011 2:10 pm

On a lark I decided to try my XT-IDE adapter on a PCjr using the ISA-to-PCjr bus adapter that I have. It doesn't work as is, but we are probably pretty close to having an IDE adapter for the Jr.

Using an older (but standard) BIOS for the XT-IDE the machine called the XT-IDE BIOS, detected the hard drive, and proceeded to boot. It died after that, probably because the card is using reserved memory space that the Jr needs. I wasn't able to boot from a floppy either; DOS 5 and DOS 3.3 were probably getting past the two hidden files, but dying a little later before making it to the command line. I have the source code and access to the author, so it should not be too hard to find and fix.

My next step is to disable the BIOS and just try some generic C code that does port I/O to the controller ports. If I can read and write sectors to the drive using a C program, then there is no reason why this won't work. The biggest challenge will be figuring out what might be different about the Jr boot process and making the card work with it.

Assuming this is going to work, we should start thinking about what else we can put on a sidecar or if it is even going to be a sidecar at all. A cable to another small project box housing the guts and a hard drive might be more appropriate. If we keep with the existing XT-IDE card design and use an ISA bus, then all we need is a custom BIOS and we don't have to design our own cards. (We'll need a good run of PCjr to ISA bus adapters, but those are generally useful anyway.)


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Re: XT-IDE on PCjr

Postby jmetal88 » Sun Feb 13, 2011 2:32 pm

Yes! I would definitely be interested in one of these, as well as the ISA adapter. :)
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Re: XT-IDE on PCjr

Postby Spankmd » Mon Feb 14, 2011 6:10 am

Me, too. It would be great to add a bootable hard drive (At this point, even an ISA slot).

Let me know if I can help (I have a few dead sidecars for housings, just in case).
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Re: XT-IDE on PCjr

Postby MattCarp » Tue Feb 15, 2011 7:36 am

I still have the original PCB layouts I did for the PCjr bus - ISA bus adapter back in 2002 (can you believe that?!). I did the layout with an older program, Circuitmaker 2000, but I do have the files.

You know, I reviewed the hi-res pictures of the other ISA adapter you got from eBay and the differences are:

- no cable, obvisouly
- they added a 14.318 MHz oscillator to supply the signal that the ISA bus has, but the PCjr doesn't (necessary for some cards, like apparently a SCSI adapter you were testing)
- they added decoupling and electrolytic capacitors on the -12V, +12V, and +5V lines (the electrolytics just store charge in case an adapter takes a fast big gulp of power; these help keep the power steady)
- they added decoupling capacitors on the signal grounds

I recall really fretting about noise in the cable and that an original long straight ribbon cable failed. When I shortened the cable, it worked, but then I was really focused on this cable noise issue. I then recall that we may have attributed cable noise as the problem with the SCSI adapter you were testing, but in the end it appears to have been the OSC signal it needed.

The decoupling capacitors are good practice, but I'm not certain they make or break the design. Doing it over again, I'd add them, they're cheap enough.

I'd also add some method to terminate the signals on the bus. I also have the oscilloscope traces from when I was investigating a few key signal lines. On these I saw a bit of ringing on the edges of the signals. I don't think those broke the design either, but by adding some resistors and maybe some diodes, we can match the impedance of the signal drivers better and be able to reduce the ringing.

In the end the adapter works fine for my XT disk controller.

I think if the group just wants to create a single adapter to run an XT-IDE board, I'd just make a custom cable. However, if the group wants to add on an ISA bus, which would be more time and money, I'd start from the old project and make a few refinements.

Probably the biggest problem with the original adapter was the lack of a suitable enclosure. That's where we should start. Find an enclosure capable of housing the adapter, power supply, hard disk, and 2-4 ISA boards.
Attachments
PCjr-ISA card 002s.jpg
Here's the adapter from 2002!
(896.38 KiB) Not downloaded yet
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Re: XT-IDE on PCjr

Postby Brutman » Tue Feb 15, 2011 8:41 am

The original adapter from 2002 was a fine piece of work. But given what I know now about the decoupling capacitors I'd add them too. The OSC signal is an easy add to make.

The biggest complaint I have about both the 2002 adapter and my older adapter featured on the web page is that the adapter and cards are just floating in space. It's ok for a quick test and hardware hacking, but not for long term use. I guess it's time to seriously start looking for external enclosures with some sort of card mounting options.

As for the hard disk - CF cards might be a good alternative, although I don't like that they wear out on writes. 1 million writes isn't a lot. IBM Microdrives are older now but might work too.


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Re: XT-IDE on PCjr

Postby Brutman » Sun Mar 06, 2011 4:57 pm

What do you guys think of this:

Image Image Image

(Bigger pics can be found in http://brutman.com/pics/Small_case/)

It has a power supply, four ISA slots on a backplane, and power connectors for a hard drive or other devices. And it's available on eBay (item # 250437893003 ) still, but the price is borderline nasty. (I bought mine years ago.)


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Re: XT-IDE on PCjr

Postby jmetal88 » Sun Mar 06, 2011 5:29 pm

That looks like it would do the trick. You're right, though, $40 is a little off-putting. Hmm.
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Re: XT-IDE on PCjr

Postby Brutman » Tue Mar 15, 2011 5:06 pm

I have been digging into the code on the existing XT-IDE adapter. To be more precise, I am looking at the older beta level code from the original designer of the code on the card. The intent is to figure out what code we don't need on a PCjr and strip it from the card. That leaves us room to do other things.

The basic idea is to install a hard drive BIOS that augments the existing INT 13 support for the floppy drives. The upgraded INT 13 service uses the ancient CHS addressing from the beginning of time which is capable of addressing up to 8GB of storage when done correctly. Under the covers the BIOS can talk to the drive using either CHS or LBA addressing. If it does LBA that won't be surfaced to the user or operating system in any way.

This means that if you plug a 40GB drive into the controller you can only use the first 8GB of it. But unless you have an operating system that uses extended INT 13 to use Logical Block addressing you can't use more than 8GB anyway. And none of the versions of DOS or Windows that we might use use extended INT 13 so it doesn't matter too much. (Win 95 is the first OS to use extended INT 13 to support drives larger than 8GB.)

I'm in the market for an EPROM burner than can handle 8 to 64KB chips. Our target chip is the Amtel AT28C64 series. Does anybody have any advice on what to get? (I have classic DOS with a parallel port and Win XP with USB available to me.)
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Re: XT-IDE on PCjr

Postby jmetal88 » Tue Mar 15, 2011 5:29 pm

The only one I've ever used is an A.R.T. EPP1 that I found out in my garage. Since it's the oldest model of theirs, it's really slow, but I'm sure the company has speedier, more modern offerings these days (I know they're still around 'cause I had to get the software from their website). I don't know how they are in price, though. Expensive, I imagine. I'm just glad mine was free.
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Re: XT-IDE on PCjr

Postby jmetal88 » Tue Mar 15, 2011 5:46 pm

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