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The homebrew IBM PC/XT compatible system - future directions poll

sergey

sergey

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Jul 15, 2010
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Silicon Forest, Oregon, USA
Hi,

I need some advice regarding future directions for my PC-compatible computer project.

First of all some background information:
- The first revision is a processor card that goes into an ISA backplane, together with other adapters/controllers (see http://www.malinov.com/Home/sergeys-projects/sergey-s-xt)
- It seems that people like the "motherboard" approach more than processor card + backplane. But at the same time it doesn't make much sense to build XT / Baby AT form factor motherboard, as there is no new cases for this form factor. Instead I was thinking about doing a MicroATX board, that will fit in modern MicroATX and ATX cases.
- The problem with MicroATX form factor is that it have space for only four ISA extension slots. But this form factor has enough real estate to integrate some controllers that builders will want to have anyway. For example an XT-IDE, a floppy disk controller and a 16550 UART (or a couple of them) could be integrated.
- Controllers' intergation can be taken to an extreme - it is possible to integrate a Sound Blaster compatible audio (say AD1815 or ESS1868 ), and an Ethernet controller (RTL8019AS). These chips come in 100-pin PQFP packages, so they can be a little bit tricky to deal with (but still it should be doable with a regular soldering iron).
- If using PQFP packages, it is possible to use a multi I/O controller chip with keyboard, mouse, floppy, RTC, serial, and parallel controllers, thus saving more space on the board, and possibly having a cheaper solution (I guess a multi I/O chip will cost about the same as FDC+keyboard controller+RTC it replaces)

What of the following options do you prefer:
1. Continue the current "processor card" form factor. Possibly developing some additional ISA adapters, for example XT-IDE + floppy, a MicroATX compatible backplane.

2. Design a basic MicroATX system putting whatever I have on the processor board in a MicroATX form factor board. Hopefully 4 slots will be enough (graphics, multi I/O card [floppy, serial, and parallel ports], XT-IDE, audio or network, but not at the same time?!)

3. Design a MicroATX system with a few additions - an XT-IDE controller, floppy controller (based on Intel 82077 chip), and a 16550 UART. In this case only through-hole components will be used.

4. Design an advanced MicroATX system using an integrated Multi-IO chip, optional audio and Ethernet controllers (optional in sense that builders can omit them when building the system).

5. Some another option (please describe).

6. Why in the world somebody will want to have an 8088 computer in ATX form factor!? :)

Thanks,
Sergey
 
I prefer option 1 (the current "processor card" form factor).I think is better because you can use it in AT and in ATX boxes,and preserves the original ISA XT architecture.The only thing I don't know is the availability of the ISA backplane
 
sergey said:
6. Why in the world somebody will want to have an 8088 computer in ATX form factor!? :)
Click to expand...

This. I actually was thinking of doing the exact thing you are talking about. However my mental progression went from what you are thinking of to:

1) I could use a Super-I/O chip to eliminate a ton of parts.

2) I could use a FPGA or large CPLD to eliminate just about everything else except for the CPU itself and the Super-I/O

3) If I did either 1 or 2, it would more than likely lock the design into the SMT realm and prevent most people from building boards

4) If I build it, would they come?

So the end of my list matches the end of yours.

The only real difference between what you described and what I had in mind was the inclusion of XT-USB instead.

I even did a lot of research on both legacy CPUs and PLDs that were still in production that used DIP/PLCC/PGA packaging and only +5V. There are options, but I could never find a happy medium in my head I thought people would want. I figured I'd get my USB project off the ground then revisit. Then my new job kicked in. :(
 
angel_grig said:
I prefer option 1 (the current "processor card" form factor).I think is better because you can use it in AT and in ATX boxes,and preserves the original ISA XT architecture.The only thing I don't know is the availability of the ISA backplane
Click to expand...

ISA backplanes are readily available, and not very expensive (something like $30, maybe less on eBay). Some of them will fit in Baby AT (or XT) cases, but I never seen a pure ISA backplane that will fit in ATX case (there are some with PCI and ISA slots).

The Micro ATX motherboard form factor probably will make the PCB a bit more expensive (say $40 instead of current $30), plus cost of ISA connectors (something like a dollar or two per connector), but it will save headache of finding and buying backplane and suitable case, probably will result in more reliable system (less connectors), and also ATX will allow having additional I/O ports on the back without adding flat cables and additional I/O brackets.
 
Not sure where it's at or if it's still being worked on/played with but the C-One (Commodore One) used an fpga for the processor as well. The only reason I mention this is because at least when it was first out they also wrote a Z80 FPGA emulation for the Amstrad not just the C64. From there I think a few other kernels were written for other emulations, though I'm not sure what limitations it has.
 
Another idea I had - going down the FPGA rabbit hole - was to add connectors for a mezzanine daughter card that could carry either a 8088/7, 80286/7, 80386/7, or 80486/7 daughter board. I just wasn't sure of the appeal of such an idea.

When you start building anything in ultra-small unit quantities, the price tends to skyrocket. Unless the project had a real conservative scope, I think the price targets would put most people off.

But regardless of which way you go, I would love to help (and front some of the time and materials equity).
 
Is an 8088 CPU (clone) still available?

It would be interesting (to me anyway) to have a kit available to build your own XT. I mean, one that runs with the XT BIOS. and BASIC. Integrated XT-IDE, SoundBlaster Pro and CGA would make it very much usable in terms of getting software on it and working - a compact flash slot onboard would be kind of ideal I guess.
 
Intersil makes/made various 8086/8, 80186/8, and 80286 new processors and new replacement 8237a, 8284, 8255, 8259, 8288, and other support chips found most PC/XT/AT clones. Most are still available new, however because they are the only supplier of new replacement silicon, the prices are pretty steep. Still for some people it might be easier than salvaging from old motherboards.

Intel shutdown their 386ex production line only a couple years ago. They are still selling new ones from back production stock in PLCC, DIP, and TQFP packages. I heard NASA and other organizations that needed hardened EMI processors have been buying them.
 
sergey said:
5. Some other option (please describe).

6. Why in the world somebody will want to have an 8088 computer in ATX form factor!? :)
Click to expand...

5: I like your mATX concept, but placing the CPU on an expansion card would create a BTX crossover and probably confuse beginners. If you kept the bare essential components on the board (Processor, memory, keyboard and maybe mouse connectors) and offered everything else as an expansion card, that would be more in-keeping with the original PC compatible systems.

However, as this project would require extra controllers to use modern hardware, you could take advantage of the available real estate on the board and put 4 more expansion slots behind the backplate area. In that configuration it would look like an XT board, plus it would be the perfect place to put internal hardware controller cards with blank PCI brackets.

6: Extra expansion options. This could actually tie in with #5, but spreading outward rather than inward.

Also, most of the world's bad-ass cases are windowed ATX, some people don't like seeing the unused space a small motherboard would create.
 
pearce_jj said:
Is an 8088 CPU (clone) still available?

It would be interesting (to me anyway) to have a kit available to build your own XT. I mean, one that runs with the XT BIOS. and BASIC. Integrated XT-IDE, SoundBlaster Pro and CGA would make it very much usable in terms of getting software on it and working - a compact flash slot onboard would be kind of ideal I guess.
Click to expand...

Intersil still produces CMOS equivalents for most Intel 8xxx chips. And they are compatible with n-MOS chips that were used in the original XT design. Intersil prices are a bit high (although I don't think 8xxx ICs were cheaper back in 80's). But there is a lot of other cheap sources for new-old-stock and used 8xxx and 8xCxx chips.

It is a hobby project, and I am not trying to make any profit on it, so don't really have kits (and I don't intend to do kits) but I do have PCBs for the first version and some (more rare) chips for it, just to make life easier for potential builders. I guess other ICs (74ALS*, flash, SRAM and such) are not hard to find. My system does run my own BIOS implementation (very compatible with XT and AT), and most if not all programs that would run on an XT, including MS DOS, and GWBASIC on top of it. For now I am just using regular ISA cards, such as XT-IDE, ES1868 audio, and video cards (I tried a CGA clone, Hercules clone and a few VGA cards).

Speaking about the next design ideas, it shouldn't be too hard to integrate audio and Ethernet (using single chip solutions, like ES1868 mentioned above). But I don't want to integrate the video controller. Even if it would be possible to find cheap and available single-chip video controller, there is a good reason not to integrate it: Video controller is a very personal preference. Some like original CGA, some like EGA, and some like advanced SVGA with 2MB of RAM...

Compact flash works now, just connect it to the XT-IDE :) And it is my preferred storage for this project - cheap, reliable and fast...
 
eeguru said:
This. I actually was thinking of doing the exact thing you are talking about. However my mental progression went from what you are thinking of to:

...

The only real difference between what you described and what I had in mind was the inclusion of XT-USB instead.

I even did a lot of research on both legacy CPUs and PLDs that were still in production that used DIP/PLCC/PGA packaging and only +5V. There are options, but I could never find a happy medium in my head I thought people would want. I figured I'd get my USB project off the ground then revisit. Then my new job kicked in. :(
Click to expand...

Well, the intention is to have fun building things :) so option 6 is not really an option, it rather something to check public interest on this project :)

I thought about using CPLD, something like Xilinx XC9500 - they are convenient as they will work with +5V power supply, they are available in PLCC packages that can be installed in through-hole sockets, and the best of all Xilinx offers development tools for free. But few folks mentioned that it will make project harder to build - as one will need to have a programmer. It is also possible that particular CPLD will be discontiuned in the near future and it will be hard to find component (if theoretically someone will want to build this project in a few years). The general purpose logic, such as TTL 74ALS or 74F series is still produced by a few manufacturers, and there is a huge stock available, so it's cheap and easy to find.

Speaking about FPGA implementation - it is completely possible to implement the entire XT or even 386/486-AT with everything including CPU, graphics, audio and video on a single FPGA, maybe only adding some SRAM to it. But it's not cool for me :) It's also possible to buy an Atom based motherboard for $70, and run DOSbox on it - again no fun...
 
barythrin said:
Not sure where it's at or if it's still being worked on/played with but the C-One (Commodore One) used an fpga for the processor as well. The only reason I mention this is because at least when it was first out they also wrote a Z80 FPGA emulation for the Amstrad not just the C64. From there I think a few other kernels were written for other emulations, though I'm not sure what limitations it has.
Click to expand...

It is possible to implement an XT or AT computer entirely in FPGA (maybe except the memory), but it is not the intention of my project. I was trying to use "real" 8xxx chips found in XT or AT, and also have a compatibility with existing ISA adapters.
 
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hello
personally a find accelerator cards very interresting. Therefore - project:
make the ultimative add on card , with a ??? cpu, Giga RAM..and?... ;)
Purpose of my project - to make a PC that runs the first games for PC and newer multi-tasking OS in one box !:)
regards cimonvg
 
sergey, if you're certain about making it through-hole only, you still need an initially programmed ROM or 2. And quite a lot of ROM programmers will also fuse PAL/GALs. A few 16v8's or 22v10's go along way in a design. Atmel also has a few more complex PLDs in programmer friendly DIP form factors (ATF750, 15xx and 2500 series). Could significantly cut your 74xx chip count.
 
eeguru said:
sergey, if you're certain about making it through-hole only, you still need an initially programmed ROM or 2. And quite a lot of ROM programmers will also fuse PAL/GALs. A few 16v8's or 22v10's go along way in a design. Atmel also has a few more complex PLDs in programmer friendly DIP form factors (ATF750, 15xx and 2500 series). Could significantly cut your 74xx chip count.
Click to expand...

Will see, I might give GALs a try... Unfortunately my programmer (cheaper Willem type) doesn't support them. But it should be pretty easy to build something like GALBlast. I'll check Atmel PLDs too. Thank you for this recommendation.
 
I have one that will do most JEDEC PAL/GALs as well as the Atmel DIP PLDs (ATF 750/15xx/2500). I'd be happy to program them for folks for materials and actual shipping only. I also have about 80 27c256 surplus EPROMs - again free, programmed, labeled, for the cost of postage (Paypal).

Also most programmable logic devices now days have in-circuit programming interfaces. You could the pin them to a header. Not sure about other manufacturers, but Lattice has support in their free ispVM programming tools for a parallel port -> JTAG cable. Of course M$ dropped parallel port support in their 64-bit OSs. But ispVM runs well on 32-bit Windows and some older flavors of Linux. And since the programming interface is JTAG, it gives you boundary scan functions too.
 
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