There are a lot of 16BIT VGA cards that work perfectly in 8BIT slots. And there are always the Paradise 8BIT VGA cards. You can easily swap a VGA for an EGA/CGA card when the need arises.

http://www.vintage-computer.com/vcforum/showt … y-ISA-VGA-cards

I found a a nice Trident card based on the 8900C chipset which auto-detects 8 bit operation. So that should be out of the way.

Now i'm only missing a XT-IDE or XT-CF card.

carlostex wrote:

I found a a nice Trident card based on the 8900C chipset which auto-detects 8 bit operation.

When I researched my own 8900C, I remember a warning that the auto-detection doesn't always work.

Since the jumper choice is usually 8-bit or auto 8/16-bit, you're assured to have at least one that works. The remaining mysteries are only if it is SW3 or SW4 on your card, and whether it is Open or Closed for 8-bit.

pewpewpew wrote:

Since the jumper choice is usually 8-bit or auto 8/16-bit, you're assured to have at least one that works. The remaining mysteries are only if it is SW3 or SW4 on your card, and whether it is Open or Closed for 8-bit.

SW3 does the VGA scan rate, and SW4 will do the BIOS detection. Closed will enable auto BIOS detection and open will disable.

Unfortunately i didn't have enough time nor i could gather all needed parts for this build. I have little experience with XT systems so i'm gonna throw some questions to whom might know an answer to:

1-Can i replace the V20 by the V20HL? It's pin compatible right?

2-This board has 2 clock crystals. One 24 MHz and a standard 14.318MHz one. It's safe to assume the CPU clock comes from dividing the 24MHz crystal by 2, so is it also safe to assume the ISA bus gets its clock by dividing the 14.318MHz crystal by 2 as well?

3-Can these Juko boards work with AT keyboards? I think i read somewhere that some might do...

Iam using these boards myself in my own XT build topic here on vogons.. Building an XT is a little bit different, then building an AT system.

An AT keyboard wont work, you really need an XT compatible one. The board is also limited on IRQ adresses.. it only uses 0-9.. And some are already occupied but excisting devices..

On the soundblaster part, you will definitely using IRQ 7. Because this was the standard back in the days when this soundcard was released.. If you use IRQ 5 instead, not al of the games would detecting your SB card..
IRQ7 is really the one you want to use..

V20 or V20HL, could be the last one a high clocked one? It should be pin compatible with the V20 and the 8088. But higher then 10Mhz wouldnt really usefull on that system. Then i would get to speedy. So it will more annoying then usefull.

Your floppy i/o card will probably only exept 360KB and 720KB floppy drives only! there is also a manual on minuszerodegrees.net you could check out.

NEC V20 XT SUPER TURBO 8088 BUILD

Robin4 wrote:

V20 or V20HL, could be the last one a high clocked one? It should be pin compatible with the V20 and the 8088. But higher then 10Mhz wouldnt really usefull on that system. Then i would get to speedy. So it will more annoying then usefull.

The V20HL is a low power version that can obviously clock higher.

The ISA bus on these boards are tied to the CPU clock. Since your board is running at 12MHz that puts your expansion bus pretty far out of spec. This will likely cause problems for cards like your old SB 1.5. There's absolutely no point using a V20HL unless you are able to build a clock doubling circuit. Also keep in mind that the fastest 8087 runs at 10MHz, and I highly advise you to not bother running these in a system faster than that, because they don't overclock at all and will make your CPU vomit even if you don't use it.

I have a 10MHz Juko board as well, but mine uses a V30 at 10MHz. What's strange is that your board divides the clock crystal by 2 (24MHz), whereas mine divides it by 3 (30MHz). Not sure why that is, but I guess it doesn't matter much. I advise you to replace your crystal with a 20MHz one if you can't make the system stable at 12MHz.

So what is the 14.318MHz crystal in there for? Is there something else that will use its clock signal? Could it be that the Board allows the CPU and the ISA bus to be clocked by the 14.318 crystal divided by 3 to run at default 4.77MHz when Turbo is not pressed on?

If not i fail to see what is that crystal doing there.

that crystal is divided by 3 to provide ntsc color clock. By 12 for the 8253 programmable 3 channel timer chip used for Timing / DMA. etc.

appleiiguy wrote:

that crystal is divided by 3 to provide ntsc color clock. By 12 for the 8253 programmable 3 channel timer chip used for Timing / DMA. etc.

Yes i know that it was spec since the 5150, the NTSC color signal was one was the biggest reasons to choose a 14.318MHz crystal. What i don't see is why these motherboard makers did not use the 14.318MHz crystal to drive the ISA bus instead. Oh well...

I am not aware of any XT motherboards that have an asychronus ISA bus.

Anonymous Coward wrote:

I am not aware of any XT motherboards that have an asychronus ISA bus.

Right. I will most probably get a 20MHz and a 16MHz crystal to experiment with them , if the board is not stablt at 12. It probably won't be.

I just remembered that messing up with the crystal might not be a good idea. If the system uses the 24MHz crystal for CPU clock and divides it by 2 to give 12MHz it should divide by 5 to give 4,8 CPU clock (close enough to 4,77MHz) compatibility. If i swap the 24MHz osci for a 20MHz one i could be effectively hampering making the system actually slower when turbo is off.

Did you notice the board actually has two Crystals? The other one should be a 14.31818MHz crystal. On the original XT motherboard this crystal was divided by 3 for 4.77MHz. It's quite possible with turbo off the board uses this crystal for the clock speed.

Anonymous Coward wrote:

Did you notice the board actually has two Crystals? The other one should be a 14.31818MHz crystal. On the original XT motherboard this crystal was divided by 3 for 4.77MHz. It's quite possible with turbo off the board uses this crystal for the clock speed.

It would be great if with Turbo off it would divide the clock by 3 to give the original 4.77MHz speed for compatibility sake, although i find it an awkward motherboard design. They might as well have chosen to run the ISA bus via the 14.318 crystal as well and run it async with the CPU clock but...
Anyway, i haven't tested this system yet so i don't know if it behaves well running at 12MHz. If it doesn't, i will be certainly swapping the 24MHZ crystal by a 20MHz one.

I'm wondering if i can use the empty socket below the JUKO BIOS to program an EPROM or compatible EEPROM to provide my own BIOS extensions.

It is visible the silkscreen 27256 which indicates a 32KB EPROM should be used. It seems like if all IBM cassete BASIC ROMS are glued together it will fill the 32KB.
I'm not interested in cassete ROM, but if the empty socket on the motherboard was intended for cassete BASIC i'm wondering if i can use it for other stuff, like BIOS extensions like HD floppy support. HD extension BIOSes are only 8192 bytes long.

I also wonder what memory addresses does that epty socket use. For cassette BASIC it should start at F000h. If the motherboard searches for adapter ROMS past F000h then i could use the socket for BIOS extensions.

My concern is that IBM XT BIOSes only scan for adapter ROMS between C800h and E000h, which make the placement of a BIOS option ROM starting at F000h simply useless. So the question is where does these XT mothernboards place their option ROM socket.

My guess is if there is a socket in there the motherboard BIOS should scan it for BIOS extensions.

I managed to assemble my XT-IDE board just fine, and i piggybacked HD Floppy extensions to the XT-IDE BIOS. The system supports all floppy formats from boot time. Despite the Board not detecting the CF cards, something worse happened. My BTC-5060 XT keyboard stopped working. I think i killed it because i thought it was an AT-XT switching keyboard and i used it with an AT system. Funnily enough it did work with the AT and it worked with my XT Juko Board right after, but after a reboot it wasn't working anymore.

I Don't get any keyboard errors, the system just does not respond to any key presses anymore. Dammit, all was going quite well.

Did you try it in the AT system again?

I had a lot of problems getting keyboards to work on my Juko board. It doesn't like certain types of switchable keyboards and works intermittently.