VOGONS


Short on ISA slots? Try this.

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First post, by jwt27

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- THE PROBLEM:
Do you always keep swapping cards around all the time?
Have you ever discarded a mainboard, just because it only had one single slot?
Do you have trouble deciding which cards to install in your time machine?

Wouldn't you rather be able install ten sound cards in one machine?

Nope. That's impossible. Or, is it?

- THE UNIVERSAL SOLUTION:
Add more ISA slots! 🤣

EH0D1Wc.jpg

- TUTORIAL:

Step 0:
Get a passive ISA backplane with the number of slots you need. Some backplanes will fit standard PC cases, you'll probably want one of these. An example is the 8-slot Advantech PC-6108 I have here, which fits in both AT and ATX cases.

S0BeQ0h.jpg

Step 1:
Get some of these cards. Cost: $24.45 for three (minimum order, free shipping).

PDTpEeo.jpg

Step 2:
Get all this stuff. Cost: $17.49 for a set of 2 cards, with 2ft (60cm) cable. (ex VAT & postage)
You can probably find these parts elsewhere too, for less. The right-angle pin headers, for example: I just got fifty 2x40 headers for $9 from ebay (China).

0SCq2G6.jpg

Step 3: (optional)
If you have any cards that need -5V, but your ATX power supply doesn't provide it, get this stuff too. Cost: $2.44 (ex VAT & postage)

JKXi3Z3.jpg

Step 4:
When you receive the cards you'll find breakaway tabs on the ISA slot edges, where the boards were attached to each other on the panel. Break them off carefully, then use sandpaper to remove the rough edges. Be careful not to damage the gold plating!
Then, solder it all together. For -5V, you only need the 7905 on one card. Installing the caps on both cards is okay (although 2x100µF polymer is some serious overkill already).
You can solder the pin headers to either the top or bottom side, it doesn't matter which, as long as they're on the same side on both cards.

I wanted to use shrouded headers at first, with a center notch so you could never accidentally plug the cable in the wrong way around. But I can't seem to find right-angle versions of these with the required number of pins. Using a straight header is not possible, as it would obstruct the slot right above it.
I also found that shrouded headers don't fit on my prototype cards. This is resolved in rev1 which you can order above, although now a shrouded header would obstruct the holes for the 7905...
Oh well, these generic break-away headers work fine too, just be careful not to plug it in the wrong way around.

c7mYgU8.jpg

Step 5:
Assemble the cables. Note the correct orientation: You'll find a triangle mark on the IDC connectors, which should line up with the red wire on your flatcable. When assembled, the center bumps should both point in the same direction.
For the 36-pin cable, you'll need to tear 4 wires from the 40-wire one. For some reason 36-wire cable is rather expensive.

Pressing these things together is harder than it looks, especially that 64-pin plug! I would recommend doing this in a vise. On my first attempt I used water-pump pliers; gets the job done, but won't make your connectors look any prettier.

Don't make the cables any longer than necessary, to prevent crosstalk and other interference. As a rough guideline, 40-wire IDE (non-UDMA) cables are limited to 450mm (18"), and support a 12.5MHz clock speed. ISA clock can vary between 1 to 20MHz (according to google, on PCI boards this is usually PCI clock divided by 4, or 8.33MHz), and you'll want a shorter cable at higher clock speeds.
"So how does this huge 20-slot backplane even work, then?", you might ask. Well, on a circuit board the traces are spaced wider apart and may be separated by ground planes, so you'll have less crosstalk. This is why 80-wire IDE cables allow faster data rates, too 😉

snWnfvL.jpg

Step 6:
Install and test. Your backplane will likely have AT power connectors. It'll be powered by your mainboard, so there's no need to connect these.

You may have noticed your backplane has termination resistors, on one or both ends. Some motherboards have these too, near the lower ISA slot. These are probably necessary if your ISA bus runs at a high clock speed, or is very long (as in our case). Termination resistors should be located on the far end of the bus, away from the southbridge. If your computer randomly locks up or shows any other erratic behaviour while using the backplane, try experimenting with these resistors. You may have to remove the resistors from your mainboard (add SIP sockets for easy replacement!), and only install them on the backplane instead. Another option would be to increase the I/O recovery time setting in your BIOS; This may make the system less susceptible to interference, but it's not an ideal solution since this will slow the bus down a bit.
Must say though, I haven't had any problems with this yet. I even tried running the 20-slot board with all resistors removed and it worked just fine.

I haven't really tested the -5V part with any cards that use it, but both my voltmeter and the BIOS hardware monitor say it works fine. If you do use it, keep an eye on the 7905's temperature for a while, especially if using multiple card on the -5V rail. It may be a good idea to mount it on a heatsink if it's running hot. With the small currents involved, I don't expect this to happen though.

You'll probably want to build a case around your backplane. I haven't done this yet, but I think the easiest way is to stack up two ATX cases, bolt them together, and cut holes in the top and bottom for the cables to run through.

40ecbL4.jpg

Step 7:
Have fun assigning IRQs and all that stuff... 😀

Last edited by jwt27 on 2014-12-01, 05:26. Edited 1 time in total.

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Reply 2 of 75, by dogchainx

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Uh......That's my worst nightmare with IRQ's. 🤣

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Reply 4 of 75, by Bullmecha

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That is an awesome idea. Multiple IDE controllers anyone 🤣

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Reply 5 of 75, by Sutekh94

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dogchainx wrote:

Uh......That's my worst nightmare with IRQ's. 🤣

🤣

Also, you can install all the sound cards you'll ever want using this method! GUS, PAS, LAPC-1, SB16, AWE64, and so on!

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Reply 6 of 75, by jwt27

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dogchainx wrote:

Uh......That's my worst nightmare with IRQ's. 🤣

Yeah, IRQ hell is more or less inevitable if you'd fill all those 20 slots.. I didn't try it with that many cards yet, just filled it up to take that picture 😀

The reason I came up with this idea is because I'm looking to upgrade from an Asus P2B to a P3B-F board, which is superior in almost every aspect... but only offers one ISA slot. And I was already struggling with three slots on the P2B. Now with the 8-slot board and two adapters, I would end up with 7 usable slots; more than enough for an MDA/Herc and some sound/MIDI cards!

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Reply 7 of 75, by smeezekitty

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The reason I came up with this idea is because I'm looking to upgrade from an Asus P2B to a P3B-F board, which is superior in almost every aspect... but only offers one ISA slot. And I was already struggling with three slots on the P2B. Now with the 8-slot board and two adapters, I would end up with 7 usable slots; more than enough for an MDA/Herc and some sound/MIDI cards!

But how do you work that into a case?

Reply 8 of 75, by jwt27

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I think I'll bolt two small ATX cases together, and cut holes for the cables to run through.

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Reply 9 of 75, by JayCeeBee64

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[Darth Vader Voice]Impressive....... Very Impressive[/Darth Vader Voice]

Now that's what I call expanding a PC's capabilities 😲 . Nicely done jwt27.

Of course, as you and others have said, IRQ hell awaits (as well as DMA hell 😵 ). Still, it's worth it just to give your old PC some extra ISA slots to play with 😁

Bullmecha wrote:

That is an awesome idea. Multiple IDE controllers anyone 🤣

Sure, why not. How many would you like - 4, 5, 6 ? 🤣

Better yet, how about multiple SCSI controllers? Just think of the possibilities 😁

Ooohh, the pain......

Reply 10 of 75, by mr_bigmouth_502

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Doesn't running that many cards on the ISA bus all at once slow things down a lot? Also, since it's all being powered through the motherboard, aren't you worried that something's going to fry or blow up?

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Reply 11 of 75, by jwt27

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mr_bigmouth_502 wrote:

Doesn't running that many cards on the ISA bus all at once slow things down a lot?

That's right, it'll slow down a bit if you run several cards simultaneously. Realistically though, under normal use, how many cards would you use at once? For myself, I might fill the backplane with sound cards but I'll never use more than two at the same time. Even if you run several HDD controllers, you're only going to use two at most when copying files from one to another.

This is different in a multi-tasking environment, of course. But then, the only reason to use ISA cards nowadays is for DOS compatibility.

mr_bigmouth_502 wrote:

Also, since it's all being powered through the motherboard, aren't you worried that something's going to fry or blow up?

That's an interesting point. And I must say, I haven't given this much thought yet. Since the 5V pin is most likely connected to a power plane on your mainboard, I wouldn't worry too much about frying that. If anything does burn up, I fear the traces on my adapter board will be first to go.

If you're worried about your boards smoking indoors, just power your backplane from the PSU directly. 😀

The only thing I've fried so far is the light bulb I used to take pictures for this thread. RIP Sylvania 225V 1000W Made in West-Germany 🙁

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Reply 12 of 75, by jxhicks

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This is awesome. Everyone is talking about the IRQ hell, what about the wiring hell? I hope you have a huge switchbox. Sidenote, I think I have that same Compaq keyboard.

Reply 14 of 75, by jwt27

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mr_bigmouth_502 wrote:

Also, since it's all being powered through the motherboard, aren't you worried that something's going to fry or blow up?

Digging some more into this:

I can't find any specifications online about maximum power draw from the ISA bus, so I presume there's no real standard for this (does anyone know?). Looking on the P3B-F board I'm planning to use, the only power pins I can see with traces leading to them are +12V and -12V, and these traces are about 0.8mm wide. Let's assume my measurement is off, and the trace width is really 0.635mm (25mil).
Google gave me this nice web calculator to determine the maximum current these traces can carry. Assuming 1oz copper thickness (worst case, I think) on the mainboard, and allowing a maximum temperature rise of 10°C, we can pull 1.7 amps from the +12V and -12V rails. That's 20.4W for each. Not too much, but not directly a cause for concern either: most ISA cards take power from the 5V rail.
Since the 5V trace is not visible, I assume it's connected directly to an internal power plane. I don't know if all Pentium 3 boards have a reasonably sized 5V plane but Pentium and earlier boards certainly do. How much current you can pull from this depends on it's thickness and shape, which we can't see. Power planes tend to be huge though so I wouldn't worry too much about frying it.
I can check some other (older) boards, see if there's some kind of standard trace width for power rails.

The power rail traces on my adapter board are 50mil (1.27mm) wide and 1oz thick. The traces leading to the 7905 are 25mil (0.635mm) wide.
There are three 5V pins in the ISA bus, so according to the calculator, the adapter board should be able to handle:
+12V: 2.8A, 33.6W
-12V: 2.8A, 33.6W
+5V: 8.4A, 42W
-5V: 2.8A, 14W
-5V: 1.7A, 8.5W (when using the 7905)

For the negative rails, most PSUs only supply a few hundred milliamps, so we're definitely safe there. I don't think 12V is much used, except for analog stuff like op-amps on sound cards. Most power is drawn from the 5V rail, which I assume is beefy enough for at least an 8-slot board. But then, I have no idea how much power the average ISA card takes.

Long story short; if you're worried about this, connect at least the 5V and 12V power pins on the backplane to your PSU, and you won't risk any damage.

jxhicks wrote:

This is awesome. Everyone is talking about the IRQ hell, what about the wiring hell? I hope you have a huge switchbox. Sidenote, I think I have that same Compaq keyboard.

Yeah, I still don't have a decent solution for this... Will have to resort to swapping cables for now. At least that's much less of a problem than swapping out sound cards all the time.
The keyboard is a Compaq Enhanced II. On a side-side note, if you happen to have a 5150 or XT, it'll work with that too! 😉

smeezekitty wrote:

Sylvania 225V 1000W

😲

Did a pretty good job of simulating daylight. All pictures in this thread were shot at night 😀
Problem is, it's a 118mm bulb. Replacements for these are not cheap, and very difficult to find...

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Reply 15 of 75, by meljor

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P3b-f has a version with 2 isa slots, just like p2b and p2b-f.

Back in the days i bought the p3b-f new and could choose between both versions: one extra isa slot or one extra pci.

Google ''p3b-f 2 isa slots'' and you will find pictures of it.

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Reply 16 of 75, by jwt27

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meljor wrote:

P3b-f has a version with 2 isa slots, just like p2b and p2b-f.

Back in the days i bought the p3b-f new and could choose between both versions: one extra isa slot or one extra pci.

Google ''p3b-f 2 isa slots'' and you will find pictures of it.

That's right, and I already knew: you can clearly see the holes where the second slot would be located.
I have no P3Bs with two slots though, and even if I did have one, two slots are still not enough for me 😉

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Reply 17 of 75, by jxhicks

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jwt27 wrote:

The keyboard is a Compaq Enhanced II. On a side-side note, if you happen to have a 5150 or XT, it'll work with that too! 😉

Thanks, that's good to know.

Reply 18 of 75, by anthony

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Have to mention that chipsets with native isa support is much more tolerate to ribbon cables connection. Think it's due to more signal strength on isa lines. If you will try same thing (backplane connection thru ribbon cable) on pci-isa bridge chip (ite or winbond at least) you'll face with some problems, especially with gus gf1 cards. From strange sounds to initialization fail at all.

Reply 19 of 75, by jwt27

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anthony wrote:

Have to mention that chipsets with native isa support is much more tolerate to ribbon cables connection. Think it's due to more signal strength on isa lines. If you will try same thing (backplane connection thru ribbon cable) on pci-isa bridge chip (ite or winbond at least) you'll face with some problems, especially with gus gf1 cards. From strange sounds to initialization fail at all.

Interesting. Did you try using termination resistors (or installing them in different positions) or shorter cables? And did that change anything?
(I don't think many people here have boards with a separate ISA bridge though)

And seeing as you've tried this before, do you happen to know if the adapter boards I made already exist or not? I couldn't find any so I had to resort to drawing my own, but then maybe I didn't search hard enough.

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