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First post, by j^aws

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I'm thinking of tinkering with a 486 system, probably a PCI/ VLB/ ISA board, and was wondering if you can switch between multiple VGA cards, like you can with AGP/ PCI switching via BIOS?

I'm looking at a 486 build with a pair of slow/ fast VGA cards. Thanks in advance.

Reply 1 of 17, by 5u3

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No, switching primary VGAs on different buses is usually not possible on 486 machines, the exception may be boards with integrated/onboard VGA:

However: If you have a PCI mainboard and install an ISA(or VLB) VGA alongside a PCI VGA, the ISA card will always be the primary VGA, so you can "switch" by just inserting the ISA card and leave the PCI card installed.

I once hacked one of my ET4000 cards by cutting all the +5V lines on the ISA connector and providing +5V power through a switch. This seemed to work well, but one of my more electronics-savvy friends mentioned that this was a very bad idea and would possibly damage the VGA or the mainboard. [Edit:] As it turns out, this is really not a good idea, so don't do this! (see Jepael's post below).

Last edited by 5u3 on 2014-01-19, 20:19. Edited 2 times in total.

Reply 3 of 17, by 5u3

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The type of the PSU should be irrelevant. The +5v was taken from one of the molex connectors of the PSU.
One reason why I don't use this solution any more is that no single ISA VGA is compatible with all the games/demos/apps I run, so I'll have to swap it anyways.

Reply 4 of 17, by j^aws

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Okay, I see.

If you need more than one ISA VGA card to cover compatibility, then you should be able to use an appropriate switch (a multi-selector type) to divert +5V to just one ISA card, with multiple ISA VGA cards installed. So you shouldn't need to swap out VGA cards.

Reply 5 of 17, by 5u3

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Good idea, but I've only got one ISA slot available (the rest is filled up with sound cards 😉). Besides, cards might use other voltages, not just +5V, which could make this rather complicated.

Also, I'm still not sure about possible damages (as far as I understood my friend, some components connected to the bus might get damaged over time if they are "left floating" [if anyone reading this understands what he meant, please explain 🤣]).

Reply 6 of 17, by Jepael

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ISA cards were never intended to be left in the slot unpowered. I am surprised it did work, instead of jamming the PC or blowing up with a smoke.

Usually, if components are not specially designed for this, leaving unpowered chips sitting on a bus either sags the bus signals down when the chips would like to draw power through CMOS chip protection diodes. But that era does not use CMOS chips, more like LS and F series TTL chips, and they do not seem to have these protection diodes.

So just don't do this 😀

Reply 8 of 17, by j^aws

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*Bump*

5u3 wrote:

...
However: If you have a PCI mainboard and install an ISA(or VLB) VGA alongside a PCI VGA, the ISA card will always be the primary VGA, so you can "switch" by just inserting the ISA card and leave the PCI card installed.
...

FYI, I've managed to switch between PCI and ISA cards whilst both cards are installed in a motherboard, and without disabling power connectors. As you mention, the ISA slot takes precedence as the primary VGA slot if multiple buses have VGA cards installed. However, this only worked because the ISA VGA card has an undocumented jumper, which I have not determined its exact function, but when enabled/ disabled, it allows the PCI VGA card to take precedence and boot as the prminary VGA card instead. And vice-versa, whilst both are installed, giving the functionality of AGP/ PCI switching.

So, I'm thinking this jumber is disabling the VGA BIOS or something else that allows the other card to boot? And could be adapted to many ISA VGA cards?

Reply 10 of 17, by vetz

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Early Matrox PCI cards have a switch on them to disable use in DOS and only work in Windows 3.1. DOS performance prior to the Millennium was pretty bad, so Matrox probably incorporated this so users could use another card for DOS applications.

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Reply 11 of 17, by j^aws

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5u3 wrote:

Nice find! Which ISA card is it? Could you upload a photo?

It's a Trident card, loathed by many, but it has its uses! Besides the aforementioned jumper, it has another jumper that forces the card to use 8bit or 16bit ISA bus widths.

So, with a PCI VGA card and appropriate CPU, I can build the ultimate speed sensitive system that's both CPU and GPU limited, and everything in-between... Also, in theory, should be able to build a triple bus system with VGA on AGP, PCI and ISA cards. I've attached a picture and the jumper is the bottom left one.

I'm wondering if this could be applied as a mod to other ISA cards?

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Vetz wrote:

Early Matrox PCI cards have a switch on them to disable use in DOS and only work in Windows 3.1. DOS performance prior to the Millennium was pretty bad, so Matrox probably incorporated this so users could use another card for DOS applications.

Have you tried an ISA card alongside it? Looks like this setup would use the Matrox as a dedicated daughter card for GUI acceleration.

Reply 12 of 17, by 5u3

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j^aws wrote:
It's a Trident card, loathed by many, but it has its uses! Besides the aforementioned jumper, it has another jumper that forces […]
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5u3 wrote:

Nice find! Which ISA card is it? Could you upload a photo?

It's a Trident card, loathed by many, but it has its uses! Besides the aforementioned jumper, it has another jumper that forces the card to use 8bit or 16bit ISA bus widths.

So, with a PCI VGA card and appropriate CPU, I can build the ultimate speed sensitive system that's both CPU and GPU limited, and everything in-between... Also, in theory, should be able to build a triple bus system with VGA on AGP, PCI and ISA cards. I've attached a picture and the jumper is the bottom left one.

I'm wondering if this could be applied as a mod to other ISA cards?

It may very well be possible to mod other ISA cards to do the same trick, since your Trident seems to be a standard design, just as most other cards of the time. The only difference is that it has a lot more jumpers, typically there are only jumpers to disable waitstates and IRQ. Maybe it has something to do with 8/16bit BIOS selection.
BTW, your card has a TVGA8900D chip, which should actually perform quite decently and can hold a candle to a Tseng.
I should have an old Trident stashed away somewhere, I'll check it out.

Reply 13 of 17, by j^aws

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5u3 wrote:

It may very well be possible to mod other ISA cards to do the same trick, since your Trident seems to be a standard design, just as most other cards of the time. The only difference is that it has a lot more jumpers, typically there are only jumpers to disable waitstates and IRQ. Maybe it has something to do with 8/16bit BIOS selection.
BTW, your card has a TVGA8900D chip, which should actually perform quite decently and can hold a candle to a Tseng.
I should have an old Trident stashed away somewhere, I'll check it out.

This is the closest entry I found on TH99 for the Trident - it's unidentified:

http://www.uncreativelabs.de/th99/v/U-Z/50813.htm

The jumper in question is J10, which is in a block assigned for "8/16-BIT BUS SELECT". However, the combination I'm using is not documented. I tried it on my VL/ EISA system but it doesn't work like a PCI/ ISA system - the ISA Trident card refuses to boot. Moreover, the J6 jumper works on both systems and is the one that toggles 8/16 bit buses, which is reflected accordingly when running benchmarks.

That's interesting about the TVGA8900D version being near Tseng speeds. I haven't benched a "D" version against other cards yet, but I'll check it out.

Reply 14 of 17, by vetz

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j^aws wrote:

Have you tried an ISA card alongside it? Looks like this setup would use the Matrox as a dedicated daughter card for GUI acceleration.

Should work with any card. Haven't tried this feature myself, but it's documented in the user manual-

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3D Acceleration Comparison Episodes

Reply 15 of 17, by 5u3

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j^aws wrote:

The jumper in question is J10, which is in a block assigned for "8/16-BIT BUS SELECT". However, the combination I'm using is not documented. I tried it on my VL/ EISA system but it doesn't work like a PCI/ ISA system - the ISA Trident card refuses to boot. Moreover, the J6 jumper works on both systems and is the one that toggles 8/16 bit buses, which is reflected accordingly when running benchmarks.

The "switching trick" won't work with VLB because it is just an add-on to the ISA bus, so when you have a VLB and an ISA video card installed, it's just like having two ISA VGAs at the same time, which results in conflict.
None of my ISA cards has 8/16-bit selectable via jumpers, so I can't really test this. Your Trident card seems to be special in the way 8/16-bit mode is switched (three jumpers instead of just one or autodetect).

j^aws wrote:

That's interesting about the TVGA8900D version being near Tseng speeds. I haven't benched a "D" version against other cards yet, but I'll check it out.

In Eliandas ISA VGA card roundup, a TVGA8900D poses next to all the good cards, while all the other Tridents perform rather like their reputation.

Reply 16 of 17, by j^aws

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5u3 wrote:
j^aws wrote:

The jumper in question is J10, which is in a block assigned for "8/16-BIT BUS SELECT". However, the combination I'm using is not documented. I tried it on my VL/ EISA system but it doesn't work like a PCI/ ISA system - the ISA Trident card refuses to boot. Moreover, the J6 jumper works on both systems and is the one that toggles 8/16 bit buses, which is reflected accordingly when running benchmarks.

None of my ISA cards has 8/16-bit selectable via jumpers, so I can't really test this. Your Trident card seems to be special in the way 8/16-bit mode is switched (three jumpers instead of just one or autodect.

I've managed to get another VGA card working, and this one is a Cirrus Logic chipset (410/420) , and only an 8bit card, so it doesn't have this 8/16bit bus selection feature. It's made by Video Seven (V7):

http://www.uncreativelabs.de/th99/v/U-Z/52859.htm

... The jumpers in question are the p2/ p8 block, which are unidentified. They may still be related to some bus function. At least this 'feature' is not only on Trident cards. If I get some time, I maybe able to check datasheets and figure something out that could be a mod for other cards.

Reply 17 of 17, by j^aws

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Okay, getting this to work seems easier than previously thought!

In addtion to getting the previous 2 cards working, I've managed to get a further 4 ISA cards working - two Tseng ET4000AXs, a Paradise WD90C31 and a different Trident 8900D). And it looks like these jumpers are, intentionally or unintentionally, disabling the ROM BIOS from booting/ initialising. So, it should be possible to rig a jumper setup and mod existing ISA cards to disable these BIOS ROMs...

A quick way that enabled me to discover this feature was:

1) Make a note of all jumper settings then disable jumpers until the ISA card refuses to POST.
2) Ignore obvious jumpers like IRQ, and pay attention to jumpers that have 3 pins.
3) With jumpers that have a state of pins 1/2 closed, or 2/3 closed, remove the jumper completely, i.e. so that no jumper is closing any of pins 1-3. This has a high chance of stopping the card from booting, thus enabling the trick to dual boot with a PCI card.

... I've even managed to get two ISA cards dual booting - a Tseng and a Paradise, alongside an ATI PCI card. This combo worked whilst various others failed to POST.

Anyone else getting this to work, please post with results - much appreciated.