Reply 31160 of 31165, by wierd_w
Shponglefan wrote on Today, 12:06:dormcat wrote on Today, 08:22:Shponglefan wrote on Today, 01:15:For the ISA card, a switch is hooked up to what I think is the bus timing jumper. With the jumper closed, the ISA card is enabled and boots. With the jumper open, the ISA card won't boot. Then either the AGP or PCI card will function depending on the BIOS initialization priority setting.
So the ISA card can override AGP/PCI cards with just a switch? That's something new to know; good work! 👍
Normally ISA video will always take priority. In this case removing the bus timing jumper seems to prevent the card's video bios from loading, so it's effectively disabled.
And I can't take full credit for this, I came across the idea in an 11-year old thread here: 486 Mobos: Can you install multiple VGA cards - like AGP/ PCI switching?
Disabling the video bios sounds like a more universal approach?
Something that just disables the address pins of the chip if disabled?
A latch chip / bus transciever tucked in 'dead bug' style inside a socket, with jumper pushed out one side, that goes in the vbios socket, with the vbios on top, perhaps?
When jumper is on, latch is on, and vbios address pins can be raised. When jumper is off, latch is off, and address pins cannot be raised.
Parasitically uses sockect's vcc and gnd to power the latch?
If the vbios uses CSelect, then we could just use a single transistor? (With the jumper on the 'on' signal for the transistor, with CSel signal on the collector, CSel status would go right through,and the vbios would behave normally. With the jumper removed, the transistor is 'off', and it wont.)
