I've built 6502 computer on an ISA card:

enter image description here

To access the bus it decodes the following 6502address ranges

  • $0200-$03FF mapped at I/O address $00200-003ff (IOR; IOW)
  • $4000-$BFFF mapped at ISA address $04000-$07fff for memory access (smemr;smemw)
  • $8000-$BFFF mapped at ISA address B8000h-BBFFFh

Intention is to operate a VGA card via a passive ISA backplane.

I get no response from registers or buffers.

Are there any signals (beside IOR/IOW/MEMR/MEMW) it needs to manage?

Schematics of the CPU card: . 6502sbc

enter image description here


2 Answers 2


Another possibility for VGA to not work - the chipset is not enabled at all until cpu runs the code from the card's bios. The code typically writes few i/o registers before the card will start responding. At the very least there must be a write to 'VGA enable' register at i/o port 3C3h. Then the rest of VGA registers are enabled and you can continue the card's setup (memory layout, etc).

You can also try to read the VGA bios at ISA addresses starting at 0C0000h to make sure the whole your setup is at least valid.


The schematics are impossible to follow as for example there is no way to double-check if the CPU RW/E signals are correctly decoded to IOR/IOW/MEMR/MEMW.

There are also other things that may cause the card to not work.

For example, there is no clock fed to the OSC pin, which the card may use.

  • you cant see it on the schem i posted for some reason but aen does go to BE on the proccessor
    – Carl
    Jul 2 at 16:31
  • If so, what does the BE do? AEN should be low on IO and MEM cycles. It would be high during a DMA cycle, and I don't see any support for DMA.
    – Justme
    Jul 2 at 16:50
  • i included it for future dma there is no dma yet
    – Carl
    Jul 2 at 17:05
  • @Carl So if resitor R4 pulls ALE down and it inverted with U2C to BE input it should not be an issue. Is it a 16-bit VGA card? Make and model, or at least tell which chipset it has?
    – Justme
    Jul 2 at 17:32
  • yes on the r4 and inverter. i wish i could upload or link to pdf of schem so you could see better. The card is a Renaissance GRX 20-0009 8-bit
    – Carl
    Jul 2 at 17:42

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