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I've built 6502 computer on an ISA card:
To access the bus it decodes the following 6502address ranges
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:
.
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.
