multiple CGA cards on ISA bus?

Richard legalize at
Sat Dec 23 14:31:27 CST 2006


Does anyone know if its possible to have multiple CGA cards on an ISA
bus?  I have a feeling the answer is no...

If the answer is no, can anyone recommend a good set of docs on the
bus interface of a CGA (memory mapped regions, relevant BIOS vectors,
ISA bus transactions, etc.)?

I have a project where I want to gang drive a group of CGA displays.
Initially my thoughts were to just have N computers with CGA cards and
gang drive the computers over a serial port.

However, lately the talk of hardware on this list has got me thinking
that it might be better to create a single FPGA implementation that
gang drove N cards directly and dropped the machines in the middle.

I'm thinking that on standard ISA bus only a single CGA card could be
present because its vectors and memory map are fixed and not
relocatable.  So I'd need to gang up the cards by creating my own
motherboard with ISA card edge connectors where each card thinks it is
the only thing on the bus and then select individual cards and drive
them with the FPGA.

Things that would help me in this project are:

- detailed information about the CGA video card,
    both electrically and software wise.

- detailed electrical information about ISA bus cycles

- existing FPGA designs that might have useful building blocks I could
    steal: ISA bus cycles, CGA implementation, etc.
"The Direct3D Graphics Pipeline" -- DirectX 9 draft available for download

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