ISA support

[-N2]

Hi @Kevin Williams

I see the awesome project is moving along nicely. I have a feature request for a memory mapped 16 bit ISA slot on the ATX board version(or perhaps a single board computer w/ISA between your ITX and raspberry pi board formats), it would seem most useful as your already placing the motherboard in a PC case.  I would like it for sound cards, but I think there may be a few other PC cards of use too. 

of notable mention for your onboard sound chips, Silicon labs makes a very nice series of PLL clock generator chips in the Si5351 series. they offer 3 to 8 individually configurable outputs between 3KHz to 290Mhz. I mention it as it could be useful if you need to slow the machine down for compatibly reasons with fussy hardware.  

 

 

 

 

[TomXP411]

ISA is not really practical. The bus cycles are too different on 8088, and therefore the ISA bus. The 6502 reads and writes memory in a single clock cycle. The ISA bus needs 4 clock cycles to do the same. 

If you really want to interface ISA devices, you'd need to build an ISA bridge board, like the Amiga had, where the ISA slots sit on their own backplane, and you would have to build some sort of FPGA interface to drive the ISA bus and pass data back and fort between them. Even so, you'll end up with 3 wait states for every ISA transaction. A single 6502 memory operation is actually 4 clock ticks on an ISA bus. In the end, you'd end up basically build a PC on an FPGA (something like AO486) and using GPIO to drive the ISA bus, using the Commander as a terminal. 

This is how the Amiga Bridge board worked - it was a complete PC on a board, sans storage and video hardware. The bridge board could access video and storage via the Amiga Zorro bus or the ISA sockets built in to the Amiga 2000 motherboard... but the thing is, even when using Amiga storage and video, the Amiga was never more than a terminal and/or disk drive for the PC hardware. 

 

[-N2]

1 minute ago, TomXP411 said:

ISA is not really practical. The bus cycles are too different on 8088, and therefore the ISA bus. The 6502 reads and writes memory in a single clock cycle. The ISA bus needs 4 clock cycles to do the same. 



If you really want to interface ISA devices, you'd need to build an ISA bridge board, like the Amiga had, where the ISA slots sit on their own backplane, and you would have to build some sort of FPGA interface to drive the ISA bus and pass data back and fort between them. Even so, you'll end up with 3 wait states for every ISA transaction. A single 6502 memory operation is actually 4 clock ticks on an ISA bus. In the end, you'd end up basically build a PC on an FPGA (something like AO486) and using GPIO to drive the ISA bus, using the Commander as a terminal. 



This is how the Amiga Bridge board worked - it was a complete PC on a board, sans storage and video hardware. The bridge board could access video and storage via the Amiga Zorro bus or the ISA sockets built in to the Amiga 2000 motherboard... but the thing is, even when using Amiga storage and video, the Amiga was never more than a terminal and/or disk drive for the PC hardware. 



 

ah good to know thank you!