exxos's Atari, Amiga & retro forum

Ive often pondered the slow decay of good available ISA graphics cards for the various ST 68000 DIP/MegaBus/VME slot solutions available and the possible remedy.

Recently,I came across this open source ISA FPGA graphics card: https://github.com/schlae/graphics-gremlin

Whilst it currently only emulates MDA and CGA, there seems the possibility of the author extending to 256 colour VGA or better at some point.

I dont expect anything to come from this, but I remain hopeful that the right person sees this and runs with it. :D

@mrbombermillzy, there is a ET4000 project out there as well

viewtopic.php?f=29&t=4916

I think emulating these graphics cards on the 68k bus using a Pi or a CPLD is the way to go.

I remember reading about that one a while back @Icky

While thats a very fine project too, theres the risk of any one of the several components becoming pricey or unobtainable at any time, along with the inherent limitations of the ET4000w32i chip: i.e. amount of RAM onboard and addressable/DAC frequency/etc.

A fully customizable FPGA solution would truly be able to avoid the pitfalls of the old ISA cards.

As an aside, there was a rarely used option of ISA (I/O channel ready line: pin A10) which could allow the bus to run at up to 16Mhz. Not mind blowing, but it would certainly help if we could use this on the higher bus speed machines. (TT, Mega STe)

While Im dreaming, IMHO the biggest gain would be working in some sort of system like TIGA (Texas Instruments Graphics Architecture), which sought to combat the slow bus speed with on board acceleration primitives.

However, I will be quiet now, as I have neither the skills or time to accomplish this grand task. :lol:

Steve wrote: 23 Sep 2023 19:16 I think emulating these graphics cards on the 68k bus using a Pi or a CPLD is the way to go.

Ive yet to see one of these in action.

I will be very interested to see it in the flesh and how well it works at the show @Steve

Also, I dont know if anyone has seen AlanH's monsterbo card, but theres a MegaBus compatible slot on that which can allow the bus to be be sped up with a faster clock on the relevant pin.

So perhaps hiking up the MegaBus and ISA bus is a good way forward?

For the Atari there is no need to bother with ISA or any other obsolete PC standard. A simple, yet ET4000 class GFX card can be done just by using fast enough SRAM, large CPLD or FPGA, crystal oscillator and resistor network based video DAC. Then write a HDL state machine that will map the SRAM to the usual 10-14MB address space and generate the RGB/HSYNC/VSYNC from the SRAM space. Sure, that will need to write a new VDI driver.

ctirad wrote: 29 Sep 2023 22:39 For the Atari there is no need to bother with ISA or any other obsolete PC standard. A simple, yet ET4000 class GFX card can be done just by using fast enough SRAM, large CPLD or FPGA, crystal oscillator and resistor network based video DAC. Then write a HDL state machine that will map the SRAM to the usual 10-14MB address space and generate the RGB/HSYNC/VSYNC from the SRAM space.

great idea

ctirad wrote: 29 Sep 2023 22:39 Sure, that will need to write a new VDI driver.

probably ET4000 VDI (or a bit modified) could be used

I actualy designed a couple of such devices already. The last project is for the ST ROM Port. It contains 2MB of SRAM, XC95144XL CPLD, 18bit R2R DAC and VGA out. I finished it to the stage where it is possible to write pixels into videoram. Due to small CPLD space it is limited to a single VGA mode with 16bit falcon compatible hicolor and no burst modes. Also no driver was written, yet.

Acording to the driver, the proper way is to write a new driver for fvdi. The fvdi driver can be just a simple put pixel/get pixel function. Fvdi will cover the rest using its internal functions. It will be slow but definitely good enough as a proof of concept. Then I have a couple of ideas how to speed the ROM ort access to near the native bus speed. The bursts with an automatic incrementation are just the part of them.

@ctirad A very promising project!

PM sent ;)