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XT motherboard identification

640KB said:
I've actually been looking for proper documentation on the TD3300A for quite some time now (including asking recently) and have not come across a true data sheet or programming manual. What I did manage to find was from pieces here and there on forums and guesswork. I can't seem to find the exact post that tipped me off to port 0x90, but the process was really just trial and error working with a very patient tester who had the board, sending test builds back and forth until we figured out exactly what does what. It isn't all that complicated and I documented what I eventually settled on in the source.

The rest of the info I was able to find is basically in the ST-12/UX-12 manual which describes the function of the 0x70 register that controls the wait state. This is also this thread that describes the function (at a high level) of the high memory bank switching register 0xe0. Later on, I came across this post which details all three control registers, and the diagrams were clear enough that it did confirm my findings. I was always hoping there would be more info out there but haven't found any other references that mention anything else.

To answer your question, I actually had neither the data sheet nor had I even had an ST-12 or Juko BIOS at the time to play with. After working with that person who did have one, I realized that this board was really cool and so managed to find one! :)

If you come across anything else though, please share it. If there's any cool or hidden features that could be added for that board, I'd absolutely do it!
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Thanks. Great research! This is exactly what I saw when disassembling my BIOS:

1681168918616.png

And that IN didn’t make any sense to me, as they are overwriting the value in next instrucción. But I guess this is the exact sequence the asic expects.
 
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vwestlife said:
Now the question is... do the ISA slots run at 12 MHz as well?
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charlysan said:
That seems to be the case:
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So is it more of a question of "does the ISA bus run at 12 MHz" or is it "does it run reliably at 12 MHz"?

In that second post, the OP mentioned that the board was successfully overclocked to 13.5 MHz, however, did not work at 15 MHz. Now that is something I'd like to see reproduced. :)
 
I also suggest you to join this group. The guy that published the above project is a member, and Jorge has done some overclocking tests using Juko boards. After you join, you can check this thread for more details.
 
@640KB, I've pushed a small tool that can be used to write to TD3300A registers. It could be useful to discover undocumented features. I haven't had the time to play around with it (I just checked reg 0x90 features). This is beta, just as a POC; don't expect any error handling. Adding extra feature to loop through all possible registers and values should be doable. Feel free to push a PR if you want.

View attachment juko_tool.mov
 
For what it's worth, I've done some experimentation with an XT-alike using a 20MHz-rated NEC V40 with somewhat modern support parts. (XT-IDE and CH375/6 cards instead of legacy hard drive controllers, 55ns SRAM/70ns ROM, new build serial/FDC card)

For high bus speeds, you could get to 20MHz with maximum (3) wait states enabled and *some* stuff would work. Sometimes the CH375/376 modules didn't like it, sometimes the XT-IDE didn't. If you need to appear on a benchmark list somewhere, you'll get it.

At about 14-16MHz you were more or less usable.

The interesting thing is there is a distinct tender spot in my system-- once you get above 8MHz, TOPBENCH would crash with weird NMIs. Might be more common as you crank the clocks up, but even a relatively modest timing set-- say 10MHz with wait states-- would fail.

I suspect it might be the video card, a circa-1991 Realtek RTG3106, as it's one of the few things I can't remove or replace with a more modern equivalent (one day, I'll build that TVGA9000 card kit...)
 
I have a similar XT mobo, it is not working and trying to revive it. Checked the power to all the ICs. The thing that gives me an idea that the chipset (TD3300A) is bad is that it does not pass the reset signal to the rest of the ICs.
Sometimes the reset output is HI sometimes is low for no apparent reason, and the input reset does not control the output. Also the clock signal to the CPU is intermittent. When present is a 6.66Mhz. Both BIOSes checked and are ok.
Any input would be helpful.
Thank you
 
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felix19 said:
I have a similar XT mobo, it is not working and trying to revive it. Checked the power to all the ICs. The thing that gives me an idea that the chipset (TD3300A) is bad is that it does not pass the reset signal to the rest of the ICs.
Sometimes the reset output is HI sometimes is low for no apparent reason, and the input reset does not control the output. Also the clock signal to the CPU is intermittent. When present is a 6.66Mhz. Both BIOSes checked and are ok.
Any input would be helpful.
Thank you
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Have you tried different BIOS? Like GlaBIOs?
 
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