Mega 4 two chip TOS with no 74LS11

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troed
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Re: Mega 4 two chip TOS with no 74LS11

Post by troed » Thu Nov 25, 2021 9:17 am

exxos' hunch is likely correct - an "advanced" TOS 2.06 decoder. Pasting in my FB comment:
Def. a TOS switch of sorts. "New TOS" or "Old 2 chip" / "Old 6 chip". It's picking up ROM2 from the G19 connection to GLUE. However, there's more involved that I'm not sure I understand (like G24 - 'DMA'). It's picking up the whole top address byte from the 68000 so I'm assuming it's able to do some software switching too. Or just an interesting way of getting TOS 2.06 running through address space changing.
edit: The other three CPU connections are AS, RW and DTACK

/Troed

satjanand
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Re: Mega 4 two chip TOS with no 74LS11

Post by satjanand » Thu Nov 25, 2021 12:30 pm

rubber_jonnie wrote:
Thu Nov 25, 2021 8:59 am
Ok, so looking at your pictures above, none of the jumpers are correctly set for 6 chip TOS.
Thank you, I am aware of that. In that same picture you can see that the mega was running on a two chip setup, so the jumper setup is correct. Trust me I have set all three jumpers correctly while switching from 2 chip to 6 chip setup (multiple times).

What should strike you as odd in that picture though, is not that the 2 chip TOS is being accompanied by a 2 chip jumper setup, but that there is no 74LS11 installed, while the 2 chip setup actually requires one to be. Installing one ordered from the webshop did not affect behaviour.

That oddity was already kind of answered bij Exxos. He said quite early that it's weird the 2 chip setup worked without the 74LS11. He then already predicted the chip select signals had to come from elsewhere. By turning it over, I think I've found it.

Following Exxos' and Troed's suspicion and by judging the label on the contraption itself:

This has probably been a switchable TOS, of which the new part was later removed. The old part still works, but only through 2 chip setup (black wire connected to "Old 2"?) and without the need of 74LS11. Since there is no wire connected to "Old 6", I think this is why I can't run a 6 chip setup. I have two directions to go:

1) see what happens when I remove "Old 2" wire and connect it to "Old 6". (This is quite easy and if it works, it works for me.)

2) Remove the whole contraption. (This means a lot more work if I have to revert removal)

After which I set the jumper configuration to six chip setup and install six chip TOS 1.4.

So I will try 1) first. The mega is already closed again, so it won't be before the weekend. Will get some pictures for you as well.

Thank you for thinking with me, I have only joined the online Atari community this year, but wow you guys are great! Please keep sharing your thoughts.

satjanand
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Re: Mega 4 two chip TOS with no 74LS11

Post by satjanand » Thu Nov 25, 2021 12:33 pm

troed wrote:
Thu Nov 25, 2021 9:17 am

edit: The other three CPU connections are AS, RW and DTACK

/Troed
Thank you Troed, although I am not aware of what that does.

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rubber_jonnie
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Re: Mega 4 two chip TOS with no 74LS11

Post by rubber_jonnie » Thu Nov 25, 2021 1:19 pm

satjanand wrote:
Thu Nov 25, 2021 12:30 pm
rubber_jonnie wrote:
Thu Nov 25, 2021 8:59 am
Ok, so looking at your pictures above, none of the jumpers are correctly set for 6 chip TOS.
Thank you, I am aware of that. In that same picture you can see that the mega was running on a two chip setup, so the jumper setup is correct. Trust me I have set all three jumpers correctly while switching from 2 chip to 6 chip setup (multiple times).

What should strike you as odd in that picture though, is not that the 2 chip TOS is being accompanied by a 2 chip jumper setup, but that there is no 74LS11 installed, while the 2 chip setup actually requires one to be. Installing one ordered from the webshop did not affect behaviour.

That oddity was already kind of answered bij Exxos. He said quite early that it's weird the 2 chip setup worked without the 74LS11. He then already predicted the chip select signals had to come from elsewhere. By turning it over, I think I've found it.

Following Exxos' and Troed's suspicion and by judging the label on the contraption itself:

This has probably been a switchable TOS, of which the new part was later removed. The old part still works, but only through 2 chip setup (black wire connected to "Old 2"?) and without the need of 74LS11. Since there is no wire connected to "Old 6", I think this is why I can't run a 6 chip setup. I have two directions to go:

1) see what happens when I remove "Old 2" wire and connect it to "Old 6". (This is quite easy and if it works, it works for me.)

2) Remove the whole contraption. (This means a lot more work if I have to revert removal)

After which I set the jumper configuration to six chip setup and install six chip TOS 1.4.

So I will try 1) first. The mega is already closed again, so it won't be before the weekend. Will get some pictures for you as well.

Thank you for thinking with me, I have only joined the online Atari community this year, but wow you guys are great! Please keep sharing your thoughts.
I suspect that the guys are right regarding the mod, just be sure to fully document it in case you ever want to re-add it.

Good luck and you're welcome.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...

troed
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Re: Mega 4 two chip TOS with no 74LS11

Post by troed » Thu Nov 25, 2021 3:26 pm

satjanand wrote:
Thu Nov 25, 2021 12:33 pm
Thank you Troed, although I am not aware of what that does.
I'm quite confident it's a TOS 2.06 mod (it can DTACK) - although it has a lot of unnecessary connections for one. I still wonder if it does something else too - but for your purposes it's not important.

On this forum we both help as well as document as much as possible for posterity :) In the future someone else with the same mod will come upon this thread.

/Troed

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