MegaBus retrofit possible?

[IngoQ]

Hiya :)

In my (limited) understanding, the MegaBus ist more or less the CPU bus. Since there are numerous expansions that make use of the MegaBus, the question arises, if it could be retrofitted in machines, that have no expansion port, like the ST and STE.

Would it really be as simple as just using the CPU lines, or would it need driving and/or additional logic?

[exxos]

In my (limited) understanding, the MegaBus ist more or less the CPU bus. Since there are numerous expansions that make use of the MegaBus, the question arises, if it could be retrofitted in machines, that have no expansion port, like the ST and STE.

Would it really be as simple as just using the CPU lines, or would it need driving and/or additional logic?

Megabus is basically just a "break out" for the CPU. Myself and Rodolphe did talk about using it on my STF remake motherboard, but problem is space, better to use a smaller expansion connector to allow more expansion options.

[IngoQ]

I did some digging and found this documentation:

http://dev-docs.atariforge.org/files/Me ... 2-1987.pdf

It says in there, that "excessive capacitance may cause improper ST operation". So I guess trace and/or wirelength are an issue if building an adapter.

In addition: If the STE is boosted, I guess it is entirely dependant on the expansion, whether it would work or not, right?

And my third thought: The STE used the MC68000FN8. Besides being the PLCC version, does it differ in any way to the "original" CPU used in the MegaST?

[exxos]

It says in there, that "excessive capacitance may cause improper ST operation". So I guess trace and/or wirelength are an issue if building an adapter.

They are talking about "load" on the bus. It's one reason I change the 10K/4.7K pull ups to 2.2K now.

Noted in that doc, 64 clocks for bus error.. always wondered that!

In addition: If the STE is boosted, I guess it is entirely dependant on the expansion, whether it would work or not, right?

The STE booster switches to 8mhz, so the idea being any previous expansion could still be used and be happy with the 32mhz booster, because it will act as a stock 8mhz machine.

The exception seems to be alan's monster.. so no idea how he managed to get it to malfunction. Saying that, I never got the IDE working on my STFM as a stock machine either and I have 2 monsters as well :roll:

And my third thought: The STE used the MC68000FN8. Besides being the PLCC version, does it differ in any way to the "original" CPU used in the MegaST?

DIP vs PLCC ? For TTL based stuff.. I doubt there is any real difference. PLCC likely new production run over DIP.. but nothing noteworthy.

PLCC 68HC00 is what I use though. Then its CMOS vs TLL.

[keli]

Noted in that doc, 64 clocks for bus error.. always wondered that!

Yup. I think it's handled by the GLU chip. If I remember correctly from the ASIC schematics, it looks like it monitors the /DTACK line and issues /BERR after the timeout has expired if /DTACK is not asserted.

[IngoQ]

Found this schematic in the Atari Profibuch. Appearantly it is not only CPU signals, _NMI, _INT5 and _INT3 are used also.

2017-09-09 09_07_02-ATARI Profibuch ST-STE-TT.pdf (GESCHÜTZT) - Adobe Acrobat Reader DC.png