Megabus to 68000 socket adaptor

[frank.lukas]

Internal is nicer but I think you have a PAK inside the Stacy ...?

[Darklord]

Internal is nicer but I think you have a PAK inside the Stacy ...?

Hehehehe, I do. However <cough> I have enough spare parts to make
at least 1 more STacy, maybe 2.

Really tied up with several other things at the moment. Who knows,
maybe something to look at down the road...or a project for some
other ambitious soul...

[mrbombermillzy]

Ive been hunting for an easy to use donor machine for any future Pistorm shenanigans and really the only suitable candidate is a MegaST4, but that has a soldered CPU.

I then remembered that I had a Megabus to 68000 DIP adapter kindly given to me by @PhilC at CL4.

After successfully hunting it down, Ive had a look and need some help with the connectors:

P1050307.JPG (256.62 KiB) Viewed 61 times

Im guessing I need to bridge /BR or /HALT on the 'BUS' 8 pin connector?

@PhilC I wonder if you tell me if my assumptions are incorrect, what pins I actually need to bridge and anything else I may need to do?

(Normally I wouldnt be so specific with asking for help, but the adapter has your name on it, so I guess you are the best to ask!)

[PhilC]

@mrbombermillzy it's been years since I did that but I think one lot of headers is +5v and gnd and the others are cou pins that aren't duplicated on the Megabus

/Halt needs to be ran to /halt on the cpu socket or another /halt location. /Br can be linked to +5v as it's pulled high anyway. The other pins need to be connected to their respective signals on the cpu socket or elsewhere.

[Badwolf]

If you're going to do what I think you're going to do: be careful.

HALT doesn't send the onboard 68000 high-z. If you try to become a bus master on the megabus port (by connecting up the interrupt lines somewhere), you will need some logic to go through the bus arbitration dance else bus contention will ensue.

If you wanted to test without any DMA (ie. no blitter, floppy, ACSI) then it *may* be enough to hold the 68000's BGK low, but you'll have a quite limited experience.

I have been toying with making 68000 expansion to plug into the megabus for a little while using a CPLD or GAL to do the BR/BG/BGK negotiation, but you know how it is. :/

BW

[PhilC]

@Badwolf so much easier to pick up the original signals, just doesn't make it plug and play though.

[mrbombermillzy]

@mrbombermillzy it's been years since I did that but I think one lot of headers is +5v and gnd and the others are cou pins that aren't duplicated on the Megabus

/Halt needs to be ran to /halt on the cpu socket or another /halt location. /Br can be linked to +5v as it's pulled high anyway. The other pins need to be connected to their respective signals on the cpu socket or elsewhere.

Ok, thanks. I could have probably handled the +5v, GND, /HALT and /BR. But...

If you wanted to test without any DMA (ie. no blitter, floppy, ACSI) then it *may* be enough to hold the 68000's BGK low, but you'll have a quite limited experience.

I could have got away with that and have most of the DMA devices being absent. Unfortunately, I REALLY do need that ACSI port working.

I have been toying with making 68000 expansion to plug into the megabus for a little while using a CPLD or GAL to do the BR/BG/BGK negotiation...

That sounds like a great idea. For maximum effectiveness, I would mirror the connection so it works the other way too (i.e. MegaBus connections available in 6800 DIP based STs). I suppose it was naive of me to wonder why those adapters were not more mainstream/available: Its a tad more work for true 'plug and play'.

, but you know how it is. :/

Yes; like me, you have one of those 'children' things now. They do take up time dont they?

I wonder if anyone else has got one of these working? (for an expansion device that actually needs bus mastering)

[Badwolf]

@Badwolf so much easier to pick up the original signals, just doesn't make it plug and play though.

Yes, if cutting tracks or lifting pins is an option then that would be the easiest route but then not easily reversible and kind of half way to just socketing the CPU anyway.

Still would need the (two?) interrupt lines sniffed from somewhere anyway so never truly plug and play, but a negotiated bus master board could probably get away with only extremely minor bits of soldering.

BW

[PhilC]

To be honest, the PCB was designed to be used with things like H4 & H5 upgrades in my Mega ST and not as something to allow the CPU to be moved to the Megabus. I never tried it and used the H4 & H5s instead.

[Badwolf]

To be honest, the PCB was designed to be used with things like H4 & H5 upgrades in my Mega ST and not as something to allow the CPU to be moved to the Megabus. I never tried it and used the H4 & H5s instead.

Makes sense. That would work straight off the bat as it's what megabus was designed for: bus clients.

BW