Technical Explanation for Power Base Converter not working on the 32X?

[DogP]

UPDATE, modification instructions added in post 24: http://www.sega-16.com/forum/showthr...l=1#post687032 .

Hey,

Quick question... I figure if anyone knows the REAL answer, it's probably here.

It's common knowledge that the PBC doesn't work with a 32X, but the typical reasoning I see is that it's because the 32X doesn't pass the Master System select pin through. That's false (pin B30 is connected), and I know some people know that... but does anyone know the actual reason that it's incompatible? I see that the Genesis, 32X, etc. service manuals have been made available, and maybe the answer lies in there.

It seems like there could be a few reasons for it... not signaling the Genesis to tell it to run SMS properly, or not passing the address/data through properly, or conflicting hardware, or...? Before I start digging, I figured I'd check in with the experts.

Thanks,
DogP

[Guntz]

I don't know the absolutely true answer, but it could be as simple as the 32X being completely incapable of setting its self to redirect SMS signals. That's all the Genesis does, it literally just re-arranges where the signals go when SMS mode is enabled. The 32X probably only has support for Genesis mode.

[DogP]

Thanks for the reply, but I'm not sure that really makes sense. As far as I understand it, the SMS signals weren't anything special. It was just the same address and data lines that the Genesis used... but rather than having the 68000 be the bus master (running Genesis code from the cart) and the Z80 run the audio, instead the Z80 was the bus master (running SMS code from the cart), and the 68000 was disabled.

I'm not sure whether the 32X actually does something special with the data when it's not being used, but I was under the impression that when disabled, it'd essentially just pass the address and data straight through, and the extra processors, memory, etc. would be disabled. Whether 68000 or Z80 data is going across the 32X, I wouldn't think it'd matter, and the 32X wouldn't even really know, since either one would be driven from the Genesis side. But that's one possiblility of where my assumptions are incorrect.

DogP

[madmax2069]

Its been asked before http://www.sega-16.com/forum/showthr...e-with-the-32X

That topic link should answer your questions especially this post by TmEE on that same topic http://www.sega-16.com/forum/showthr...l=1#post266681

[Guntz]

There's a lot of hardware in the 32X, there's no room for direct connection to just pass through a Genesis or SMS cart to the Genesis system slot. The 32X probably only has hardware support for making a Genesis cartridge work, not SMS.

[DogP]

Its been asked before http://www.sega-16.com/forum/showthr...e-with-the-32X

That topic link should answer your questions especially this post by TmEE on that same topic http://www.sega-16.com/forum/showthr...l=1#post266681

I know the question of whether it works or not has been asked many times... I know that it doesn't. My question is what exactly causes it not to work? I had come across that post, and saw TmEE's reply, but he doesn't really go into any of the reasoning of why it doesn't work, or what he has tried (just that he tried some things and they didn't work). I'm not sure if he knows the exact reason, or if he's also not sure. I was hoping maybe one of the Sega technical gurus (including TmEE) knew what the actual limitation was. That post was from long before the service manuals (with schematics) were posted, so I wondered if someone may have noticed the limitation after looking at the schematics.

Thanks,
DogP

[Chilly Willy]

Very simple example... not exactly correct, but gives you a way to visualize what it going on. Let's say that in Genesis mode, the data lines carry data, while the address lines carry the address. Now let's say that in SMS mode, that changes so the the data lines are the SMS address, while a small number of the address lines now carry the SMS data. The Genesis interface chip understands and supports this. The 32X interface chip does not. The exact change in the lines is unimportant - what is important is the 32X doesn't support the change in what the lines carry. If you wish to know EXACTLY which lines change to what, get the pinout for the SMS cart, the pinout for the Genesis cart, and the schematic for the PowerBase converter, all of which are easily found online via google.

[Joe Redifer]

So there's probably no way to mod a 32X to do this without a firmware update or a massive switch, I'm assuming.

I wish I could get composite video out of my Virtual Boy.

[DogP]

Very simple example... not exactly correct, but gives you a way to visualize what it going on. Let's say that in Genesis mode, the data lines carry data, while the address lines carry the address. Now let's say that in SMS mode, that changes so the the data lines are the SMS address, while a small number of the address lines now carry the SMS data. The Genesis interface chip understands and supports this. The 32X interface chip does not. The exact change in the lines is unimportant - what is important is the 32X doesn't support the change in what the lines carry. If you wish to know EXACTLY which lines change to what, get the pinout for the SMS cart, the pinout for the Genesis cart, and the schematic for the PowerBase converter, all of which are easily found online via google.

Well... I understand that SOMETHING doesn't happen properly, otherwise it'd work. But I'm fairly certain that nothing like address and data lines swapping places happens.

I have looked at what is involved in making a PBC, and for the most part, the address and data lines are connected as they are on a Genesis... plus there are a few control signals. But the SMS and Genesis pinouts and details of a PBC don't answer the question of what specifically the 32X doesn't do, which causes a PBC not to work.

One thought I have is related to how the PBC has A21 held high and A22 held low (and A23 is used for the pause button). Typically, an address line would be unidirectional (the bus master would control it), so the 32X may have those as output buffers only (i.e. whatever is on the Genesis side is driven out the 32X cart side). So by connecting A21 high and A22 low, you're shorting outputs of the 32X, and not causing those pins to actually do what they're supposed to do on the Genesis side. But that's just one possibility. I was wondering if maybe someone had already looked into this to say that's the problem, or that one or more of the control signals aren't connected, or that the 32X doesn't disable completely and sits in the SMS memory space, or...

DogP

[DogP]

I wish I could get composite video out of my Virtual Boy.

Heh, I will finish that SOME day.

DogP

[DogP]

One thought I have is related to how the PBC has A21 held high and A22 held low (and A23 is used for the pause button). Typically, an address line would be unidirectional (the bus master would control it), so the 32X may have those as output buffers only (i.e. whatever is on the Genesis side is driven out the 32X cart side). So by connecting A21 high and A22 low, you're shorting outputs of the 32X, and not causing those pins to actually do what they're supposed to do on the Genesis side.

So... looking at the service manual, it looks like that's certainly a possibility of at least part of the problem. The schematics show that A21-A23 of the cartridge slot connect to RA21-RA23 on IC5 (315-5805) - which the service manual lists as outputs... and A21-A23 of the cartridge edge connect to VA21-VA23 of IC5 - which the service manual lists as inputs. If that's the case, then there's no way that the 32X would pass those through, which presumably would prevent the PBC from working (assuming those are actually needed for it to work in the first place)... and could be bad for the 32X. It wouldn't take much to hack a workaround for this though.

Of course there may be other things preventing this from working... which I'm hoping maybe someone that knows can chime in with.

DogP

[DogP]

Success! At least I think so. I don't actually have a PBC, but I have an Everdrive MD (that's not Arnold Palmer Tournament Golf in the system ), which can play games in SMS mode. I simply cut the traces to A21 and A22 on the cart socket and jumpered them to A21 and A22 on the cart edge, and it seems to work.

DSC02681.JPG

Of course I probably broke 32X functionality... but it means that a simple logic chip switching A21-A23 based on the SMS mode pin should make it work.

DogP

[Guntz]

Very nice work! That's probably exactly why Sega never tried to make the SMS compatible, or why there aren't any factory fixes for that like there is for Virtua Racing support.

[Chilly Willy]

Having a mod to allow SMS through the 32X would be a great thing. Don't know how many people would be able to use it, but that's beside the point. Getting it going with the minimum amount of hacking is a noteable achievement.

[DogP]

Yeah, it shouldn't be too tough to mod it for full functionality (just gotta dig through my logic chips and see what I have laying around). I have a long and convoluted reasoning for wanting this... basically, I have the Everdrive MD, which lets me play SMS games on the Genesis, so I don't need a flash cart for my SMS. But I want to use it through the 32X so I get the S-Video that I already modded the 32X for. I need S-Video because my TV has a comb filter, which adds a few lines of delay, which ruins gun games, which are some of my favorite SMS games... and the comb filter isn't used on the S-Video ports (plus S-Video just looks better ).

If I just had a good way to run the 3D games on the Genesis with the Everdrive, I'd be all set.

DogP