I have a Cross Products Genesis/SCD combo unit... how does it work?

I suspect the "emulator" port is meant to connect to an ICE (In-Circut Emulator) for the M68000 CPU. There is an M68000 processor inside the Mega Drive itself, as well as a secondary M68000 inside the MegaCD unit. For your device ElBarto, I think you'll find it's basically an "off the shelf" MegaCD, which someone took and modified to insert the emulator port. That port is designed to override the second M68000 processor inside the MegaCD unit, allowing it to be run by a virtual emulated processor through the attached ICE. The ICE would then generally by connected to a PC, through which you could debug crashes or other such bugs. Essentially, it provides a hardware debugger on the physical system. Pressing that button on the front of the unit is disconnecting the real CPU inside the unit, and attempting to enable the virtual CPU attached via the emulator port, which will just lock up the unit in your case since there is no ICE attached.

Now, if anyone has the name, or any pictures of one of these ICE boxes these systems were meant to connect to, that could be useful. I haven't been able to find any information on them as of yet.

 

So if you're right, which processor the emulator port will be affected, the megadrive one or the mega-cd one ?
And, if you're always right, that mean I was right about the SCSI port, it's for read disc image from a computer.
Anyone got a full Cross Product with sotware and cable ? I only have the unit itself. Assembler maybe ?

 

For your machine, it can only be the secondary M68000 inside the MegaCD unit itself, since you can't disconnect the primary CPU inside the Mega Drive except through a modification to the Mega Drive unit itself. In the case of the combined unit which Kitsune Sniper found, I would suspect the emulator port is also for the sub CPU in the MegaCD unit, since the unit was obviously built for MegaCD development, and the sub-cpu would be the main focus of debugging when developing for that system. There would also be standalone Mega Drive units which were made with emulator ports on them also, and in that case, the emulator port would be for the primary CPU inside the Mega Drive itself. If you were to come across one of these systems and combine it with your standalone MegaCD unit, you could attach an ICE to either the primary or the secondary CPU.

I'm not sure about that yet. It could be. I certanly can't think of anything else it could relate to. I would think it is either to override the internal cdrom drive, and provide the data from another source, or it's a method of accessing the internal drive externally. I should be able to figure it out once I get my hands on the unit itself.

Yeah, according to the following thread, Assembler managed to get his hands on a complete system:
http://www.assemblergames.com/forums/showthread.php?t=3765

I couldn't find any pics of the rest of the system though, just the main unit itself, which seems pretty much the same as the one Kitsune Sniper found. Assembler said he got the instructions, software, and several other boxes with it. Any chance of getting some scans of the manuals, or even a copy of the software, or is that asking too much? At the least, some pics of the rest of the hardware would be great.

 

indeed, El Barto's unit seems to be an off-the-shelf MegaDrive modded (with the middle-piece) to sit on that dev (?) MegaCD unit. Interesting how they use a dedicated port for debugging.

From my experience with dev.kits I would suspect that SCSI is indeed destined to be for Optical Disk Emulation purposes, exactly like it is on the GDEV for example.

Considering the MegaDrive had two serial ports in the front, it's stupid how they were never used (officially) for RS-232 signals to show debug status on a host computer.

 

They're all modified off-the-shelf MCDs, but they have an additional PCB with presumably a debugger (no external ICE), which is why the case is expanded vertically. Cross debuggers generally connect to the PC via SCSI, and the CD emulator via the big connector. The emulator ALSO connects to the PC via SCSI.

The MD also doesn't have serial ports by any means! The two on the front and one on the back are multiplexed 8 to 4-bit parallel ports.

 

..in the shell of standard serial ports? why?!

 

That's wrong, the three DB9 connector support parrallel and serial mode.

 

You mean DE9? You are right, there is a UART, but is it RS232 which is what Barc0de was referring to?
The level isn't at least.

 

Um, are you referring to the controller ports there? Surely they would be using those for, you know, helping with the debugging process?

 

Yes, I am reffering to the 9-pin DB ports found in front of the unit, used for the controllers.

From little research that I did online, there's no way to make it work natively on the PC without an adaptor, although it fits nicely inside a standard PC serial port.

RS-232 is just a protocol, you can emulate it on any sort of port, even USB, as many cellphones do (eg P800)


http://en.wikipedia.org/wiki/RS-232

http://en.wikipedia.org/wiki/Serial_Cable

often does not mean always.

 

That's what I want to say.
As the three communication port of the megadrive support serial/parrallel you can use it for RS-232 communication.

 

RS232 is a *physical* protocol. You can make any port into a synchronous serial line, but you can't make an asynchronous serial port without a UART, either in software or in hardware. USB can't emulate RS232, a microcontroller connected to USB, connected to a MAX232 transceiver can. Sure you could do a software UART for the MD (granted it's interfaced through a MAX232 or similar), but then you couldn't have a game also running to debug since time is eaten trying to encode the clock signal. Since there is a hardware UART though, that's not necessary. Really old console games aren't debugged with "printf()", that is why they debuggers exist which are far more useful...

My point is that it's not often, if ever. The only thing I can think of that probably uses the serial mode is the Mega Modem. Since games always utilize the two front ports as controller ports, they are always set in parallel mode. Serial mode is so insignificant I had to dig into the SOA doc to find anything about it. I was wrong, but by chance

Realize that just because two devices use the same connector they do not necessarily work together, to assume that the ports are serial is a fallacy since for a 1988 console's controller to have a built in UART to serialize buttons is ludicrous.

 

I was aware of that, hence why I said one needs an adaptor in order to make MD controllers useable on a PC, despite the perfect fit of the serial port =)

So, in theory, one could utilize the serial side-port of the MD to debug? (any protocol) - provided there's debug support code inside the (in development) game title/gamepak?

sorry for my questions, I m just a lawyer :$

 

Side port? The edge pins under the expansion cover or the DE9 on the back of the console that was removed later on?

The latter (back DE9) is capable of serial output (and could be utilized with a debug build of a game to send messages to a PC) but the former (expansion connector) isn't really a port, it's more or less the same as the cartridge connector which is connected to multiple system buses.

Whether or not it can debug depends on what kind of debugging. Anything (port/register on cartridge etc) is pretty much capable of outputting a message to a PC. That's a common form of debugging but it's not very powerful at all. The more useful way is to have full control over the CPU which can only be done by hijacking it, either virtually using interrupts/bankswitching/tricky code or with an ICE replacing the CPU. It must be able to be done virtually from the cartridge port since PsyQ exists.

If you had to debug without any hardware other than a ROM emulator and a serial cable to your PC, it's possible to design a powerful debugger in software which would be linked to your debug build of your game (which would have to periodically give the debugger control.)

 

I dump my carts this way -- I run some homebrew code on Sega CD that transfers data from the cartridge through one of the connector ports (using a cable designed by a person called MoD)